I wrote a function that reverses an integer (123 becomes 321) with recursion, and I was told that using global variables is bad practice, but I'm having trouble figuring out how to get the result from the function without using one. I tried declaring the variable inside the function, but then I get undesired results.
#include <stdio.h>
int result = 0;
int reverse_num(int num)
{
int remainder = num % 10;
if (num == 0) return 0;
result *= 10;
result += remainder;
reverse_num(num / 10);
return result;
}
int main(void)
{
int num, reversed_num;
printf("Insert number: ");
scanf("%d", &num);
reversed_num = reverse_num(num);
printf("Inverted number: %d", reversed_num);
return 0;
}
The common way to do this is to have another argument just for the accumulated result. It can be accomplished in C by splitting your function into the one called by the user and the one that does all the work.
int reverse_num_helper(int num, int result)
{
int remainder = num % 10;
if (num == 0) return result;
result *= 10;
result += remainder;
return reverse_num_helper(num / 10, result);
}
int reverse_num(int num)
{
return reverse_num_helper(num, 0);
}
(In C++ you could combine the two into int reverse_num(int num, int result = 0).)
Notice how your function is essentially unchanged.
We made only two minor modifications:
result instead of zero when num reaches zeroresult value