how to determine the length of array

[csxia]

Hi, all

i am a newbie to cprogramming , could you please point the difference of following code segments of determing length of array ??

#include <stdio.h>
#include <stdlib.h>



void print_length(void * p)
{
        int len = sizeof(p) / sizeof(p[0]);
        printf("the length of str is %d\n",len);
        return ;
}




int main()
{
        char str[11] = "hello world";

        int len = sizeof(str) / sizeof(str[0]);
        printf("the length of str is %d\n",len);

        print_length(&str);
        return 0;
}
linux@localhost:/data/learning/C/variantLen.c> gcc -o length length.c 
linux@localhost:/data/learning/C/variantLen.c> ./length 
the length of str is 11
the length of str is 4

[laserlight]

Hint: when passed as an argument, an array is converted to a pointer to its first element. The sizeof an array is not necessarily the same as the sizeof a pointer to the first element of the same array.

By the way, sizeof(p[0]) does not make sense: you are effectively dereferencing a pointer to void, and then trying to find sizeof(void).

[csxia]

if so, could you please give me example function which can determine the length of an given array.


thanks a lot

[laserlight]

You already have an expression that does that, in the main function. You just need to recognise when it can, and cannot, be used.

[cas]

You cannot. As laserlight pointed out, once an array is passed to a function, it's no longer an array (in that function).

What you can do is mark the end of the array somehow (as C does by using a null character at the end of a string) and search for that. However, your code implies that you want to be able to pass in any array type and find its length. Sorry, but that's not going to work. You could use a macro, though:

#define ARRSIZE(a) (sizeof (a) / sizeof *(a))

[csxia]

Hmm. i understand it
thanks to all of you for your quick reply

[Elysia]

Pass along the size of the array to the function.

[noobiept]

btw, char str[11] = "hello world";
should be: char str[12] = "hello world";

don't forget to add space for the '\0' character.

[Elysia]

Better yet,
char str[] = "hello world";

[quzah]

btw, char str[11] = "hello world";
should be: char str[12] = "hello world";

don't forget to add space for the '\0' character.

Actually both of those are legal. One is a string, the other isn't.


Quzah.

[noobiept]

Isn't it always a string when you declare with "" ?

[laserlight]

Isn't it always a string when you declare with "" ?

It is a string literal, yes, but the array initialised with the string literal might not constitute a string.

[noobiept]

could you elaborate? in what circumstances it doesn't evaluate to a string?

Whenever I see a initialization with the "" I normally assume its a string.

[laserlight]

in what circumstances it doesn't evaluate to a string?

There is no null character since the array does not have enough space to hold it.

[noobiept]

Ok, but isn't that like, a mistake?
Do people do that sort of thing?