Thread: code understanding

Hey I m reading K&R but this code there some stuff in it whish i dont understand it suppose to be an space counter and num from 0 to 9 thingy so please anyone help thanks

Code:

#include <stdio.h>
/* count digits, white space, others */
int main(void)
{
    int c, i, nwhite, nother;
    int ndigit[10];
    nwhite = nother = 0;
    for (i = 0; i < 10; ++i)
        ndigit[i] = 0;
    while ((c = getchar()) != EOF)
        if (c >= '0' && c <= '9')
            ++ndigit[c-'0'];
        else if (c == ' ' || c == '\n' || c == '\t')
            ++nwhite;
        else
            ++nother;
    printf("digits =");
    for (i = 0; i < 10; ++i)
        printf(" %d", ndigit[i]);
    printf(", white space = %d, other = %d\n",nwhite, nother);
    return 0;
}

i dont understand this line
++ndigit[c-'0'];
why did he do c-'0'
i have read K&R carefully but still didnt understand why did he do that