Trouble using pointers with array

Question

I'm new to C and having a problem with pointers. I've been unable to find the answer online or through my peers, so here I am.

I've been given an assignment to:

Create an array of 20 random integers
print out the integers
sort the array in ascending order
print it out again

When I compile the program with GCC and run, I get a segmentation fault. I've narrowed it down to happening when I try and set the value of number[i] or number[k] in the sort function. Any help would be appreciated.

#include <stdio.h>

void sort(int* number, int n){
     /*Sort the given array number , of length n*/
    int temp, min;
    int i, k;
    for(i=0; i<n; i++){
        min = i;
        for(k=i+1; k<n; k++){
            if(number[k]<min){
                min = k;
            }
        }
        temp = number[i];
        number[i] = number[k];
        number[k] = temp;
    }   
}

int main(){
    /*Declare an integer n and assign it a value of 20.*/
    int n=20;

    /*Allocate memory for an array of n integers using malloc.*/
    int *array = malloc(n * sizeof(array));

    /*Fill this array with random numbers, using rand().*/
    srand(time(NULL));
    int i;
    for(i=0; i<n; i++){
        array[i] = rand()%1000+1;
    }

    /*Print the contents of the array.*/
    for(i=0; i<n; i++){
        printf("%d\n", array[i]);
    }

    /*Pass this array along with n to the sort() function of part a.*/
    sort(&array, 20);

    /*Print the contents of the array.*/
    printf("\n");
    for(i=0; i<n; i++){
        printf("%d\n", array[i]);
    }

    return 0;
}

Here are the compile errors I get:

Q3.c: In function âmainâ:

Q3.c:31: warning: implicit declaration of function âmallocâ

Q3.c:31: warning: incompatible implicit declaration of built-in function âmallocâ

Q3.c:34: warning: implicit declaration of function âsrandâ

Q3.c:34: warning: implicit declaration of function âtimeâ

Q3.c:37: warning: implicit declaration of function ârandâ

Q3.c:46: warning: passing argument 1 of âsortâ from incompatible pointer type

Q3.c:9: note: expected âint *â but argument is of type âint **â

You don't need dynamic allocation for your problem. An array of 20 integers is int[20]. — Kerrek SB
– Kerrek SB, Commented Apr 15, 2013 at 11:50
sizeof(array) is not rite you meant sizeof(*array)?. moreover "<stdlib.h>" is necessary for malloc() — Koushik Shetty
– Koushik Shetty, Commented Apr 15, 2013 at 11:58

Daniel Fischer · Accepted Answer · 2013-04-15 11:53:30Z

2

At the point where you swap elements,

temp = number[i];
number[i] = number[k];
number[k] = temp;

k == n because it's after the end of

for(k=i+1; k<n; k++){

You meant to use min instead of k in the swap.

In main,

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

allocates enough space for n pointers to int, not space for 20 int. That should be

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

With regard to the compiler warnings/errors,

#include <stdlib.h>

and call

sort(array, 20);

instead of passing &array.

answered Apr 15, 2013 at 11:53

Daniel Fischer

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1 Comment

Tanner Over a year ago

Thank you, that worked. I also missed that if(number[k]<min) should have been if(number[k]<number[min])

msam · Accepted Answer · 2013-04-15 11:54:20Z

0

array is an int*, your sort function is expecting an int*, yet you pass &array, the address of the int pointer

answered Apr 15, 2013 at 11:54

msam

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