Yeah, so one of my friends said we can use indexes to traverse a stack, but I think he's wrong. Basically, I have a homework in which I had to write an algorithm using an array. I had to use two for loops to do it, so I was wondering how to do something like this with a stack:
for(int i = 0; i < n; i++)
{
for(int j = 0; j < n; j++)
{
x = A[i]+A[j]
}
}
There is no way right? And I have to use pop() and push() only to do whatever I need to do, right? Because I used an array and stack concurrently, but one of my friends told me I can't do that. I know we can implement a stack using an array, but the stack ADT doesn't have indexes (although they just said stack and not stack ADT).
said we can use indexes to traverse a stack, but I think he's wrong.
You're right, he's wrong.
There is no way [to do two nested loops] right?
You can access element at index if you have enough space for a temporary stack: pop to the index while storing the popped values onto a temp stack, remember the value, and then push the values back:
int GetAt(Stack s, int index) {
Stack temp;
while (temp.size() != index) {
temp.push(s.pop());
}
int res = temp.top();
while (!temp.empty()) {
s.push(temp.pop());
}
return res;
}
Yes, that's very, very slow.
Yes, a pure stack abstraction would not have indices. But pure abstractions rarely exist outside of Comp Sci classrooms and Haskell User's Groups, and most stack implementations can admit to something like this, because indeed, they are usually implemented using an array. At the end of the day, you don't get a prize for how "pure" something is, but rather for getting the job done. I can certainly imagine a situation in which you build up a stack, and then at some point, you need to process all the elements as given in your loop. Welcome to the real world!
"Stack" is an abstract concept, not something that exists in reality. In the real world, they are generally implemented as data in consecutive memory addresses, and so indexing them is certainly possible, depending on the language/library/API you're using. Without a specific language or library, there's really no way to answer a question like yours.
If what you really mean is, is there a way to do the calculation you mention with two data collections that can only be accessed by push/pop, then probably not without an intermediate array. But why would you want to? A stack is, by its very nature, intended to be used for algorithms that only need to access its data in a LIFO way. Why else would you use one?
Actually you're all wrong(tm)! With two stacks ("pure" stacks) you can implement an array, inefficiently. Suppose stack S1 has 10 items pushed on it, and you want item 5; just pop 4 items off of S1, pushing each on to S2 in turn. Then pop off one more and that is your item. Just keep track of the index of the item that is currently in use (on neither stack), and you can always retrieve any of the other items when you need them.
But this idea is absurd.
Look at Reverse Polish Notation for how to use stacks for computing.
http://en.wikipedia.org/wiki/Reverse_Polish_notation
Basically push the values, then pop them off and apply a function.
Stack have index. Depending on how you implement stack we can use array or link list to implement stack in array the indexing is start from left to right from zero. But we can do indexing in both ways either from
stack top element to bottom most element
bottom most element to top most element
Related
The .split_off method on std::collections::LinkedList is described as having a O(n) time complexity. From the (docs):
pub fn split_off(&mut self, at: usize) -> LinkedList<T>
Splits the list into two at the given index. Returns everything after the given index, including the index.
This operation should compute in O(n) time.
Why not O(1)?
I know that linked lists are not trivial in Rust. There are several resources going into the how's and why's like this book and this article among several others, but I haven't got the chance to dive into those or the standard library's source code yet.
Is there a concise explanation about the extra work needed when splitting a linked list in (safe) Rust?
Is this the only way? And if not why was this implementation chosen?
The method LinkedList::split_off(&mut self, at: usize) first has to traverse the list from the start (or the end) to the position at, which takes O(min(at, n - at)) time. The actual split off is a constant time operation (as you said). And since this min() expression is confusing, we just replace it by n which is legal. Thus: O(n).
Why was the method designed like that? The problem goes deeper than this particular method: most of the LinkedList API in the standard library is not really useful.
Due to its cache unfriendliness, a linked list is often a bad choice to store sequential data. But linked lists have a few nice properties which make them the best data structure for a few, rare situations. These nice properties include:
Inserting an element in the middle in O(1), if you already have a pointer to that position
Removing an element from the middle in O(1), if you already have a pointer to that position
Splitting the list into two lists at an arbitrary position in O(1), if you already have a pointer to that position
Notice anything? The linked list is designed for situations where you already have a pointer to the position that you want to do stuff at.
