Using C to implement radix sort - c

Question

I got stuck in the below question for a few days. I used C to implement radix sort, everything was fine except for one line of code. Could you please help me to solve this issue?
My problem is in the first line of radix_sort function. While I use int semi_sort[12], I can run the program correctly. However, I wanna use the size variable that I passed into the function, but when I use int semi_sort[size] instead of int semi_sort[12], my program would crash. Could anyone tell me why is that? BY the way, I referred to this link, in this author's codes, he did int semiSorted[size]. Why does this line of code work this time?
Thank you in advance!!
#include <stdio.h>
#include <stdlib.h>
#define bucket_size 10
int find_the_largest(int arr[],int size);
void display(int arr[],int size);
void radix_sort(int arr[],int size);
int main()
{
printf("------------------------------------------------------\n");
printf(" Hey! This is a radix sort algorithm!\n");
printf("------------------------------------------------------\n\n");
int array[] = {10, 2, 303, 4021, 293, 1, 0, 429, 480, 92, 2999, 14};
int size = sizeof(array)/sizeof(int);
int largest_num = find_the_largest(array,size);
printf("The unsorted array:");
display(array,size);
printf("The radix sort algorithm:\n\n");
radix_sort(array,size);
display(array,size);
return 0;
}
int find_the_largest(int arr[],int size){
int i,max_num=0;
for(i=0;i<size;i++){
if(arr[i]>max_num)
max_num = arr[i];
}
return max_num;
}
void display(int arr[],int size){
int i;
for(i=0;i<size;i++){
printf(" %d",arr[i]);
if(i==size-1)
printf("\n\n");
}
}
void radix_sort(int arr[],int size){
int semi_sort[12];
int max_num = find_the_largest(arr,size);
int i,significant_num = 1;
while(max_num/significant_num>0){
int bucket[bucket_size] = {0};
for(i=0;i<size;i++){
bucket[(arr[i]/significant_num)%10]++;
}
for(i=1;i<size;i++){
bucket[i] += bucket[i-1];
}
for(i=size-1;i>=0;i--){
semi_sort[--bucket[(arr[i]/significant_num)%10]] = arr[i];
}
for(i=0;i<size;i++)
arr[i] = semi_sort[i];
significant_num *= 10;
}
}

Answer 1

You have problem with code:
for(i=1;i<size;i++){
bucket[i] += bucket[i-1];
}
Because size can be greater then bucket_size.
And probably you have problem with:
for(i=size-1;i>=0;i--){
semi_sort[--bucket[(arr[i]/significant_num)%10]] = arr[i];
}
Because --bucket[(arr[i]/significant_num)%10] can be greater then 11 or less then 0.
And find_the_largest does not work correct on negative numbers.
You can dynamically allocate memory for your buffers, like this:
semi_sort = malloc(size * (sizeof *semi_sort));
And don't forget to free memory on end (free(semi_sort)).