How to decide whether it is an English letter or a number?

[QQ]

for instance, I read a char from the input
and I need to decide whether it is a letter or a number
What I am doing is
char a;
...... // read a
int true = false;
if(( (a >='0') && (a <='9')) | | ((a >='a') && (a <= 'z')) ||((a >='A')
&& (a <= 'Z')))
true = 1;

Is there any easier way for it?
Thanks a lot!

 

[Dale]

for instance, I read a char from the input
and I need to decide whether it is a letter or a number
What I am doing is
char a;
..... // read a
int true = false;
if(( (a >='0') && (a <='9')) | | ((a >='a') && (a <= 'z')) ||((a >='A')
&& (a <= 'Z')))
true = 1;

Is there any easier way for it?
Thanks a lot!

<hint>
Try looking in ctypes.h.
</hint>

 

[Fred Kleinschmidt]

for instance, I read a char from the input
and I need to decide whether it is a letter or a number
What I am doing is
char a;
..... // read a
int true = false;
if(( (a >='0') && (a <='9')) | | ((a >='a') && (a <= 'z')) ||((a >='A')
&& (a <= 'Z')))
true = 1;

Is there any easier way for it?
Thanks a lot!

isdigit(), isalpha(), etc.

 

[Old Wolf]

<hint>
Try looking in ctypes.h.
</hint>

If that doesn't work, try <ctype.h> .

Also, read the documentation carefully: those functions accept
a value in the range [0, UCHAR_MAX]; if you have some plain
or signed chars then you have to cast them to unsigned char
before passing them.

 

[Dale]

<hint>
Try looking in ctypes.h. ^^^^^^^^
</hint>

If that doesn't work, try <ctype.h> .

Also, read the documentation carefully: those functions accept
a value in the range [0, UCHAR_MAX]; if you have some plain
or signed chars then you have to cast them to unsigned char
before passing them.

<excuse>
It was late and my mind was elsewhere.
</excuse>

I'll do better next time.

 

[Default User]

for instance, I read a char from the input
and I need to decide whether it is a letter or a number

Pick a language and newsgroup. The same question was multi-posted to
comp.lang.c.



Brian

 

[Default User]

Pick a language and newsgroup. The same question was multi-posted to
comp.lang.c.

Scratch this. When I saw the same question again, I thought it was in a
different newsgroup. I'll attempt a cancel.




Brian

 

[Simon Biber]

for instance, I read a char from the input
and I need to decide whether it is a letter or a number
What I am doing is
char a;
...... // read a
int true = false;

That's terribly confusing. You have defined false previously, hopefully
as 0, and are now defining a new variable 'true' and giving it the value
of 'false'!

It's poor style to define your own variables with names 'true' and

if(( (a >='0') && (a <='9')) | | ((a >='a') && (a <= 'z')) ||((a >='A')
&& (a <= 'Z')))
true = 1;

Because digits are guaranteed to be consecutive and in order from 0 to
9, you can determine digit by (a >= '0') && (a <= '9').

However, letters are not guaranteed to be consecutive and in order. You
should never check for letters by (a >= 'a') && (a <= 'z').

Is there any easier way for it?

Absolutely!

#include <ctype.h>

char a;
// read a
if(isdigit((unsigned char)a))
{
printf("it's a number\n");
}
else if(isalpha((unsigned char)a))
{
printf("it's a letter\n");
}
else
{
printf("it's neither a number nor a letter\n");
}

Alternatively, if want a boolean value that expresses whether it's
either a number or a letter:

bool number_or_letter = isdigit((unsigned char)a)
|| isalpha((unsigned char)a);

It's important to include the cast to (unsigned char), since the is*
functions expect a non-negative argument in the range 0 .. UCHAR_MAX, or
else EOF.

 

[Dave Thompson]

Because digits are guaranteed to be consecutive and in order from 0 to
9, you can determine digit by (a >= '0') && (a <= '9').
Right.

However, letters are not guaranteed to be consecutive and in order. You
should never check for letters by (a >= 'a') && (a <= 'z').

Not preferred, but I wouldn't go as far as 'never'. There are some
cases where the limited unportability is a reasonable tradeoff.

Alternatively, if want a boolean value that expresses whether it's
either a number or a letter:

bool number_or_letter = isdigit((unsigned char)a)
|| isalpha((unsigned char)a);

Or in this particular case just use (standard) isalnum().

It's important to include the cast to (unsigned char), since the is*
functions expect a non-negative argument in the range 0 .. UCHAR_MAX, or
else EOF.

It is important the argument be in that range. In some cases it
already surely is, as for example it was the value returned from
getchar(). If it isn't, or maybe not, then the cast is needed.

- David.Thompson1 at worldnet.att.net

 

[Richard Bos]

Not preferred, but I wouldn't go as far as 'never'. There are some
cases where the limited unportability is a reasonable tradeoff.

Such as those cases where you don't have a definition of isalpha()?
AFAIK that's in one of the headers which is required even for
freestanding implementations.

Richard

 

[CBFalconer]

Such as those cases where you don't have a definition of isalpha()?
AFAIK that's in one of the headers which is required even for
freestanding implementations.

Not so. All the absolutely required headers provide definitions
alone.