Rust's LinkedList, like many others, just store a pointer to the start and end. To have a pointer to an element inside the linked list, you need something like an Iterator. In our case, that's IterMut. An iterator over a collection can function like a pointer to a specific element and can be advanced carefully (i.e. not with a for loop). And in fact, there is IterMut::insert_next which allows you to insert an element in the middle of the list in O(1). Hurray!
But this method is unstable. And methods to remove the current element or to split the list off at that position are missing. Why? Because of the vicious circle that is:
LinkedList lacks almost all features that make linked lists useful at all
Thus (nearly) everyone recommends not to use it
Thus (nearly) no one uses LinkedList
Thus (nearly) no one cares about improving it
Goto 1
Please note that are a few brave souls occasionally trying to improve the situations. There is the tracking issue about insert_next, where people argue that Iterator might be the wrong concept to perform these O(1) operations and that we want something like a "cursor" instead. And here someone suggested a bunch of methods to be added to IterMut (including cut!).
Now someone just has to write a nice RFC and someone needs to implement it. Maybe then LinkedList won't be nearly useless anymore.
Edit 2018-10-25: someone did write an RFC. Let's hope for the best!
Edit 2019-02-21: the RFC was accepted! Tracking issue.
Maybe I'm misunderstanding your question, but in a linked list, the links of each node have to be followed to proceed to the next node. If you want to get to the third node, you start at the first, follow its link to the second, then finally arrive at the third.
This traversal's complexity is proportional to the target node index n because n nodes are processed/traversed, so it's a linear O(n) operation, not a constant time O(1) operation. The part where the list is "split off" is of course constant time, but the overall split operation's complexity is dominated by the dominant term O(n) incurred by getting to the split-off point node before the split can even be made.
One way in which it could be O(1) would be if a pointer existed to the node after which the list is split off, but that is different from specifying a target node index. Alternatively, an index could be kept mapping the node index to the corresponding node pointer, but it would be extra space and processing overhead in keeping the index updated in sync with list operations.
pub fn split_off(&mut self, at: usize) -> LinkedList<T>
Splits the list into two at the given index. Returns everything after the given index, including the index.
This operation should compute in O(n) time.
The documentation is either:
unclear, if n is supposed to be the index,
pessimistic, if n is supposed to be the length of the list (the usual meaning).
The proper complexity, as can be seen in the implementation, is O(min(at, n - at)) (whichever is smaller). Since at must be smaller than n, the documentation is correct that O(n) is a bound on the complexity (reached for at = n / 2), however such a large bound is unhelpful.
That is, the fact that list.split_off(5) takes the same time if list.len() is 10 or 1,000,000 is quite important!
As to why this complexity, this is an inherent consequence of the structure of doubly-linked list. There is no O(1) indexing operation in a linked-list, after all. The operation implemented in C, C++, C#, D, F#, ... would have the exact same complexity.
Note: I encourage you to write a pseudo-code implementation of a linked-list with the split_off operation; you'll realize this is the best you can get without altering the data-structure to be something else.
I'm creating a indicator that recognizes candlestick shapes.
To do that I created a separate class Candlestick that I include to the indicator file.
The problem is that I suffer from memory leaks.
I'm new to pointers and after reading / watching a lot, I still seem to miss something here.
This is the Indicator class. The content of the Candlestick class is irrelevant so I leave that out.
Candlestick *candles[];
void OnDeinit(const int reason)
{
for(int i = 0; i < ArraySize(candles); i++ ){
delete(candles[i]);
}
}
int OnCalculate(args here)
{
ArrayResize(candles, Bars);
for(int i = MathMax(Bars-2-IndicatorCounted(), 1); i >= 0; i--)
{
candles[i] = new Candlestick();
// Do stuff with this candle (and other candles) here e.g.
if(candles[i+1].type == BULLISH) Print("Last candle was Bullish");
}
}
When I do this I get memory leak errors. It seems that I need to delete the pointers to the candles in that dynamic array. The problem is, when and where? Because I need them in the next iteration of the for(){...} loop. So I can't delete it there.
When I delete it in the OnDeinit() function there are still candles out there and I still get the leak error.
How come?
First, Nick, welcome to the Worlds of MQL4
You might have already realised, the MQL4 code is not a C.
