I am trying to return the lowest number in an array.
Parameter: arrayOfNumbers - An array of NSNumbers.
Return: The lowest number in the array as an NSInteger.
The code I have thus far doesn't give me any errors, but does not pass the unit tests. What am I doing wrong?
- (NSInteger) lowestNumberInArray:(NSArray *)arrayOfNumbers {
NSNumber* smallest = [arrayOfNumbers valueForKeyPath:#"#min.self"];
for (NSInteger i = 0; i < arrayOfNumbers.count; i++) {
if (arrayOfNumbers[i] < smallest) {
smallest = arrayOfNumbers[i];
}
}
NSInteger smallestValue = [smallest integerValue];
return smallestValue;
}
This is the unit test:
- (void) testThatLowestNumberIsReturned {
NSInteger lowestNumber = [self.handler lowestNumberInArray:#[#3, #8, #-4, #0]];
XCTAssertEqual(lowestNumber, -4, #"Lowest number should be -4.");
lowestNumber = [self.handler lowestNumberInArray:#[#83, #124, #422, #953, #1004, #9532, #-1000]];
XCTAssertEqual(lowestNumber, -1000, #"Lowest number should be -1000.");
}
This method
NSNumber* smallest = [arrayOfNumbers valueForKeyPath:#"#min.self"];
will already determine the smallest number in the array, so the loop inside the method is superfluous (on top of being plain wrong, as #vikingosegundo notices).
you are comparing objects with c types, resulting im pointer addresses being compared with an int.
Beside the fact your smallest is already the smallest, as you used the KVC collection operator #min.self (see Glorfindel answer), the following code shows you correct comparison
if (arrayOfNumbers[i] < smallest)
should be
if ([arrayOfNumbers[i] compare:smallest] == NSOrderingAscending)
or
if ([arrayOfNumbers[i] integerValue] < [smallest integerValue])
Related
This is a pretty simple concept, but I'm not getting the results I'm wanting. I have an NSMutableArray that is populated with NSArrays, I want to loop through that NSMutableArray and remove certain NSArrays based on a key-value pair. My results have many of the NSArrays that I should be removing and I think it has something to do with the count of the NSMutableArray and the int I declare in the For Loop.
Here is my code: (restArray is the NSMutableArray)
for (int i=0; i<restArray.count; i++) {
NSArray *array = restArray[i];
if ([[array valueForKey:#"restaurant_status"] isEqualToString:#"0"]) {
[restArray removeObjectAtIndex:i];
}
}
Does someone know what I am doing wrong here?
It is not recommended to modify an array on what are you currently iterating.
Lets create a tmp array, and reverse your logic.
NSMutableArray * tmpArray = [NSMutableArray array];
for (int i=0; i<restArray.count; i++) {
NSArray *array = restArray[i];
if (![[array valueForKey:#"restaurant_status"] isEqualToString:#"0"] {
[tmpArray addObject:array];
}
}
So at the end of the iteration, you should end up with tmpArray having the arrays you needed.
Use NSPredicate:
NSArray *testArray = #[#{#"restaurant_status" : #"1"}, #{#"restaurant_status" : #"0"}];
NSArray *result = [testArray filteredArrayUsingPredicate:[NSPredicate predicateWithFormat:#"(restaurant_status == %#)", #"1"]];
When you remove an element all the elements past it shift down by one, e.g. If you remove the element at index 3 then the element previously at index 4 moves to index 3.
Every iteration you increase the index by one.
Combine the above two and you see that when you remove an element your code skips examining the following element.
The simple solution is to reverse the order of the iteration:
for (int i = restArray.count - 1; i >= 0; i--)
and then your algorithm will work.
Addendum
You can safely ignore this addendum if your arrays contain < 2^32 elements and you use Clang or GCC (and most other C compilers).
It has been raised in the comments that this answer has a problem if the array has 0 elements in it. Well yes & no...
First note that the code in the question is technically incorrect: count returns an NSUInteger which on a 64-bit machine is a 64-bit unsigned integer, the loop variable i is declared as an int which is 32-bit signed. If the array has more than 2^31-1 elements in it then the loop is incorrect.
Most people don't worry too much about this for some reason ;-) But let's fix it:
for (NSInteger i = restArray.count - 1; i >= 0; i--)
Back to the problem with an empty array: in this case count returns unsigned 0, C standard arithmetic conversions convert the literal 1 to unsigned, the subtraction is done using modular arithmetic, and the result is unsigned 2^64-1.
