Question in getint #2

[jongmin]

Can someone please help me understand the code in bold below?
I can't understand what it's doing...

 
#include <ctype.h>
#include <stdio.h>

int getch(void);
void ungetch(int);

/* getint: get next integer from input into *pn */
int getint(int *pn)
{
    int c, sign;
        
    while (isspace(c = getch()));/* skip white space */
            
    if (!isdigit(c) && c != EOF && c != '+' && c != '-') {
        ungetch(c);  /* it's not a number */
        return 0;
    }
    sign = (c == '-') ? -1 : 1;
    if (c == '+' || c == '-')
        c = getch();
     for (*pn = 0; isdigit(c); c = getch())
        *pn = 10 * *pn + (c - '0');   // This is what I don't get...
    *pn *= sign;
 

    if (c != EOF)
        ungetch(c);
    return c;
}

[Sebastiani]

Imagine the user enters:

12345

That's 5 ascii characters. If you subtract the ascii value of '0' from an ascii digit yeilds it's numerical value, ie: '8' - '0' == 8.

So with the equation:

value = (value * 10) + digit

...the number builds like this:

1 [ (0 * 10)+1 == 1 ]
12 [ (1 * 10)+2 == 12 ]
123 [ (12 * 10)+3 == 123 ]
1234 [ (123 * 10)+4 == 1234 ]
12345 [ (1234 * 10)+5 == 12345 ]

[jongmin]

Originally posted by Sebastiani
Imagine the user enters:

12345

That's 5 ascii characters. If you subtract the ascii value of '0' from an ascii digit yeilds it's numerical value, ie: '8' - '0' == 8.

So with the equation:

value = (value * 10) + digit

...the number builds like this:

1 [ (0 * 10)+1 == 1 ]
12 [ (1 * 10)+2 == 12 ]
123 [ (12 * 10)+3 == 123 ]
1234 [ (123 * 10)+4 == 1234 ]
12345 [ (1234 * 10)+5 == 12345 ]

Now it makes sense.. I had no idea about '8' - '0' == 8
Thank you very very much.

[c99]

ASCII '8' - '0' == 0x38 (decimal 56)

Unless of course we're talking about EBCDIC which
is apparently the case.

http://www.natural-innovations.com/c...ciiebcdic.html

[Thantos]

ASCII '8' - '0' == 0x38 (decimal 56)

Unless of course we're talking about EBCDIC which
is apparently the case.

Eh? '8' - '0' == 8 not 0x38. Even on the EBCDIC system.

Only time this would break down is when the characters '0' to '9' aren't continuous or sequential

[c99]

Yes, of course.

In my defense, parse error, hallucinating, and just woke up.

What I did learn though, was that EBCDIC is the reason that
the C programming language has trigraphs.