macOS Quick question about initializing arrays in C

mac2x · Oct 5, 2010

Hi all,

Quick question:

Is this a correct way to initialize all elements of an array to zero?

Code:

double name[ size ] = {0.0}

My instructor said that this works, but gcc does not like it, and returns an error. And in fact, everything I have read in books and on the 'net states that this shortcut is not available in C.

This fellow teaches C++ and Java as well, so maybe it's a mix-up?

Thanks!

lloyddean · Oct 5, 2010

That would work in 'C++' but not 'C'.

mac2x · Oct 5, 2010

Thanks!

chown33 · Oct 5, 2010

The correct answer is: It depends on the nature of 'size'.

If size is a compile-time constant, it works fine in C:

Code:

#include <stdio.h>

#define size 10

int main (int argc, const char * argv[]) 
{
	double name[ size ] = {0.0};
	
	printf( "sizeof name: %d \n", sizeof(name) );

    return 0;
}

Note that size can be any defined value, including a calculation, as long as it's an expression that is a compile-time constant.

If size is a const int, however, then it doesn't work in C:

Code:

#include <stdio.h>

const int size = 23;

int main (int argc, const char * argv[]) 
{
	double name[ size ] = {0.0};
	
	printf( "sizeof name: %d \n", sizeof(name) );

    return 0;
}

Error message:

Code:

init.c:7: error: variable-sized object may not be initialized

I think this shows one of the problems with posting too little code and no actual error messages: context has to be guessed at in order to answer the question.

mac2x · Oct 5, 2010

Thanks for the explanation. I actually didn't have any code to post as I have had very little problem with my latest assignment after clearing up a few things early on. I actually rewrote it extensively to use arrays, since we just covered that.

Just wondering about this; but sorry I didn't provide deeper context.

But, one learns something every day!

mac2x · Oct 6, 2010

Checked with instructor today. This works in C too (and I tested it):

Code:

double name[100] = {0.0};

lee1210 · Oct 6, 2010

That falls under "compile-time constant".

-Lee

gnasher729 · Oct 7, 2010

mac2x said:
Hi all,

Quick question:

Is this a correct way to initialize all elements of an array to zero?

Code:

double name[ size ] = {0.0}

My instructor said that this works, but gcc does not like it, and returns an error. And in fact, everything I have read in books and on the 'net states that this shortcut is not available in C.

This fellow teaches C++ and Java as well, so maybe it's a mix-up?

Thanks!

What is "size"?
Where is the declaration?
What error?
What shortcut are you talking about?

Did your instructor ever tell you that when you ask questions, you have to say _very precisely_ what you are asking, because slightly different questions could have very different answers, and we can't guess what you mean?

chown33 said:
If size is a const int, however, then it doesn't work in C:

Code:

const int size = 23;

Slight change, and it works in both C and C++:

Code:

enum { size = 23 };

jared_kipe · Oct 7, 2010

calloc() should zero out the memory it reserves, so in the future when you need to make variable sized arrays try to remember that.

You can also use bzero() (*deprecated*) and memset() to zero out some memory.

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macOS Quick question about initializing arrays in C

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