How to randomly pick elements from an array in C? - c

Question

I have an array of 6 rows and 20 columns :
char name[6][20];
And I enter the names with for :
puts("Enter with 6 names :");
for(i=0; i< 6 ; i++)
{
scanf("%19[^\n]%*c",name[i]);
}
After that, I need to randomly choose three names of the array and display them in the screen. How can I do that ?
PS : Different from the other questions similar to that, I want not to take just one word, but the full word of the array.

Answer 1

Here's a possible solution to your problem, assuming you've stored the array of names, just create an array of positions and then shuffle it few times so the positions will be random ones, finally pick 3 positions (for instance, the first 3 ones):
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define ROWS 6
#define COL 20
#define RND_NAMES 3
void shuffle(int *array, int n, int num_shuffles) {
srand((unsigned)time(NULL));
for (int j = 0; j < num_shuffles; j++) {
for (int i = 0; i < n - 1; i++) {
size_t j = i + rand() / (RAND_MAX / (n - i) + 1);
int t = array[j];
array[j] = array[i];
array[i] = t;
}
}
}
int main() {
int i;
char name[ROWS][COL] = {"name1", "name2", "name3",
"name4", "name5", "name6"};
int positions[ROWS] = {0, 1, 2, 3, 4, 5};
shuffle(positions, ROWS, 100);
printf("%s\n", name[positions[0]]);
printf("%s\n", name[positions[1]]);
printf("%s\n", name[positions[2]]);
return 0;
}
With this way, you're guaranteed to pick up 3 random non-repeated names.

Answer 2

Here I wrote a simple solution for what you're trying to achieve.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define ROWS 6
#define COL 20
#define RND_NAMES 3
int main()
{
int i;
char name[ROWS][COL];
// init pseudo-random number generator
srand((unsigned)time(NULL));
puts("Enter with 6 names: ");
for (i=0; i < ROWS; i++) {
scanf("%19[^\n]%*c", name[i]);
}
puts("Random names: ");
for (i=0; i < RND_NAMES; i++) {
printf("%s\n", name[rand() % ROWS]);
}
return 0;
}