A problem in declaring an array in C...

pittopitto · 08-02-2006, 10:41 PM

Hey folks!

I have the seguent code, simplified for debug purposes...

Code:

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

int main() {
    const double help = 0.0001;
    int n_pop;
    int n_ind[n_pop];
    int i;
    
    printf("How many populations?\n");
    scanf("%i", n_pop);
    printf("NOTE: It is NOT a sex-biased simulation, so you will have 1/2 males and 1/2 females!");
    i=0;
    while(i < n_pop) {
            printf("How many individuals for the population ", i ,"?\n");
            scanf("%i", n_ind[i]);
            i++;
            }
            
            
    return 0;
}

But seems that I can't declare an array like n_ind with a size declared by user input...

Can you please help me as my compiler (Dev-C++) doesn't find any errors but I can't execute it...

I am working on Windows XP system (Yeah I know it's crap, but I couldn't find any IDE C/C++ to download as the website brings me to wrong download sites!!

)

Ciao ciao

Pittopitto

vharishankar · 08-02-2006, 10:43 PM

You should probably use pointers instead. You cannot declare arrays with floating sizes on the stack as the compiler requires to know the exact size of the array. You can allocate dynamic sized arrays on the heap using the functions malloc() and free() in pure C and the new and delete operators in C++.

Code:

/* set a size for the array, say 10 or any other user inputted value */
int arraylen = 10;
..
/* Allocate a double array on the heap. */
int* array = (int*)malloc (sizeof(int)*arraylen);
/* The C++ way:  int* array = new int[arraylen]; */

..
/* do something with the array */
..
..
..
/* finished with the array. now deallocate the memory */
free (array);
/* The C++ way: delete[] array; */

It's important to "free" the memory because the program won't do it automatically as it's assigned on the heap and it can lead to memory leaks if you don't deallocate memory.

pittopitto · 08-02-2006, 10:52 PM

Wow that was quick!!

I modified the code, but I think the C language does not recognise the 'new' and 'delete' instructions...

I am very sorry but I am just learning the C language so not really an expert...

pittopitto

pittopitto · 08-02-2006, 10:55 PM

what?? before I didn't see the C instructions and now I do!!!

pittopitto

vharishankar · 08-02-2006, 10:59 PM

Sorry I edited it out because I realized you were using C. Yes, you should use malloc() and free() in C as the operators new and delete are C++ specific.

pittopitto · 08-02-2006, 11:05 PM

That's OK!

Now it's working perfectly!! Thank you very much!!
Do you know a online reference library of all the C instructions?

Many Thanks

vharishankar · 08-02-2006, 11:12 PM

There are *plenty* of good books and online manuals to learn the C programming language.

Here's a site from a quick web search:
http://www-ccs.ucsd.edu/c/index.html

Take a look at the links down at the bottom of the wikipedia page:
http://en.wikipedia.org/wiki/C_programming_language

You should probably buy Kernighan & Ritchie's book "The C Programming Language" if you want to know straight from the horse's mouth.

http://www.amazon.com/gp/product/013...lance&n=283155

paulsm4 · 08-02-2006, 11:46 PM

Hi -

A couple of suggestions (presumably, you've already found and corrected them):

Code:

#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#include <time.h>
#include <malloc.h>  /* You need this for "malloc()" */

int main() {
    const double help = 0.0001;
    int n_pop;
    int *n_ind = NULL;  /* Declare array: we'll allocate it later */
    int i;
    
    printf("How many populations?\n");
    scanf("%i", &n_pop);  /* Be sure to pass "addressof" n_pop */
    printf("NOTE: It is NOT a sex-biased simulation, so you will have 1/2 males and 1/2 females!");
    n_ind = malloc (sizeof (int) * n_pop);
    if (n_ind == NULL) {
      printf ("ERROR: Unable to malloc %d bytes!\n", sizeof (int) * n_pop);
      return 1;
    }
    i=0;
    while(i < n_pop) {
            printf("How many individuals for the population ", i ,"?\n");
            scanf("%i", &n_ind[i]); /* "addressof" n_ind[i], too */
            i++;
            }
    free (n_ind);        
            
    return 0;
}

'Hope that helps .. PSM

pittopitto · 08-03-2006, 12:04 AM

Thank you all folks!! It is working...

I need the malloc function also for a 2-D and 3-D array!!!! Is that hard to do it??

Many thanks!

vharishankar · 08-03-2006, 12:48 AM

For 2d arrays there are two strategies.

1. Allocate a bulk of memory like a normal array (but use x*y in the malloc statement instead of just x). Then based on the array index, you can think of the first x as belonging to the first array and the sceond x elements as belonging to the second array and so on. This is easier for an array of fixed length arrays.

2. Use a pointer to pointers. (like in int** x). Allocate each pointer in the pointer array to its own array. This is probably better when you want each array in the array of arrays to be of a different length.

In general try and avoid 3 dimensional arrays (x, y, z). It will usually lead to confusion and probably memory leaks if you don't use them properly.

( All this assuming you want to create dynamic arrays using "malloc" rather than using static arrays like in int a[30][40] )

spooon · 08-03-2006, 02:28 AM

Quote:

Originally Posted by paulsm4

Code:

#include <malloc.h>  /* You need this for "malloc()" */

No you don't. malloc is part of the ANSI C standard header <stdlib.h>

Quote:

Originally Posted by Harishankar

1. Allocate a bulk of memory like a normal array (but use x*y in the malloc statement instead of just x). Then based on the array index, you can think of the first x as belonging to the first array and the sceond x elements as belonging to the second array and so on. This is easier for an array of fixed length arrays.

You can actually allocate arrays of arrays if you are careful with your syntax.