Among many important differences, the key here is what does the code-execution platform ( the MetaTrader Terminal 4 ) do in what moment.
OnCalculate() is a zombie-alike process, which gets invoked many times, but anyway, definitely not under your control.
Next, OnCalculate() by-design does not mean a new Bar.
How to?
MQL4 conceptually originates from days, when computing resources were many orders smaller and much more expensive in terms of their time-sharing CPU-MUX-ing during a code execution phase.
Thus the MQL4-user-domain language retains benefits from some hidden gems, that are not accessible directly. One of these is a very efficient register-based update-processing and keeping dynamic resources allocations on miminum, for their devastatingly adverse effects on Real-Time execution predictability.
This will help you understand how to design & handle your conceptual objects way smarter, best by mimicking this "stone-age"-but-VERY-efficient behaviour ( both time-wise & memory-wise ), instead of flooding your memory-pool with infinite amount of unmanaged instances upon each call of OnCalulate() which sprinkles an endless count of new Candlestick(); // *--> candles[]
A best next step:
If in doubts, just read about best practices for ArrayResize() in the platform localhost-help/documentation, to start realise the things, that introduce overheads ( if not blocks ) in a domain, where nano$econd$ count & hurt in professional software design.
I would like to know what isEqualToArray actually does...
I have an array with size 160, each containing a dictionary with 11 entries, but I can do the comparison simply based on the first column (contains the date when the row was changed).
Now I can do that with a simple for-cycle:
BOOL different = FALSE;
for (int index = 0 ; index < [newInfo count] ; ++index)
if (![[[oldInfo objectAtIndex:index] objectForKey:#"Update"] isEqual:[[newInfo objectAtIndex:index] objectForKey:#"Update"]]) {
different = TRUE;
break;
}
if (different) {
}
else
NSLog(#"Contact information hasn't been updated yet");
Or I can use the built-in isEqualToArray method:
if ([oldInfo isEqualToArray:newInfo])
NSLog(#"Contact information hasn't been updated yet");
else {
NSLog(#"Contact information has been updated, saving new contact information");
[newInfo writeToFile:path atomically:YES];
}
Now, if assuming isEqualToArray just invokes isEqualTo for each cell, the for-loop method runs for 1/11 of the time isEqualToArray does (only need to compare one column instead of 11).
Maybe I'm just too much into optimizing... (I've been at many contests where runtime was limited and I'm feeling the after-effects).
The Documentation says:
Two arrays have equal contents if they each hold the same number of objects and objects at a given index in each array satisfy the isEqual: test.
So basically you are right.
From a design point of view I would either go for isEqualToArray:, since it makes the code easier to understand or introduce a BOOL hasUpdates if you are concern about performance, which has the additionally advantage that you don't have to hold two copies in memory.
I suspect that many people wrongly assume that performance is proportional to the number of source statements executed and that a function like isEqualToArray is blindingly fast compared to the equivalent directly-coded loop.
In fact, while sometimes the coders of these APIs do indeed know a few "tricks of the trade" that speed things up a bit (or have access to internal interfaces you can't use), just as often they must throw in additional logic to handle "oddball" cases that you don't care about, or simply to make the API "general".
So in most cases the choice should be based on which most reasonably fits the overall program and makes the logic clear. In some cases the explicit loop is better, especially if one can harness some of the logic (eg, to take a later-required "max" of the array values) to avoid duplication of effort.
Also, when there is a complex API function (more complex than isEqualToArray) you're not quite sure you understand, it's often better to code things in a straight-forward manner rather than deal with the complex function. Once you have the code working you can come back and "optimize" things to use the complex API.
When you know both objects are Arrays, isEqualTo<Class> method is a faster way to check equality than for loop.
isEqualTo<Class> is used to provide specific checks for equality.so isEqualToArray: checks that the arrays contain an equal number of objects.
So as per my knowledge i can say isEqualToArray is better option when you know that two objects are arrays.
I have a function inside a loop inside a function. The inner function acquires and stores a large vector of data in memory (as a global variable... I'm using "R" which is like "S-Plus"). The loop loops through a long list of data to be acquired. The outer function starts the process and passes in the list of datasets to be acquired.
for (dataset in list_of_datasets) {
for (datachunk in dataset) {
<process datachunk>
<store result? as vector? where?>
}
}
I programmed the inner function to store each dataset before moving to the next, so all the work of the outer function occurs as side effects on global variables... a big no-no. Is this better or worse than collecting and returning a giant, memory-hogging vector of vectors? Is there a superior third approach?