Now that unsigned value is stored into the signed i. In C converting from signed to unsigned of the same type is defined to be a simple bit-copy. However converting from unsigned to signed is only defined if the value is within range, and implementation defined otherwise.
Now 2^64-1 is greater than the maximum signed integer, 2^32-1, so the result is implementation defined. In practice most compilers, including Clang and GCC, choose to use bit-copy, and the result is signed -1. With this the above code works fine, both the NSInteger and the int (if you've less than 2^32-1 elements in your array) versions.
What the comments raise is how to avoid this implementation-defined behaviour. If this concerns you the following will handle the empty array case correctly with ease:
for (NSUInteger i = restArray.count; i > 0; )
{
i--; // decrement the index
// loop body as before
}
If the array is empty the loop test, i > 0, will fail immediately. If the array is non-empty i, being initialised to the count, will start as one greater than the maximum index and the decrement in the loop will adjust it - effectively in the loop test i contains the number of elements left to process and in the loop body after the decrement contains the index of the next element to process.
Isn't C fun (and mathematically incorrect by definition)!
I have a sorted array of times like so
[0.0, 1.2, 4.3, 5.9, 7.2, 8.0]
While an audio file plays, I want to be able to take the current time and find what the nearest, lower number is in the array.
My approach would be to traverse the array, possible in reverse order as it feels like it should be faster. Is there a better way?
The playback SHOULD be linear, but might be fast-forwarded/rewound, so I would like to come up with a solution that takes that into account, but I'm not really sure how else to approach the problem.
The method you are looking for is -[NSArray indexOfObject:inSortedRange:options:usingComparator:]. It performs a binary search. With the options:NSBinarySearchingInsertionIndex option, if the value isn't found exactly, it returns the index where the object would be inserted, which is the index of the least larger element, or the count of items in the array.
NSTimeInterval currentTime = ...;
NSUInteger index = [times indexOfObject:#(currentTime)
inSortedRange:NSMakeRange(0, times.count)
options:NSBinarySearchingInsertionIndex
usingComparator:^(id object0, id object1) {
NSTimeInterval time0 = [object0 doubleValue];
NSTimeInterval time1 = [object1 doubleValue];
if (time0 < time1) return NSOrderedAscending;
else if (time0 > time1) return NSOrderedDescending;
else return NSOrderedSame;
}];
// If currentTime was not found exactly, then index is the next larger element
// or array count..
if (index == times.count || [times[index] doubleValue] > currentTime) {
--index;
}
The fastest* way to find something in a sorted array is binary search: if there are n items, check the element at index n/2. If that element is greater than what you're looking for, check the element at index n/4; otherwise, if it's less than what you're looking for, check the element at index 3n/4. Continue subdividing in this fashion until you've found what you want, i.e. the position where the current time should be. Then you can pick the preceding element, as that's the closest element that's less than the current time.
However, once you've done that once, you can keep track of where you are in the list. As the user plays through the file, keep checking to see if the time has passed the next element and so on. In other words, remember where you were, and use that when you check again. If the user rewinds, check the preceding elements.
*Arguably, this isn't strictly true -- there are surely faster ways if you can make a good guess as to the probable location of the element in question. But if you don't know anything other than that the element appears somewhere in the array, it's usually the right approach.
I'm not sure if it's the best approach, but I think it'll get the job done (assuming your array is always ascending order).
- (NSNumber *) incrementalClosestLowestNumberForNumber:(NSNumber *)aNumber inArray:(NSArray *)array {
for (int i = 0; i < array.count; i++) {
if ([array[i] floatValue] == [aNumber floatValue]) {
return aNumber;
}
else if ([array[i] floatValue] > [aNumber floatValue]) {
int index = (i > 0) ? i - 1 : 0;
return array[index];
}
}
return #0;
}
Then call it like this:
NSArray * numbArray = #[#0.0, #1.2, #4.3, #5.9, #7.2, #8.0];
NSNumber * closestNumber = [self closestLowestNumberForNumber:#2.4 inArray:numbArray];
NSLog(#"closest number: %#", closestNumber);
I'm not sure if someone else knows a special way that is much faster.
Based on some of the other answers / comments, I came up with this, perhaps one of them can point out if a whole is somewhere.
