Keep track of index in fast enumeration

问题:

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|?
}

回答1:

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

}];

回答2:

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];
     }

回答3:

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. :)

回答4:

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;
}

回答5:

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)?