Would the answer change if I were storing the data vectors in a database rather than in memory? Ideally, I'd like to be able to terminate the function (or have it fail due to network timeouts) without losing all the information processed prior to termination.
use variables in the outer function instead of global variables. This gets you the best of both approaches: you're not mutating global state, and you're not copying a big wad of data. If you have to exit early, just return the partial results.
(See the "Scope" section in the R manual: http://cran.r-project.org/doc/manuals/R-intro.html#Scope)
Remember your Knuth. "Premature optimization is the root of all programming evil."
Try the side effect free version. See if it meets your performance goals. If it does, great, you don't have a problem in the first place; if it doesn't, then use the side effects, and make a note for the next programmer that your hand was forced.
It's not going to make much difference to memory use, so you might as well make the code clean.
Since R has copy-on-modify for variables, modifying the global object will have the same memory implications as passing something up in return values.
If you store the outputs in a database (or even in a file) you won't have the memory use issues, and the data will be incrementally available as it is created, rather than just at the end. Whether it's faster with the database depends primarily on how much memory you are using: is the reduction is garbage collection going to pay for the cost of writing to disk.
There are both time and memory profilers in R, so you can see empirically what the impacts are.
FYI, here's a full sample toy solution that avoids side effects:
outerfunc <- function(names) {
templist <- list()
for (aname in names) {
templist[[aname]] <- innerfunc(aname)
}
templist
}
innerfunc <- function(aname) {
retval <- NULL
if ("one" %in% aname) retval <- c(1)
if ("two" %in% aname) retval <- c(1,2)
if ("three" %in% aname) retval <- c(1,2,3)
retval
}
names <- c("one","two","three")
name_vals <- outerfunc(names)
for (name in names) assign(name, name_vals[[name]])
I'm not sure I understand the question, but I have a couple of solutions.
Inside the function, create a list of the vectors and return that.
Inside the function, create an environment and store all the vectors inside of that. Just make sure that you return the environment in case of errors.
in R:
help(environment)
# You might do something like this:
outer <- function(datasets) {
# create the return environment
ret.env <- new.env()
for(set in dataset) {
tmp <- inner(set)
# check for errors however you like here. You might have inner return a list, and
# have the list contain an error component
assign(set, tmp, envir=ret.env)
}
return(ret.env)
}
#The inner function might be defined like this
inner <- function(dataset) {
# I don't know what you are doing here, but lets pretend you are reading a data file
# that is named by dataset
filedata <- read.table(dataset, header=T)
return(filedata)
}
leif
Third approach: inner function returns a reference to the large array, which the next statement inside the loop then dereferences and stores wherever it's needed (ideally with a single pointer store and not by having to memcopy the entire array).
This gets rid of both the side effect and the passing of large datastructures.
It's tough to say definitively without knowing the language/compiler used. However, if you can simply pass a pointer/reference to the object that you're creating, then the size of the object itself has nothing to do with the speed of the function calls. Manipulating this data down the road could be a different story.
How does a stack overflow occur and what are the ways to make sure it doesn't happen, or ways to prevent one?
Stack
A stack, in this context, is the last in, first out buffer you place data while your program runs. Last in, first out (LIFO) means that the last thing you put in is always the first thing you get back out - if you push 2 items on the stack, 'A' and then 'B', then the first thing you pop off the stack will be 'B', and the next thing is 'A'.
When you call a function in your code, the next instruction after the function call is stored on the stack, and any storage space that might be overwritten by the function call. The function you call might use up more stack for its own local variables. When it's done, it frees up the local variable stack space it used, then returns to the previous function.
Stack overflow
A stack overflow is when you've used up more memory for the stack than your program was supposed to use. In embedded systems you might only have 256 bytes for the stack, and if each function takes up 32 bytes then you can only have function calls 8 deep - function 1 calls function 2 who calls function 3 who calls function 4 .... who calls function 8 who calls function 9, but function 9 overwrites memory outside the stack. This might overwrite memory, code, etc.
Many programmers make this mistake by calling function A that then calls function B, that then calls function C, that then calls function A. It might work most of the time, but just once the wrong input will cause it to go in that circle forever until the computer recognizes that the stack is overblown.
