Determine if char is a num or letter

Question

How do I determine if a char in C such as a or 9 is a number or a letter?

Is it better to use:

int a = Asc(theChar);

or this?

int a = (int)theChar

score 120 · Accepted Answer · answered Dec 23 '11 at 02:58

120

You'll want to use the isalpha() and isdigit() standard functions in <ctype.h>.

char c = 'a'; // or whatever

if (isalpha(c)) {
    puts("it's a letter");
} else if (isdigit(c)) {
    puts("it's a digit");
} else {
    puts("something else?");
}

answered Dec 23 '11 at 02:58

Greg Hewgill

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score 37 · Answer 2 · answered Dec 23 '11 at 03:01

37

chars are just integers, so you can actually do a straight comparison of your character against literals:

if( c >= '0' && c <= '9' ){

This applies to all characters. See your ascii table.

ctype.h also provides functions to do this for you.

answered Dec 23 '11 at 03:01

Christopher Neylan

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This is correct, but not because ASCII is guaranteed, but because of the contiguity of digits: http://stackoverflow.com/a/35936519/895245 – Ciro Santilli OurBigBook.com Mar 11 '16 at 09:33
That's the answer I was looking for. And I think most of people do need this answer when resolving any kind of exercise at university – Henrique Amaral Aug 29 '20 at 20:23

score 14 · Answer 3 · answered Dec 23 '11 at 03:00

<ctype.h> includes a range of functions for determining if a char represents a letter or a number, such as isalpha, isdigit and isalnum.

The reason why int a = (int)theChar won't do what you want is because a will simply hold the integer value that represents a specific character. For example the ASCII number for '9' is 57, and for 'a' it's 97.

Also for ASCII:

Numeric - if (theChar >= '0' && theChar <= '9')
Alphabetic -
if (theChar >= 'A' && theChar <= 'Z' || theChar >= 'a' && theChar <= 'z')

Take a look at an ASCII table to see for yourself.

The check for '0' <= x <= '9' is correct for all standard-compliant implementations (ASCII or not is irreverent). C requires that decimal digits have a contiguous representation. As for letters, all bets are off. — Wiz, Jan 22 '15 at 00:50

score 10 · Answer 4 · answered Dec 23 '11 at 02:58

10

Neither of these does anything useful. Use isalpha() or isdigit() from the standard library. They're in <ctype.h>.

answered Dec 23 '11 at 02:58

Ernest Friedman-Hill

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score 7 · Answer 5 · answered Dec 23 '11 at 02:59

7

If (theChar >= '0' && theChar <='9') it's a digit. You get the idea.

answered Dec 23 '11 at 02:59

Mike Dunlavey

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score 6 · Answer 6 · edited Apr 02 '20 at 09:36

6

You can normally check for ASCII letters or numbers using simple conditions

if ((ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z'))
{
    /*This is an alphabet*/
}

For digits you can use

if (ch >= '0' && ch <= '9')
{
    /*It is a digit*/
}

But since characters in C are internally treated as ASCII values you can also use ASCII values to check the same.

How to check if a character is number or letter

edited Apr 02 '20 at 09:36

plasmacel

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answered Jul 23 '15 at 14:45

Pankaj Prakash

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as every intermediate c text mentions, it is not guaranteed that c uses ascii so this wont work "everywhere" – Ankur S Jul 22 '18 at 06:34

score 3 · Answer 7 · edited May 23 '17 at 12:26

3

C99 standard on c >= '0' && c <= '9'

c >= '0' && c <= '9' (mentioned in another answer) works because C99 N1256 standard draft 5.2.1 "Character sets" says:

In both the source and execution basic character sets, the value of each character after 0 in the above list of decimal digits shall be one greater than the value of the previous.

ASCII is not guaranteed however.

edited May 23 '17 at 12:26

Community

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answered Mar 11 '16 at 09:32

Ciro Santilli OurBigBook.com

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Determine if char is a num or letter

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