C shuffle 2D array

Question

Want to shuffle char *array and make elements random order

I read Shuffle array in C but for int

I try

const char *words = { "one", "two", "three", "four", "five", "six" }; 
for (int i=0; i<6; i++)
{
    int r = (rand() % (6 - i)) + i;

    int temp =  words[i];
    words[i] = words[r];
    words[r] = temp;
}

error though when iterate array words

please explain

Answer 1

As said in remarks you have several problems

const char *words = { "one", "two", "three", "four", "five", "six" }; 

is invalid because the initialization (and the rest of the code) indicates words is an array, so replace it by

const char *words[] = { "one", "two", "three", "four", "five", "six" }; 

    int temp =  words[i];
    words[i] = words[r];
    words[r] = temp;

words is an array of const char * so temp cannot be an int, use

const char * temp = words[i];

Out of that

• to use the literal 6 is dangerous, use sizeof to take into account any change on words replacing 6 by sizeof(words)/sizeof(*words)
• the right type for an index is size_t,
• to not have always the same executions use srand for instance with the current time

---

For instance :

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

int main(void)
{
  const char *words[] = { "one", "two", "three", "four", "five", "six" };
  const size_t nelts = sizeof(words)/sizeof(*words);

  srand(time(NULL));

  for (size_t i=0; i < nelts;  ++i)
  {
    size_t r = (rand() % (nelts - i)) + i;
    const char * temp =  words[i];

    words[i] = words[r];
    words[r] = temp;
  }

  /* show */
  for (size_t i=0; i < nelts;  ++i)
    puts(words[i]);

  return 0;
}

Compilation and executions :

pi@raspberrypi:/tmp $ gcc -pedantic -Wall -Wextra c.c
pi@raspberrypi:/tmp $ ./a.out
four
one
two
five
three
six
pi@raspberrypi:/tmp $ ./a.out
one
three
two
four
five
six
pi@raspberrypi:/tmp $ ./a.out
six
five
two
four
three
one
pi@raspberrypi:/tmp $ ./a.out
three
one
five
four
six
two

Answer 2

There are multiple errors and flaws in OPs code.

1. array of strings:

const char *words = { "one", "two", "three", "four", "five", "six" };

Not sure how the compiler reads this (if not complaining). An array of C strings should be:

const char *words[] = { "one", "two", "three", "four", "five", "six" };

2. magic number

for (int i=0; i<6; i++)

6 is a magic number which is considered as bad style. It's better to give it a name. Even better: the value can be determined by compiler which improves maintainability of code:

int n = sizeof words / sizeof *words;
for (int i = 0; i < n; ++i) {

3. usage of rand()

int r = (rand() % (6 - i)) + i;

rand() is a pseudo random generator with certain limitations. It shouldn't be used with % n. For certain ns (e.g. 2), this may result into a rather non-random sequence. The cppreference doc. of rand() gives a better example which I turned into a function:

int randomRange(int min, int max)
{
  for (int range = max - min;;) {
    int x = min + rand() / ((RAND_MAX + 1u) / range);
    if (x < max) return x;
  }
}

which is called

int r = randomRange(i, n);

4. idiomatic swap

int temp =  words[i];

If the type of array elements to swap is const char* the temp must be as well:

    const char *temp = words[i];
    words[i] = words[j];
    words[j] = temp;

---

The complete sample code:

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

int randomRange(int min, int max)
{
  for (int range = max - min;;) {
    int x = min + rand() / ((RAND_MAX + 1u) / range);
    if (x < max) return x;
  }
}

int main(void)
{
  /* an array of strings */
  const char *words[] = { "one", "two", "three", "four", "five", "six" };
  /* let compiler determine size */
  int n = sizeof words / sizeof *words;
  /* shuffle */
  for (int i = 0; i < n; ++i) {
    int j = randomRange(i, n);
    /* idiomatic swap */
    const char *temp = words[i];
    words[i] = words[j];
    words[j] = temp;
  }
  /* print result */
  const char *sep = "";
  for (int i = 0; i < n; ++i) {
    printf("%s%s", sep, words[i]);
    sep = ", ";
  }
  /* done */
  return 0;
}

Output:

six, three, one, two, four, five

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