Recursive functions are also a cause for this, but if you're writing recursively (ie, your function calls itself) then you need to be aware of this and use static/global variables to prevent infinite recursion.
Generally, the OS and the programming language you're using manage the stack, and it's out of your hands. You should look at your call graph (a tree structure that shows from your main what each function calls) to see how deep your function calls go, and to detect cycles and recursion that are not intended. Intentional cycles and recursion need to be artificially checked to error out if they call each other too many times.
Beyond good programming practices, static and dynamic testing, there's not much you can do on these high level systems.
Embedded systems
In the embedded world, especially in high reliability code (automotive, aircraft, space) you do extensive code reviews and checking, but you also do the following:
Disallow recursion and cycles - enforced by policy and testing
Keep code and stack far apart (code in flash, stack in RAM, and never the twain shall meet)
Place guard bands around the stack - empty area of memory that you fill with a magic number (usually a software interrupt instruction, but there are many options here), and hundreds or thousands of times a second you look at the guard bands to make sure they haven't been overwritten.
Use memory protection (ie, no execute on the stack, no read or write just outside the stack)
Interrupts don't call secondary functions - they set flags, copy data, and let the application take care of processing it (otherwise you might get 8 deep in your function call tree, have an interrupt, and then go out another few functions inside the interrupt, causing the blowout). You have several call trees - one for the main processes, and one for each interrupt. If your interrupts can interrupt each other... well, there be dragons...
High-level languages and systems
But in high level languages run on operating systems:
Reduce your local variable storage (local variables are stored on the stack - although compilers are pretty smart about this and will sometimes put big locals on the heap if your call tree is shallow)
Avoid or strictly limit recursion
Don't break your programs up too far into smaller and smaller functions - even without counting local variables each function call consumes as much as 64 bytes on the stack (32 bit processor, saving half the CPU registers, flags, etc)
Keep your call tree shallow (similar to the above statement)
Web servers
It depends on the 'sandbox' you have whether you can control or even see the stack. Chances are good you can treat web servers as you would any other high level language and operating system - it's largely out of your hands, but check the language and server stack you're using. It is possible to blow the stack on your SQL server, for instance.
A stack overflow in real code occurs very rarely. Most situations in which it occurs are recursions where the termination has been forgotten. It might however rarely occur in highly nested structures, e.g. particularly large XML documents. The only real help here is to refactor the code to use an explicit stack object instead of the call stack.
Most people will tell you that a stack overflow occurs with recursion without an exit path - while mostly true, if you work with big enough data structures, even a proper recursion exit path won't help you.
Some options in this case:
Breadth-first search
Tail recursion, .Net-specific great blog post (sorry, 32-bit .Net)
Infinite recursion is a common way to get a stack overflow error. To prevent - always make sure there's an exit path that will be hit. :-)
Another way to get a stack overflow (in C/C++, at least) is to declare some enormous variable on the stack.
char hugeArray[100000000];
That'll do it.
Aside from the form of stack overflow that you get from a direct recursion (eg Fibonacci(1000000)), a more subtle form of it that I have experienced many times is an indirect recursion, where a function calls another function, which calls another, and then one of those functions calls the first one again.
This can commonly occur in functions that are called in response to events but which themselves may generate new events, for example:
void WindowSizeChanged(Size& newsize) {
// override window size to constrain width
newSize.width=200;
ResizeWindow(newSize);
}
In this case the call to ResizeWindow may cause the WindowSizeChanged() callback to be triggered again, which calls ResizeWindow again, until you run out of stack. In situations like these you often need to defer responding to the event until the stack frame has returned, eg by posting a message.
Usually a stack overflow is the result of an infinite recursive call (given the usual amount of memory in standard computers nowadays).
When you make a call to a method, function or procedure the "standard" way or making the call consists on:
Pushing the return direction for the call into the stack(that's the next sentence after the call)
Usually the space for the return value get reserved into the stack
Pushing each parameter into the stack (the order diverges and depends on each compiler, also some of them are sometimes stored on the CPU registers for performance improvements)
Making the actual call.
So, usually this takes a few bytes depeding on the number and type of the parameters as well as the machine architecture.
You'll see then that if you start making recursive calls the stack will begin to grow. Now, stack is usually reserved in memory in such a way that it grows in opposite direction to the heap so, given a big number of calls without "coming back" the stack begins to get full.
