(C) - Array length - why is it not calculated correctly?

[trilobite]

Hi all -
I've written a small program which does a "map" on an array, applying a function to each element and putting the result in a new array. Only problem is - the length of the array is not calculated correctly.
Here's the code -



#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <math.h>

/* A function to apply to an integer. */
int func(int val)
{
return pow(val,2) + 5;
}

void map(int array[], int (*fn)(int) )
{
/* Length of the array */
int len = sizeof(array) / sizeof(int);

/* An array to store the results */
int resarray[len] ;

int i;

for(i=0; i<len; i++)
{
resarray[i] = fn(array[i]);
printf("Array[%d] is %d, Result[%d] is %d\n", i, array[i], i, resarray[i]);
}

}


int main(void)
{

/* A function pointer to use */
int (*ptr)(int) ;

/* Point the function pointer at our function */
ptr = func;

int myarray[] = {2, 3, 5, 7, 11, 13, 17};

map(myarray, *ptr);

return 0;

}


When this is run, it only gives one row, as follows -
Array[0] is 2, Result[0] is 9

Why is this, and what can I do to ensure the array size is calculated correctly?
Many thanks in advance -
- trilobite

[Barrucadu]

sizeof() is evaluated at compile time, and is the size of the data-type rather than what it contains. You'll either need to pass the length of the array as a parameter, or end the array with some special marker (such as NULL) so you can calculate the length by counting the elements.

[MadCow108]

sizeof cann only determine the size of arrays (e.g. int a[5]) not of pointers to memory blocks which you are using

void map(int array[], int (*fn)(int) )
is equivalent to:
void map(int *array, int (*fn)(int) )

=> sizeof(int*) is the size of the pointer (4 or 8 )
you need to explicitly pass the size of the memory block

and remember that arrays convert into pointers when passed into functions!
this will also not work:


void f(int * a) {
for (i < sizeof(a)); // wrong! will give size of pointer
}

main() {
int a[5];
f(a);
}

[trilobite]

Hi Barrucadu and MadCow108 -
Thanks very much for your help!

Calculated at compile time - so *that's* the reason that sizeof(array) / sizeof(*array) works in main() but not elsewhere....

Ok - this is a big help - thanks again :)
Bye for now -
- trilobite