楼主似乎是要写冒泡排序。修改代码如下：
#include<stdio.h>
int main( void )
{
    int i, j, k;
    int a[5] = {21, 12, 13, 31, 37};
    for ( i = 4; i >= 0; i-- ) {
        for ( j = 0; j < i; j++ ) {
            if ( a[j] < a[j - 1] ) {
                k = a[j];
                a[j] = a[j + 1];
                a[j + 1] = k;
            }
        }
        printf( "%d,", a[i] );
    }
    return 0;
}