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What don't I get about variable declaration?

I'm writing a C program that starts like this:

#include <stdio.h>
#define ARRAY_LENGTH 20

int main(void) {
int al = ARRAY_LENGTH;
int n[al];
...

Well the code won't let me do this. It doesn't see al as equalling ARRAY_LENGTH or 20. I thought it was a Borland glitch, but MSVC gives the same error!

I know C has that voodoo that requires the variables to be declared in the program "head" instead of any old place. It's a 100 point question, so I'm not only looking for how to make the above code work, but a concrete explanation of what the rules are for declaring variables.

Thanks,

Raydot

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pjknibbs

Raydot: If what you're basically looking for is to create an array whose length is only known at runtime, it isn't possible to do it directly--a declared array in C must be of a fixed length; there are no exceptions to this rule. If you want to dynamically size an array you're going to have to do it manually, e.g.:

int *n;
int al = 100;

n = malloc(sizeof(int) * 100);
n[0] = 13;
n[99] = 127;

or something like that. You can use code like this because a C array is defined as being just a pointer to the first element of the array. Obviously you're going to have to remember to free() the array when you've finished using it!

pjknibbs

Correction: the line

n = malloc(sizeof(int) * 100);

should be

n = malloc(sizeof(int) * al);

Sorry about that...

Raydot

ASKER

I'm not sure you gave me 100 points worth of answer, considering the question, but I did actually find this exact answer this morning, so you must be right!

Thanks,

Raydot.