Distinguish between a letter and integer in C

Question

Please note that I'm NOT trying to distinguish between a single digit and a letter (or other character) as done by functions like isalpha() in ctype.h

I'm trying to distinguish between an integer and an alphabet.

The problem is that an alphabet is also treated as an integer in C and I can't think of a way out.

eg: if input is the letter a, it is the same as 'a' which is the integer 97.

And I need to find if 'a' or 97 was the input.

I tried to do this and realised it simply couldn't work.

int a;
scanf("%d", &a);
if( (a>='A' && a<='Z') || (a>='a' && a<='z') )
{
   printf("\nAlphabet");
}

Try that [`scanf`](http://en.cppreference.com/w/c/io/fscanf) call, and give some non-integer input. Check what [`scanf`](http://en.cppreference.com/w/c/io/fscanf) ***returns***. — Some programmer dude, Aug 24 '16 at 09:09
If you want to distinguish letters, then it's not a good idea to read them into an `int`... — Oliver Charlesworth, Aug 24 '16 at 09:12
`%d` always reads an `int`, never a character. There's no "alphabet" type in C — phuclv, Aug 24 '16 at 09:12

pmg · Accepted Answer · 2016-08-24T09:31:53.367

Check the return value of scanf()

int val;
int chk = sscanf("a", "%d", &val);
if (chk == 1) /* all ok */;
if (chk == 0) /* no assignments */;
if (chk == EOF) /* error */;

or

int val;
int chk = sscanf("97", "%d", &val);
if (chk == 1) /* all ok */;
if (chk == 0) /* no assignments */;
if (chk == EOF) /* error */;

or

int val1, val2, val3;
int chk = sscanf("97 b 99", "%d%d%d", &val1, &val2, &val3);
if (chk == 3) /* all ok */;
if (chk == 2) /* only val1 and val2 were assigned a value */;
if (chk == 1) /* only val1 was assigned a value */;
if (chk == 0) /* no assignments */;
if (chk == EOF) /* error */;

Garf365 · Answer 2 · 2016-08-24T09:28:30.840

You don't have to distinguish it after scanf because it had already distinguished it:

%d only input integer, so you just can enter integer => if you write 9, you will store 9 inside variable. You can't read character with this (demo). Also, if you input 97, it will store 97 inside variable, which also correspond to code associated to a
%c only input character => if you write 9, you will store associated code of character 9, ie 57

You can check return of scanf to know if it succeed read something or not.

So, if user can input both character and letter, a possible solution is to acquire only character with %c and to recreate integer during typing of user:

int i = 0;
int dec = 1;
char c;
scanf("%c", &c);
if (c >= '0' && c <= '9')
{
    i += c*dec;
    dec *= 10;
}
else
{
    // manage letter
}

Distinguish between a letter and integer in C

2 Answers2

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