Now, on older times stack overflow could occur simply because you exausted all available memory, just like that. With the virtual memory model (up to 4GB on a X86 system) that was out of the scope so usually, if you get an stack overflow error, look for an infinite recursive call.
I have recreated the stack overflow issue while getting a most common Fibonacci number i.e. 1, 1, 2, 3, 5..... so calculation for fib(1) = 1 or fib(3) = 2.. fib(n) = ??.
for n, let say we will interested - what if n = 100,000 then what will be the corresponding Fibonacci number ??
The one loop approach is as below -
package com.company.dynamicProgramming;
import java.math.BigInteger;
public class FibonacciByBigDecimal {
public static void main(String ...args) {
int n = 100000;
BigInteger[] fibOfnS = new BigInteger[n + 1];
System.out.println("fibonacci of "+ n + " is : " + fibByLoop(n));
}
static BigInteger fibByLoop(int n){
if(n==1 || n==2 ){
return BigInteger.ONE;
}
BigInteger fib = BigInteger.ONE;
BigInteger fip = BigInteger.ONE;
for (int i = 3; i <= n; i++){
BigInteger p = fib;
fib = fib.add(fip);
fip = p;
}
return fib;
}
}
this quite straight forward and result is -
fibonacci of 100000 is : 25974069347221724166155034021275915414880485386517696584724770703952534543511273686265556772836716744754637587223074432111638399473875091030965697382188304493052287638531334921353026792789567010512765782716356080730505322002432331143839865161378272381247774537783372999162146340500546698603908627509966393664092118901252719601721050603003505868940285581036751176582513683774386849364134573388343651587754253719124105003321959913300622043630352137565254218239986908485563740801792517616293917549634585586163007628199160811098365263529954406942842065710460449038056471363460330005208522777075544467947237090309790190148604328468198579610159510018506082649192345873133991501339199323631023018641725364771362664750801339824312317034314529641817900511879573167668349799016820118499077566864568450662873924856039140476051995500662888263458771894106803700918793650017330117100283104739474562560914449328213748555738640805798130282666402703542944121049199958031318768058991865134251759599115205631553377039969410355182752749199598022575079020377981030899229849963044962558140455170002502997643221934621653662108418767454282982613982344783665815880408190033073829395000821320093747154851310272208173054322648669496309879147143629255542526240439996153269798768075106468190687921182991679644091782718685617029181022126792674013626504997849688436809752547001310045741864064482994858725517447466956518791269169932445648176733222571493149677633458466238303338202397024368594782876418757885729107101337003000942293335972927791914092128049015459762627910570552481588840517794181929052167695766087488155678601288183543542923073978101547857013284386127286201766539534449930019800629538936985500723286651317181135886613537472684585432548981137176605194616937916884425342594781263103889520479565943807153019112539648471126389007133628569101551453423329441284357220996286746119420951661002309740709965531900508158669911445442647882872642845017253320486483194578920399848938236367456182203750973485668474338872490493370316338265717607297788917989136673251906232471180372801739215723908227692280772924566627505383375006926077210593619421268920302567443565378008318306375933345023502569729065152853271943677560156660399164048825639676930792905029514886934137991251748566670747175149389790386533381395346848378086126737554383821108448976538368483182588363399173104558509056638462025014631311831087429077292622159430204291594740306101839816855066950261973761508571761199475875722129872053120607918649803615960923395941041186351688548839119185179061511562752936158490008721501922265117853150892510275280451512386037921846921215338292871369243215273327141574788295902601571954853164447945467502858402360002383447905203451080332820138038807089807348326201227952633606773669875783326254859449060219173688677862411205621098369850197290177157801120404586491539351157834995461006366357454485082418882790675313599505192062229760153765297973085881648731173082370598284894044874039320535929359764541655607954724778620299692329561389719894679422187273605123365595211331087787582288795975803204596084790245063851941743126163775104599211024868794963417068620929088930685252348056925998333775103901013166178123051145719327066291671254465121517468025481903583516889717075706778656188008220346836321018130262329960275994035799977740462449521145315883703579044832931500072461734173558055678321534543411700202585608091662941986374015145695722728369219632295111877625307534025947814482046574602