我在C程序中对数组进行排序时出错

Question

I am trying to write a C program which sorts an array as follows. I am a beginner and I have very less idea about algorithms. Please help me find the error in my code. Since I am not getting any compiler errors (except few warnings) but only the program stops responding, so I am not able to detect my error.

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

void selection_sort(int n, int a[n]);

int main () {
int n,i;


printf("Enter the number of elements:");
scanf("%d",&n);
printf("Enter %d elements :",n);

int a[n];

for (i=0 ; i<n ; i++) {
    scanf("%d",&a[i]);
}

selection_sort(n, a[n]);

for (i=0 ; i<n ; i++) {
    printf("%d ",a[i]);
}

return(0);
}

void selection_sort(int n, int a[n]) {

int i,p;

p = 0;

if (n != 0) {
    for (i=0 ; i<n ; i++) {
        if (a[i]>p) p=a[i];
    }

    p = a[n];

    selection_sort(n-1 , a[n]);
}
}

Answer 1

First warnings are not to be ignored.

My compiler says:

warning: incompatible integer to pointer conversion passing 'int' to parameter of type 'int *'; take the address with & [-Wint-conversion]

  selection_sort(n, a[n]); 

The is a serious problem: you pass one single integer (and passed end of array...) when you want to pass the array (which will decay to the address of its first element) => this is enough to invoke undefined behaviour and likely to cause a crash

Trivial to fix, just call:

selection_sort(n, a);

and later in selection_sort , recursively call:

selection_sort(n-1, a);

Once this is done, your algorithm is plain wrong. For selection sort, you must exchange elements in the array, what you do not. It should be:

if (n != 0) {
    for (i=0 ; i<n ; i++) {
        if (a[i]>a[p]) p=i;            # first select the rank
    }

    if (p != n-1) {                    # then exchange elements
        int tmp = a[p];
        a[p] = a[n-1];
        a[n-1] = tmp;
    }

Answer 2

There are few errors in your code.

First, when you don't understand everything your are doing, consider warnings as errors. Because the warnings that are shown during the compilation are errors in the case, and result in segfault.

When you compile a program, always use flags like gcc -Wall -Werror -Wextra . It will treat warnings as errors.

Second, your program doesn't work because you don't really understand how pointers and arrays work. When you declare an int array, such as int a[n] , and try to have access to the array, a[n] will point to the first element in the array. So if you want to pass the array to a function, you can't pass int a[n] as a parameter. Because it is an int.

If you want to pass the array, you have to say "I want to give you the address if that first element, which is &a[0]", which is the same as saying "a".

So what should your function take as a parameter ?

If you give "int a[n]" as a parameter, you give it an int. But you want the address of that int, so int *.

Good luck