Can't properly retrieve objects from NSMutableArray

Question

I will be putting a variety of things in this mutable array, but first I am just trying to make sure it works by putting in strings, and then pulling out the strings. Here is my code

str1=@"1";
str2=@"2";
str3=@"3";
NSMutableArray *testArray;
[testArray addObject:str1];
[testArray addObject:str2];
[testArray addObject:str3];

retrieve =[testArray objectAtIndex:1];
NSLog(@"the test number is %@",retrieve);

The problem is that my string:retrieve equals "null" after receiving the string from the array. I'm not sure what I am doing wrong, I've looked at Apple's documentation but I'm having trouble making sense of it. I know I must be interacting with the array incorrectly, but I'm not sure how exactly. Help will be appreciated. -Thank you!

testArray = [NSMutableArray new] ?

EDUsta
– EDUsta

2014-11-20 18:43:25 +00:00
Commented Nov 20, 2014 at 18:43 — EDUsta
– EDUsta, Commented Nov 20, 2014 at 18:43

Sergey Kalinichenko · Accepted Answer · 2014-11-20 18:45:11Z

2

You did not initialize your testArray:

NSMutableArray *testArray = [NSMutableArray array];

You can populate the array using new syntax. If you needed mutability only to add the three items, you could use a non-mutable array instead, like this:

NSArray *testArray = @[ @"1", @"2", @"3"];

If you do need mutability, call mutableCopy:

NSMutableArray *testArray = [@[ @"1", @"2", @"3"] mutableCopy];

answered Nov 20, 2014 at 18:45

Sergey Kalinichenko

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Comments

Lewen · Accepted Answer · 2014-11-20 18:44:52Z

0

Well, that's because testArray is nil. you should change the 4th line to

NSMutableArray *testArray = [NSMutableArray array];

answered Nov 20, 2014 at 18:44

Lewen

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Kenny Lim · Accepted Answer · 2014-11-20 18:44:58Z

0

You array is nil.

You are missing NSMutableArray *testArray = [NSMutableArray array];

answered Nov 20, 2014 at 18:44

Kenny Lim

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Duncan C · Accepted Answer · 2014-11-20 19:19:35Z

0

To clarify what others are telling you:

This line

NSMutableArray *testArray;

Does not create an array. It creates a pointer variable that can be used to point to a mutable array. It starts out containing a zero value (nil, points to nothing.)

It's like a postal address that points to an empty lot.

You need to create (allocate) and initialize a mutable array object in order to use it. (Continuing our analogy, you have to build a house and put a mailbox in front of it before the address becomes valid.)

So you need to say:

testArray = [@[ @"1", @"2", @"3"] mutableCopy];

Breaking that down:

The inner part,

@[ @"1", @"2", @"3"]

Creates an immutable array that contains 3 string objects.

Then we ask the immutable array to create a mutable copy of itself. We save the address of that newly created mutable array into the pointer variable testArray.

We could do it in 3 steps:

NSMutableArray *testArray;
NSArray *tempArray = @[ @"1", @"2", @"3"];
testArray = [tempArray mutableCopy];

Or all at once, like @dasblinkenlight's code:

NSMutableArray *testArray = [@[ @"1", @"2", @"3"] mutableCopy];

answered Nov 20, 2014 at 19:19

Duncan C

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1 Comment

Andrew Savage Over a year ago

Thanks Duncan! it makes sense now, some how i didn't grasp that concept by reading apples docs.

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