Even though , I execute the below code again & again I get the same Output. But , I think it should not:
int ObjectCount =500;
NSMutableArray *mut_arr = [[NSMutableArray alloc]initWithCapacity:0];
for (int i = 0; i<ObjectCount ; i++)
{
[mut_arr addObject:[NSNumber numberWithInt: rand()%ObjectCount]];
}
NSSet* uniqueSet = [NSSet setWithArray:mut_arr];
NSLog(#"Array of %d objects generates %d Unique Objects",[mut_arr count],[uniqueSet count]);
The output is as follows:
Array of 500 objects generates 317 Unique Objects
Here, Since the array contains random numbers the unique set count should be same again & again for same ObjectCount.
You're not actually generating unique NSNumber objects; some of them are equal.
A NSArray can contain multiple objects that are equal. A NSSet can not. This is why the set created from your array has less objects.
The reason why you're always getting 317 objects is that you're using rand() without seeding: Why do I always get the same sequence of random numbers with rand()?
Consider using arc4random() instead, which is seeding automatically.
Use like this
[mut_arr addObject:[NSNumber numberWithInt:(arc4random() % (ObjectCount-1) + 1)]];
Related
I have the following object which contains an array of NSDictionary. I wonder how could I able to get the sum of Quantity from this object.
In Objective-C there is a very convenient way using KVC Collection Operators:
NSNumber *sum = [sharedData.orderItems valueForKeyPath:#"#sum.Quantity.integerValue"];
NSInteger integerSum = sum.integerValue;
The other simple operators are:
#count - count of objects
#avg - average value
#max - maximum value
#min - minimum value
int sum=0;
for(NSDictionary *item in sharedData.orderitems){
sum = sum + [[item objectForKey:#"Quantity"] intValue];
}
NSLog(#"Sum = %d",sum);
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 an NSArray filled with only NSStrings
I understand that to iterate over a NSArray of n elements, all I have to do is use for (NSString *element in arrayOfElements). However, I was wondering if there is specific function that will perform a comparison between every string element in the array with each other. For example, if I have the array:
[#"apple", #"banana", #"peach", #"kiwi"],
how would I do the comparison so apple is compared to banana, peach and then kiwi; and then banana is against peach and wiki, and finally peach is against kiwi?
Try using nested for loops, ex:
for (int i = 0 ; i < array.count ; i ++) {
for (int j = i + 1 ; j < array.count ; j ++) {
// compare array[i] to array [j]
}
}
Edit: And although wottle's suggestion would work, I'd recommend mine in this case, since it won't waste iterations going over the same comparisons multiple times. What I've done in this algorithm by setting j = i + 1 is compare each element in the array only to the ones after it.
Given "the array will not have any duplicates, and every NSString will be unique" this sounds like a great case for using NSSet classes instead of NSArray. NSMutableSet provides:
minusSet:
Removes each object in another given set from the receiving
set, if present.
and
intersectSet:
Removes from the receiving set each object that isn’t a
member of another given set.
I'm not sure which operation you're looking for but it sounds like one of those should cover your exact use case.
What you're trying to do is a bit beyond what custom comparators were meant to do. Typically when you have a list and you want to run a custom comparator, you're doing it to sort the list. You seem to want to do some specific action when certain items in the list compare to others, and for that, I think a loop within a loop is your best bet. It won't be very good performance, so hopefully you are not expecting a large array:
-(void) compareArrayToSelf
{
NSArray *array=#[#"apple", #"bananna", #"peach", #"kiwi"];
for( NSString *value1 in array)
{
for( NSString *value2 in array)
{
if( ![value1 isEqualToString:value2] && [self compareArrayValue:value1 toOtherValue:value2])
{
//Do something with either value1 or value2
}
}
}
}
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];
}
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).