Change position of an exact number of elements in an array - c

Question

Let's say we have an array of m elements and we want to change randomly the position of exactly n of them, where of course 2 <= n <= m.
For example: if we have this array of 10 ints {1 2 3 4 5 6 7 8 9 10} and we ask for 4 of its elements to change positions randomly, a result could be {3 2 1 4 5 6 10 8 9 7}
What is the simplest way to program this in ANSI C? (pseudocode will also be just fine)

Answer 1

Step1). Generate a list of n random unique numbers between 1 and m. This list should NOT be sorted. Eg, for your example, the list could have been [10,7,1,3]
Step 2) do something like :
int save = array[list[0]];
For (i=0; i<n-1; i++) {
Array[list[i]] = array[list[i+1]];
}
Array[list[n-1]] = save;
Edit: actually, you'll have to have subtract 1 from each list[whatever] to allow for zero-based arrays - but I'm sure you get the idea :)

Answer 2

since at least two numbers must be swapped, i would pick two random numbers from array first and then swap them. they are added to array that holds swapped indexes. if there's more to swap, pick another number different than swapped ones and swap it with one of the previously swapped numbers.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
void print_arr(int a[], int size) {
int i;
for (i = 0; i < size; i++) {
printf("%d ",a[i]);
}
printf("\n");
}
void swap(int a[], int i, int j) {
int temp = a[i];
a[i] = a[j];
a[j] = temp;
}
int next_idx(int swapped[], int s_count, int size) {
int n, i;
char in_arr;
while (1) {
in_arr = 0;
n = rand() % size;
for (i = 0; i < s_count; i++) {
if (swapped[i] == n) {
in_arr = 1;
break;
}
}
if (!in_arr) {
break;
}
}
return n;
}
void swap2_or_more(int a[], int size,int count) {
srand(time(NULL));
int i, j, s_count = 0;
int swapped[size];
i = rand() % size;
swapped[s_count] = i;
s_count++;
do {
j = rand() % size;
} while (i == j); // make sure indexes are different
swapped[s_count] = j;
s_count++;
swap(a, i, j);
count -= 2;
while (count) {
i = next_idx(swapped, s_count, size);
j = rand() % s_count;
swap(a, i, swapped[j]);
swapped[s_count] = i;
s_count++;
count--;
}
printf("swapped indexes:\n");
print_arr(swapped, s_count);
}
int main(void)
{
int a[] = {1,2,3,4,5,6,7,8,9,10};
swap2_or_more(a, 10, 5);
printf("array after swap:\n");
print_arr(a, 10);
return 0;
}