CODEWITHSUNDEEP
objective-cfast-enumeration
Shri
Shri

Reputation: 2139

Keep track of index in fast enumeration

I want to get the index of the current object when using fast enumeration, i.e. 

for (MyClass *entry in savedArray) {
// What is the index of |entry| in |savedArray|?
}

Upvotes: 29

Views: 12671

Answers (6)

Albert Renshaw
Albert Renshaw

Reputation: 17902

I just had a pretty bad bug because I was doing this the way everyone else in here has suggested. That is, "create an index variable and increment it at the end of your loop".

I propose that this should be avoided and instead the following pattern should be followed:

int index = -1;
for (a in b) {
   index++;
   //Do stuff with `a`
}

The reason I recommend this odd pattern, is because if you use the continue; feature of fast enumeration, it will skip the final index++ line of code at the end of your loop, and your index count will be off! For this reason I recommend starting at -1 and incrementing before doing anything else.

As for people who said just use indexOfObject: this won't work with duplicate entries.

Upvotes: 0

karim
karim

Reputation: 15599

If you want to access the index or return outside block here is a piece of code that can be useful. (considering the array is an array of NSString). 

- (NSInteger) findElemenent:(NSString *)key inArray:(NSArray *)array
{
    __block NSInteger index = -1;
    [array enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        if ([obj isEqualToString:key]) {
            *stop = YES;
            index = idx;
        }
    }];
    return index;
}

Upvotes: 3

SilkyMacGuy
SilkyMacGuy

Reputation: 9

A simple observation: If you initialize the index to -1 and then put the ++index as the first line in the for loop, doesn't that cover all bases (provided someone doesn't slip code in front of the increment)?

Upvotes: 0

GrandAdmiral
GrandAdmiral

Reputation: 1408

This question has already been answered, but I thought I would add that counting iterations is actually the technique mentioned in the iOS Developer Library documentation:

NSArray *array = <#Get an array#>;
NSUInteger index = 0;

for (id element in array) {
    NSLog(@"Element at index %u is: %@", index, element);
    index++;
}

I was sure there would be a fancy trick, but I guess not. :)

Upvotes: 7

Paul.s
Paul.s

Reputation: 38728

Look at the API for NSArray and you will see the method

- (void)enumerateObjectsUsingBlock:(void (^)(id obj, NSUInteger idx, BOOL *stop))block

So give that one a try

[savedArray enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {

    //... Do your usual stuff here

    obj  // This is the current object
    idx  // This is the index of the current object
    stop // Set this to true if you want to stop

}];

Upvotes: 65

Josh Rosen
Josh Rosen

Reputation: 1686

I suppose the most blunt solution to this would be to simply increment an index manually.

NSUInteger indexInSavedArray = 0;
for (MyClass *entry in savedArray) {
   indexInSavedArray++;
 }

Alternatively, you could just not use fast enumeration.

    for (NSUInteger indexInSavedArray = 0; indexInSavedArray < savedArray.count; indexInSavedArray++) {
       [savedArray objectAtIndex:indexInSavedArray];
     }

Upvotes: 9

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