#include<stdio.h>
 
int main(void)
 
{
	char num_string;
	int sourcebase;
	int targetbase;
	int temp
 
	do {
		printf("Enter a positive number/n");
		scanf_s("%c",&num_string);
		printf("Enter the base of that number");
		scanf_s("%d", &sourcebase);
		printf("Enter the destination base");
		scanf_s("%d",&targetbase);
		if((sourcebase<2)||(sourcebase>10)){
		printf ("You number is invalid!!!!!!!/n/n");
	}
	}
     length= //some code to get string length
 
   for(i=0;i<length,i++)
 
   temp=num_string[i]*sourcebase //here should be corresponding index

char num_string = '\0' ; // wrong
char num_string[BUFSIZ] = {'\0'} ; // correct, will now hold a number string

   while( tmp_number )
    {
        decimal += (tmp_number % 10) * pow( source_base, i ) ;
        tmp_number /= 10 ;
        ++i ;
    }

    while( decimal )
    {
        new_number[i] = decimal % target_base ;
        decimal /= target_base ;
        new_number = (int*) realloc( new_number, i + 2 ) ;
        ++i ;
    }

// result stored in the dynamically allocated integer array with
// one slot
    int* new_number = (int*) calloc( sizeof( int ), sizeof( int ) ) ;
 
    i = 0 ;
    while( decimal )
    {
        new_number[i] = decimal % target_base ; 
        decimal /= target_base ;
 
        new_number = (int*) realloc( new_number, i + 1 ) ;
// we need more memory, which will be controlled by the counter i 
// hence while reallocing we use i as reference. So when i = 1
// allocate (2 + 1) = 3 slots and so on.
 
        ++i ;
    }