Objective-c array

Question

I am slightly confused as I am using this piece of code;

NSArray *dirContents = [[NSFileManager defaultManager] contentsOfDirectoryAtPath:@"/Volumes/" error:nil];

int arraysize = sizeof dirContents;

to get an array of the contents of the "Volumes" direcoty, however when I output the array size it says that the array has 8 entries when there are only 4 files in that directory? This wouldn't be a problem but since I am using a for loop as soon as I get to;

NSString *volume1 = [dirContents objectAtIndex:4];

(4 being the value in the for loop) the application crashes and refuses to launch?

Thanks for any help

sizeof dirContents tells you that the pointer "dirContents" is eight bytes long. Has absolutely nothing to do with the array itself. — Hot Licks
– Hot Licks, Commented Jan 1, 2012 at 20:14

Damien · Accepted Answer · 2012-01-01 20:12:09Z

3

You should use int arraysize = [dirContents count] to get the correct size.

sizeof is a c-style operator that will not work correctly on Objective-C objects.

answered Jan 1, 2012 at 20:12

Damien

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4 Comments

Lennart Over a year ago

Thanks, that worked but I am curios why does the way I found on the web not work?

Damien Over a year ago

because sizeof returns the length of the pointer to the Objective-C object in memory. Just the size of the pointer not the object, and certainly not the size/count of the NSArray.

flumpb Over a year ago

sizeof will give you the size of NSArray*. sizeof pointer being 8 means you're compiling for 64 bit platform

Damien Over a year ago

Ah, I do forget about that :-)

flumpb · Accepted Answer · 2012-01-01 20:13:04Z

2

sizeof is receiving the size of the pointer. Instead use int arraysize = [dirContents count];

answered Jan 1, 2012 at 20:13

flumpb

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Comments

Sergey Kalinichenko · Accepted Answer · 2012-01-01 20:13:37Z

2

Taking sizeof is not the right way of finding the number of NSArray elements. Use dirContents.count instead.

answered Jan 1, 2012 at 20:13

Sergey Kalinichenko

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Comments

Manlio · Accepted Answer · 2012-01-01 20:18:29Z

2

sizeof does not returns the length of the array but the size your variable occupies in memory. Since dirContents is a pointer it occupies only 8 byte.

To get the length of the array you should use

[dirContents count];

Besides, in arrays, objects are stored with indexes starting from 0. Thus, if your array has only 4 elements, [dirContents objectAtIndex:4] will end in a runtime error, since you are trying to retrieve the element in the fifth position.

edited Jan 1, 2012 at 20:18

answered Jan 1, 2012 at 20:12

Manlio

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