- (NSNumber *) quartalClosestLowestNumberForNumber:(NSNumber *)compareNumber inArray:(NSArray *)array {
int low = 0;
int high = array.count - 1;
NSNumber * lastNumber;
int currentIndex = 0;
for (currentIndex = low + (high - low) / 2; low <= high; currentIndex = low + (high - low) / 2) {
NSNumber * numb = array[currentIndex];
if (numb.floatValue < compareNumber.floatValue) {
low = currentIndex + 1;
}
else if (numb.floatValue > compareNumber.floatValue) {
high = currentIndex - 1;
}
else if (numb.floatValue == compareNumber.floatValue) {
return numb;
}
lastNumber = numb;
}
if (lastNumber.floatValue > compareNumber.floatValue && currentIndex != 0) {
lastNumber = array[currentIndex - 1];
}
return lastNumber;
}
I'm really bored right now, so I'm trying to test the fastest method. Here's how I did it.
NSMutableArray * numbersArray = [NSMutableArray new];
for (int i = 0; i < 100000; i++) {
float floater = i / 100.0;
[numbersArray addObject: #(floater)];
}
// courtesy #RobMayoff
NSDate * binaryDate = [NSDate date];
NSNumber * closestNumberBinary = [self binaryClosestLowestNumberForNumber:#4.4 inArray:numbersArray];
NSLog(#"Found closest number binary: %# in: %f seconds", closestNumberBinary, -[binaryDate timeIntervalSinceNow]);
// The Quartal Version
NSDate * quartalDate = [NSDate date];
NSNumber * closestNumberQuartal = [self quartalClosestLowestNumberForNumber:#4.4 inArray:numbersArray];
NSLog(#"Found closest number quartal: %# in: %f seconds", closestNumberQuartal, -[quartalDate timeIntervalSinceNow]);
// The incremental version
NSDate * incrementalDate = [NSDate date];
NSNumber * closestNumberIncremental = [self incrementalClosestLowestNumberForNumber:#4.4 inArray:numbersArray];
NSLog(#"Found closest number incremental: %# in: %f seconds", closestNumberIncremental, -[incrementalDate timeIntervalSinceNow]);
And here's the output:
Found closest number binary: 4.4 in: 0.000030 seconds
Found closest number quartal: 4.4 in: 0.000015 seconds
Found closest number incremental: 4.4 in: 0.000092 seconds
And another test case:
Found closest number binary: 751.48 in: 0.000030 seconds
Found closest number quartal: 751.48 in: 0.000016 seconds
Found closest number incremental: 751.48 in: 0.013042 seconds
I am making a program where I need to loop through an array with a list of letters. I want the letters to be shown on their specific label. I have therefore created an outlet of each (about 38) and given them the name "alif01", "alif02", etc.
for (int i = 0; i < [arabicLetters count]; i++) {
int num = i;
NSString *letterString = [arabicLetters objectAtIndex:i];
NSLog(#"alif0%d is %#", num, letterString);
alif0**[i]**.text = arabicLetters[i];
}
is it possible to use the index [i] instead of writing it all manually?
You should not have 38 IBOutlet properties for this. You should have an array (possibly an IBOutletCollection) so that you can loop over the array / index into the array.
While technically you can create a key name and use KVC valueForKey: (appending strings / string format), the array approach is a much better solution.
Indeed, as you already have a loop, you would be better served by creating the labels in the loop directly, then you know you have the correct number. This is particularly beneficial later, when you change the contents of arabicLetters (though that sounds like it isn't a concern in this particular case).
Try with below code:
for (int i = 0; i < [arabicLetters count]; i++) {
NSString *letterString = [arabicLetters objectAtIndex:i];
NSString *propertyName = [NSString stringWithFormat:#"alif0%d.text",i];
[self setValue:letterString forKeyPath:propertyName];
}
I have a problem that I can not understand
NSArray *emptyArr = #[];
for (int i=0; i < ([emptyArr count]-1) ; i++) {
NSLog(#"Did run for1");
}
[emptyArr count] - 1 is -1 but my app still runs NSLog command!
If I use a int variable:
NSArray *emptyArr = #[];
int count = [emptyArr count]-1;
for (int i=0; i < count ; i++) {
NSLog(#"Did run for1");
}
then my app doesn't run NSLog command.
Anyone help me please!
This is because the return type of count is an unsigned int. When you substract 1 from 0, you do not get -1. Instead you underflow to the highest possible unsigned int. The reason it works in the second version is because you cast it (implicitly) to an int in which the value -1 is legal.
[emptyArr count]-1 is never less than 0 since it is unsigned. I'm guessing if you do ((int)[emptyArr count]-1), you will get the correct behavior.
