A calculator recognizing addition and multiplication

addmulcal.c

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

#define MAX 128

bool is_digit(char ch) { return (ch >= '0' && ch <= '9'); }

int main(void)
{
	bool mul = false;

	int number[MAX], cnt = 0;
	char expression[MAX];
	scanf_s("%s", expression, MAX);
	
	int i, sum = 0;
	for (i = 0; i <= strlen(expression); i++)
	{
		if (is_digit(expression[i]))
		{
			sum *= 10;
			sum += (int)(expression[i] - '0');
		}
		else
		{
			if (mul)
			{
				number[cnt - 1] *= sum;
				sum = 0;
				mul = false;
			}
			else
			{
				number[cnt] = sum;
				cnt++;
				sum = 0;
			}

			if (expression[i] == '*')
			{
				mul = true;
			}
		}
	}

	for (i = 0; i < cnt; i++)
	{
		printf("\n%d", number[i]);
		sum += number[i];
	}

	printf("\n%d", sum);
}