将字符数据类型打印为整数数据类型

Question

#include<stdio.h>
main()
{
    char a[12];
    scanf("%s",a);
    int s=0;
    s=s+a[1];
    printf("%d",s);
}
example:
a=1234
output:50

This is a basic c program.When i try to print the value of s,it displays 50 but when i replace a[1] with a[1]-'0' ,it displays the exact value of character present at the index (output: 2) .Any reason why is it happening ?

Answer 1

Character constant '2' in the ASCII table has code 50. So using the format specifier %d the character is displayed as an integer that is its value 50 is displayed.

As for this expression

a[1] - '0'

then as it has been said a[1] that represents the character '2' stores the ASCII value 50 . The character '0' has the ASCII code 48 . So the difference yieds 2 .

Answer 2

lets take a look what happened at run time.

s is initialized to 0

and a[] is scanned with "1234"; 

which is

a[0] = '1';
a[1] = '2';
a[2] = '3';
a[3] = '4';

first case

s=s+a[1]; // 0 + asci value of 2 which evaluates to 50

second case

s=s+a[1]-'0' //  0 + asci value of 2 i.e 50 - asci value of 0 i.e 48 = 2

man -a ascii

Answer 3

C specifies that the codes for characters '0' , '1' , '2' , ... '9' are sequential.
'0' + 3 must equal '3' .
This applies for any character set: the common ASCII or others.

Subtraction from '0' yields the numeric difference.

printf("%d",'2' - '0'); // must print 2

... the value of each character after 0 in the above list of decimal digits shall be one greater than the value of the previous. ... C11dr §5.2.1 3