Finding the index/location of an object inside an array without looping through all its contents

Question

Take the code below:

 NSPredicate *predicateSource = [NSPredicate predicateWithFormat:@"SELF = %d",5];
 NSArray *filteredArraySource = [[[self.myArray copy]autorelease] filteredArrayUsingPredicate:predicateSource];

myArray (NSMutableArray) contains 100 integers (numbers 1-100 in random order). What I'd like to do is somehow find out which index contains the number 5 without looping through each object in the array. The above code only extracts the object and places it into an array.

Any suggestions?

I think there are methods to get the index - did you check the documentation ? — Legolas
– Legolas, Commented Apr 18, 2012 at 15:45
I think that NSArray is a wrapper for a linear array, so with every method you can use, you will always have to scan the whole array (independently from who does it). And with 100 numers array it's not a problem sobig. Then, what if you have more than 1 item containing the number "5" , or more generically the Value "X" ? — kappa
– kappa, Commented Apr 18, 2012 at 15:51
Luckily in my case, there will never be a duplicate (each is unique). — sooper
– sooper, Commented Apr 18, 2012 at 15:52
@Kappa: You're (almost certainly) wrong. NSArray is opaque, but seems to be a front-end for a hash table: ridiculousfish.com/blog/posts/array.html — jscs
– jscs, Commented Apr 18, 2012 at 18:31
@IuliusCæsar probably not linear.. but very strange that beginning insertion graph.. seems like Std C++ vector, even worst. Anyway something like BST would be faster both in walking,searching and insertion — kappa
– kappa, Commented Apr 18, 2012 at 22:14

Ingolmo · Accepted Answer · 2012-04-18 15:49:00Z

3

You can use indexOfObject:.

answered Apr 18, 2012 at 15:49

Ingolmo

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Sergey Kalinichenko · Accepted Answer · 2012-04-18 15:48:40Z

1

You can use indexOfObjectPassingTest:. If you need to use NSPredicate, call your predicate's evaluateWithObject: inside the block; otherwise, simply check the object's integer value to be 5.

answered Apr 18, 2012 at 15:48

Sergey Kalinichenko

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2 Comments

sooper Over a year ago

I was working with an NSMutableArray and didn't think to just copy it into an NSArray. Thanks! (Will accept after "6 minutes")

Sergey Kalinichenko Over a year ago

@sooper You do not need to copy NSMutableArray into NSArray to make it work: whatever works for NSArray, works for NSMutableArray as well.

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