The value returned by [emptyArr count] is unsigned integer.
In the first case,
[emptyArr count]-1 is 0-1 represented in 2's compliment, which is a huge number.
So it prints the log many times.
In the second case,
[emptyArr count]-1 -> You are casting the result of this to int.
0-1 -> -1 signed int.
Hence does not print.
can anyone suggest a faster approach to the following:
I have an array of 5,000 managed objects (faulted) (an array of car.h objects)
Each object has a set of items (toCarParts.h). This set can have any number of objects.
Now i want to sort these out by the most matches in my search query carpart array.
I search for wheel, seat, window, mirror.
The method will go through each car and find the closest match, and calculate a percentage. So if car a has wheel, seat, window, mirror, mat, tire, wiper, pipe --> the % should be 50%. (Matched 4/8 parts.
This is simple enough, but the problem is with 5,000 items the search takes a long time (even using coredata).
The logic i am using goes something like: (Pseudocode)
For each Car*car in array.
NSMutableArray *x=[car tocarparts]allobjects];
For the count of objects in x.
Carpart*part=objectatindex...i.
If the name of this matches one of my parts
add a count to my counter.
At the end of the loop counter/[x count] =%.car.percent=%.
There has to be a better way, any suggestions? (I think its the dividing and checking each part that takes forever.
Thank you in advance.
Edited, added code below:.
-(NSMutableArray*)calculatePercentagePerFind:(NSMutableArray*)CarArray:(NSMutableArray*)partsArray{
NSArray*defaultParts =[NSArray arrayWithArray:[[[HelperMethods alloc]init]getObjectUserDefault:#"AvailableDefaultParts"]];
int lowestPercentMatchInt=[[[HelperMethods alloc]init]getIntegerUserDefault:#"lowestPercentageMatch"];
NSMutableArray*partsFromCarArray=[[NSMutableArray alloc]init];
NSMutableArray*returnArray=[[NSMutableArray alloc]init];
NSMutableArray *partsWithDefaultParts =[NSMutableArray arrayWithArray:partsArray];
[partsWithDefaultParts addObjectsFromArray:defaultParts];
for (int i=0; i<[CarArray count]; i++) {
double matchCount=0;
Car *CarResult =(Car*)[CarArray objectAtIndex:i];
//Check if it will at least be 30% match
double number1 = [partsWithDefaultParts count];
number1 =(number1/[CarResult.numberOfParts doubleValue])*100;
if (number1>lowestPercentMatchInt) {
partsFromCarArray =[NSMutableArray arrayWithArray:[[CarResult toParts]allObjects]];
NSMutableArray *faultedParts=[[NSMutableArray alloc]init];
for (int i =0; i<[partsFromCarArray count]; i++) {
CarPart*part = (CarPart*)[partsFromCarArray objectAtIndex:i];
[faultedParts addObject:part.name];
}
// for each part in the Car
for (NSString *partInCar in partsWithDefaultParts){
//if the search parts contain that part, add one to count
if ([faultedParts containsObject:partInCar]) {
matchCount++;
}
}
//Calculate percent match
double percentMatch = matchCount;
percentMatch =(percentMatch/[CarResult.numberOfParts doubleValue])*100;
//if at least 30%(user default) then add the percent match to Car result
if (percentMatch >lowestPercentMatchInt) {
if (percentMatch>100) {
CarResult.percentMatch = [NSNumber numberWithDouble:100.00];
}else{
CarResult.percentMatch = [NSNumber numberWithDouble:percentMatch];
}
[returnArray addObject:CarResult];
}
}
}
NSLog(#"Percent Matched Cars = %i",[returnArray count]);
return [self arrangeByHighestPercentMatch:returnArray];
}
Try this, which I believe will minimize the strain on core data.
NSSet *selectionSet; // contains the selected parts
NSPredicate *filter = [NSPredicate predicateWithFormat:
#"self IN %#", selectionSet];
float percentageSum = 0;
NSSet *parts;
for (Car *car in fetchedObjects) {
parts = car.parts; // so the relationship is retrieved only once
percentageSum +=
[parts filteredSetUsingPredicate:predicate].count*1.0f
/ (parts.count*1.0f);
}
return percentageSum/fetchedObjects.count;
This would average out the percentages across all cars. There are other methods to weigh the parts differently in the aggregate.
It is not clear from your question, but if you do not need a total percentage but one percentage for each car there would be no need to loop through all cars - you could just calculate the percentage on the fly when displaying it (e.g. with a transient property).