Null pointer using const instead of variable declaration - c

Question

I have a problem I can't figure out. I wrote this code to shuffle the elements of an array:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
const char *array[]={"a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"};
int main(int argc, char **argv)
{
int i, tmp, randomize, size;
size = sizeof(array)/sizeof(*array);
srand(time(NULL));
for(i=size;i>0;i--){
randomize=0+(rand()%size);
tmp=(int)array[i];
array[i]=array[randomize];
array[randomize]=(char*)tmp;
}
for(i=0;i<size;i++)
printf("%s", array[i]);
return 0;
}
When I run the program, this is the iutput:
azlngiwexbv(null)uscphqjyrodmtk
I can't understand why the pointer is sometimes null and I can't understand why, changing the source code in this way:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(int argc, char **argv)
{
char *array[]={"a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"};
int i, tmp, randomize, size;
size = sizeof(array)/sizeof(*array);
srand(time(NULL));
for(i=size;i>0;i--){
randomize=0+(rand()%size);
tmp=(int)array[i];
array[i]=array[randomize];
array[randomize]=(char*)tmp;
}
for(i=0;i<size;i++)
printf("%s", array[i]);
return 0;
}
Everything works fine.
Thanks.

Answer 1

Start loop from size-1, you are pointing out of array in array[size] (first loop iteration).
for (i=size-1; i>=0; i--)

Answer 2

You should avoid casting whenever possible, and it is rarely necessary to do so.
Unless you plan on using argc and argv[], use the simpler form for main(): int main(void). This will remove some compiler warnings, and you should have these enabled; remove all warnings emitted by your compiler.
You cast a pointer to char to int to store it in tmp, and then you cast tmp back to (char *). Just declare tmp as a pointer to char to start with. The first loop counts down from size to 0; this is out of array bounds to start, but why not just use the conventional loop construction: for (i = 0; i < size; i++)? Also, you should consider using size_t for array indices; it is an unsigned integer type guaranteed to be able to hold any array index, and is also the type returned by the sizeof operator.
Further, in your first version you have declared array as an array of pointers to const chars. You can do this, but you need the temporary storage variable tmp to agree with the const type qualifier.
Here is your code with the above changes. There are no casts, it compiles without warnings, and it works.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(void)
{
const char *array[] = {"a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"};
const char *tmp;
size_t i, size;
int randomize;
size = sizeof(array) / sizeof(*array);
srand(time(NULL));
for(i = 0; i < size; i++){
randomize = (rand() % size);
tmp = array[i];
array[i] = array[randomize];
array[randomize] = tmp;
}
for(i = 0; i < size; i++)
printf("%s", array[i]);
putchar('\n');
return 0;
}