Please explain!

Question

This is a code based on checking whether the input of an array of parentheses is balanced or not. but i dont undertand a part of the code. Can you plz explain it? what's the function of this part?

#include<stdio.h>
#include<string.h>

int is_balanced(char input[])
{
char stack[100], last_char;
int top, i, len;

top = 0;
len = strlen(input);

for(i = 0; i < len; i++){
if(input[i] == '(') {
stack[top] = '(';
top++;
} else if (input[i] == ')') {
if(top == 0){
/// stack empty
return 0;
}

top--;
/******** THIS PART *********/
last_char = stack[top];
if(last_char != '(') {
return 0;
}
/********************************/
}
}

if(top == 0) return 1;
else return 0;
}
int main()
{
char input[100];
scanf("%s", input);

if (is_balanced(input)) printf("%s is balanced!\n", input);
else printf("%s is not balanced!\n", input);
return 0;
}

Answer 1

The author is trying to using a number line of 100 to determine if the numbers of ( and ) are equal. In the part your looking at does an unnecessary check to see if the value in stack[top] is a ( then moves the indices back one.

Input (ab)

Input[0] goes in stack[0] top goes to 1
Input[3] is ) so the part you looking after the check of the value of top which is currently 1 first moves top to 0 the checks the value of stack[0] to verify it is ( which it is. The since we have reached the end of the loop since top is at 0 the author says the ( and ) are balanced.