Format Specifiers in C and their Roles?

Question

wrote this code to "find if the given character is a digit or not"

#include<stdio.h>
int main()
{
    char ch;
    printf("enter a character");
    scanf("%c", &ch);
    printf("%c", ch>='0'&&ch<='9');
    return 0;
}

this got compiled, but after taking the input it didn't give any output. However, on changing the %c in the second last line to %d format specifier it indeed worked. I'm a bit confused as in why %d worked but %c didn't though the variable is of character datatype.

https://stackoverflow.com/questions/17307275/what-is-the-printf-format-specifier-for-bool — Mitch Wheat, Jul 11 '22 at 07:49
@MitchWheat I don't understand, sorry, I'm basically just starting with C. — Priyanuj Bora, Jul 11 '22 at 07:52
@Lundin Oops ... need more coffee. I'll defer posting an answer, as you can probably do better than I would. — Adrian Mole, Jul 11 '22 at 07:59
@AdrianMole Well I guess I have to, now that I reopened it... :) — Lundin, Jul 11 '22 at 08:01
`ch>='0'&&ch<='9'` evaluates to `0` or `1` neither of which are printable characters. — Weather Vane, Jul 11 '22 at 08:01
I got it that, to print boolean i.e. 1 for true and 0 for false, we need to use %d format specifier right? — Priyanuj Bora, Jul 11 '22 at 08:04

score 5 · Accepted Answer · answered Jul 11 '22 at 08:04

Characters in C are really just numbers in a token table. The %c is mainly there to do the translation between the alphanumeric token table that humans like to read/write and the raw binary that the C program uses internally.

The expression ch>='0'&&ch<='9' evaluates to 1 or 0 which is a raw binary integer of type int (it would be type bool in C++). If you attempt to print that one with %c, you'll get the symbol table character with index 0 or 1, which isn't even a printable character (0-31 aren't printable). So you print a non-printable character... either you'll see nothing or you'll see some strange symbols.

Instead you need to use %d for printing an integer, then printf will do the correct conversion to the printable symbols '1' and '0'

As a side-note, make it a habit to always end your (sequence of) printf statements with \n since that "flushes the output buffer" = actually prints to the screen, on many systems. See Why does printf not flush after the call unless a newline is in the format string? for details

Yep, got it! Thanks for clearing the doubt and the tip! – Priyanuj Bora Jul 11 '22 at 08:11 — Priyanuj Bora, Jul 11 '22 at 08:11

score 1 · Answer 2 · answered Jul 11 '22 at 08:32

1

In a memory, int take 4 bytes of memory you are trying to storing the int values in a character which will return the ascii value not a int value in %c if you are using a %d which will return the int value which are storing in a memory of 4 bytes memory.

answered Jul 11 '22 at 08:32

vishal meena

11
2

1

"int take 4 bytes of memory" is [*not* always true](https://stackoverflow.com/a/11438985). – JASLP doesn't support the IES Jul 11 '22 at 08:49

Format Specifiers in C and their Roles?

2 Answers2