printing randome choices from an array - c

Question

I am working on a function that picks random numbers from a given array and prints them to stdout. These numbers should not repeat and how many numbers are picked is given to the function along with the array. I have a separate test file for the function and a header file as well. Everything compiles fine but when I run the program I get a hang up in the pickNumbers function, nothing is printed and I don't even know if anything is being chosen to begin with.
#include <stdio.h>
#include <stdlib.h>
#include "head.h"
//program that picks random numbers from the given array
int alreadyPicked(int choices[], int choice);
void pickNumbers(int myArray[],int max)
{
// delcare/initilize variables
int i;
int choices[max];
int length = sizeof(myArray)/sizeof(myArray[0]);
// seed rand
srand(time(NULL));
// pick a random choice until that given number of choices is reached
// to make sure non repeat run against alreadyPicked function
for (i=0; i <= max; i++) {
do{
choices[i] = (rand() % max);
}while (alreadyPicked(choices, choices[i]) == TRUE);
}
for (i=0; i <= max; i++) {
printf("%d", myArray[choices[i]]);
}
printf("\n");
}
int alreadyPicked(int choices[], int choice)
{
int i;
int answer = FALSE;
for (i=0; i <= (sizeof(choices)/sizeof(choices[0])); i++) {
if(choices[i] == choice)
answer = TRUE;
}
return answer;
}

Answer 1

Perhaps
for (i=0; i <= max; i++) {
must be:
for (i=0; i < max; i++) {
and
for (i=0; i <= (sizeof(choices)/sizeof(choices[0])); i++) {
must be:
for (i=0; i < (sizeof(choices)/sizeof(choices[0])); i++) {

Answer 2

In your first "for" loop you have a nested while/do. You increment "i" in your for loop, instead you should increment the variable inside of while/do, otherwise it will hang forever performing such loop because "i" is never incremented.
Replace:
for (i=0; i <= max; i++) {
by:
for (i=0; i < max;) {
And also replace:
choices[i] = (rand() % max);
By:
choices[i++] = (rand() % max);
This way you make sure "i" is being incremented. Also your construction "i<=max" is incorrect since you start from 0, use the way David RF did.

Answer 3

Apart from the aforementioned wrong loop tests, the reason for the endless loop is that in alreadyPicked() you compare the new choice index with every choice index in choices[], including uninitialized ones and the new one itself; thus, alreadyPicked() always returns TRUE. I suggest to change to call of alreadyPicked() to
alreadyPicked(choices, i)
and its implementation to
int alreadyPicked(int choices[], int choice)
{
for (int i = 0; i < choice; i++)
if (choices[i] == choices[choice])
return TRUE;
return FALSE;
}