c语言中将char类型数据转换成int型 得用-‘0’，不能用+‘0’

#include <stdio.h>

//这个是输入两串15位的数字相加再输出

int main() {

int i = 0, j = 0, n;
char ch1, ch2;
int a[15] = { 0 }, b[15] = { 0 }, c[15] = { 0 };

while ((ch1 = getchar()) != '\n')
{
a[i] = ch1 - '0';
i++;
}
while ((ch2 = getchar()) != '\n')
{
b[j] = ch2 - '0';
j++;
}

for (n = 14; n >=0; n--)
{
if (a[n] + b[n] < 10)
{
c[n] = a[n] + b[n];
}
else
{
c[n] = (a[n] + b[n])%10;
a[n - 1] = a[n - 1]+1;
}
}

for (n = 0; n < 15; n++)
{
printf("%d", c[n]);
}
printf("\n");

return 0;
}