884855000628069348113982760168555840795421620575435572915106415375929390228843561207926437055600623679865443824643739469724719459965557955058380348255978396827760847315302517889517186307227611036305093600742622617173630586132915440246954329046162586917746305785076749374879923291817501634840688134655343709975893536074051729094126976575932951568186247471276364688365517570183534172746626073065104511957628663499228486787805910851189856535554349587616640164475880286336297040462890970677362565843002353147494612339120686321466370878446992104275415694109122465685712047172411333784898167640969249816334211768571503116710400681753031921154156119580425706586931272762137106974722260296555246110537155545324997508432752001992143019105053629960070429632978051030666506387862681576587726837451289768507963663710593809112254288358391941211547737599813019216509521401333060709873137329265181692268450634439540567298120315463923249817937804691037934221694952291007930299492375072993250630509428139027930841344730614116433556147640931044259184813639305423693789765205264563476483182726333715121120306292338892864879492097378478618848682608046473195392008403983080088038690495574197562192939221108257663976813610444900247209483403267967688376213967440757138872928630798218493143438797780887379588968409461434159271317578365114578289355818599029235343888888465874521308381377794436361197628390368945957601203165022798579015453447473527069728514545998614229027372911314637820455162254475353567736227936485450357102086445412089842350389087702230398493802147348096874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Now another approach I have applied is through Divide and Concur via recursion
i.e. Fib(n) = fib(n-1) + Fib(n-2) and then further recursion for n-1 & n-2.....till 2 & 1. which is programmed as -
package com.company.dynamicProgramming;
import java.math.BigInteger;
public class FibonacciByBigDecimal {
public static void main(String ...args) {
int n = 100000;
BigInteger[] fibOfnS = new BigInteger[n + 1];
System.out.println("fibonacci of "+ n + " is : " + fibByDivCon(n, fibOfnS));
}
static BigInteger fibByDivCon(int n, BigInteger[] fibOfnS){
if(fibOfnS[n]!=null){
return fibOfnS[n];
}
if (n == 1 || n== 2){
fibOfnS[n] = BigInteger.ONE;
return BigInteger.ONE;
}
// creates 2 further entries in stack
BigInteger fibOfn = fibByDivCon(n-1, fibOfnS).add( fibByDivCon(n-2, fibOfnS)) ;
fibOfnS[n] = fibOfn;
return fibOfn;
}
}
When i ran the code for n = 100,000 the result is as below -
Exception in thread "main" java.lang.StackOverflowError
at com.company.dynamicProgramming.FibonacciByBigDecimal.fibByDivCon(FibonacciByBigDecimal.java:29)
at com.company.dynamicProgramming.FibonacciByBigDecimal.fibByDivCon(FibonacciByBigDecimal.java:29)
at com.company.dynamicProgramming.FibonacciByBigDecimal.fibByDivCon(FibonacciByBigDecimal.java:29)
Above you can see the StackOverflowError is created. Now the reason for this is too many recursion as -
// creates 2 further entries in stack
BigInteger fibOfn = fibByDivCon(n-1, fibOfnS).add( fibByDivCon(n-2, fibOfnS)) ;
So each entry in stack create 2 more entries and so on... which is represented as -
Eventually so many entries will be created that system is unable to handle in the stack and StackOverflowError thrown.
For Prevention :
For Above example perspective
Avoid using recursion approach or reduce/limit the recursion by again one level division like if n is too large then split the n so that system can handle with in its limit.
Use other approach, like the loop approach I have used in 1st code sample. (I am not at all intended to degrade Divide & Concur or Recursion as they are legendary approaches in many most famous algorithms.. my intention is to limit or stay away from recursion if I suspect stack overflow issues)
Considering this was tagged with "hacking", I suspect the "stack overflow" he's referring to is a call stack overflow, rather than a higher level stack overflow such as those referenced in most other answers here. It doesn't really apply to any managed or interpreted environments such as .NET, Java, Python, Perl, PHP, etc, which web apps are typically written in, so your only risk is the web server itself, which is probably written in C or C++.
Check out this thread:
https://stackoverflow.com/questions/7308/what-is-a-good-starting-point-for-learning-buffer-overflow
Stack overflow occurs when your program uses up the entire stack. The most common way this happens is when your program has a recursive function which calls itself forever. Every new call to the recursive function takes more stack until eventually your program uses up the entire stack.