integer input check in C

Question

I would like to ask you what's the most efficient way for checking user's input in C. I want the user to enter a 4 digit integer. How can I make sure that the user has entered exactly 4 digits and no letters?

for(int i=0;i<4;++i)if(!isdigit(in[i])) panic(); if(in[4]) panic(); ;-) — Michael Krelin - hacker
– Michael Krelin - hacker, Commented Apr 2, 2012 at 21:12
I thought to subtract the number 999 from the integer, because if the user has added a letter in the input C will neglect it and will save a three digit integer. then I check if the output it's positive or negative — Dimitris
– Dimitris, Commented Apr 2, 2012 at 21:14
Get input from the user as a string. Loop over the string and check that each character is an integer using isdigit from <ctype.h> — Charles Salvia
– Charles Salvia, Commented Apr 2, 2012 at 21:15

P.P · Accepted Answer · 2012-04-02 21:15:05Z

2

One way:

1) read as string or char*

2) check each char falls within 49-58 ASCII range.

Finally, convert into int using atoi() (or atol() in case of long) if there are only four chars in the string and satisfy (2)

answered Apr 2, 2012 at 21:15

P.P

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4 Comments

ouah Over a year ago

@MichaelKrelin-hacker a string. A data format as defined in the C Standard in 7.1.1p1 (C99)

Keith Thompson Over a year ago

Don't check for the range 49-58; use '0' - '9', or better yet call isdigit((unsigned char)s[i]).

Jack Over a year ago

Well, char * in C is called of string too

Michael Krelin - hacker Over a year ago

@ouah, "a string" or `string'? ;-)

Emmanuel N · Accepted Answer · 2012-04-02 21:20:07Z

1

All input are taken as strings(console). what you can do is check if lengh is less than four, if so loop through and use isdigit() for each char to see if is digit.

For checking numeric you can do something like like:

 int isnumeric(char *str)
 {
     while(*str)
     {
        if(!isdigit(*str))
            return 0;
            str++;
     }
     return 1;
 }

edited Apr 2, 2012 at 21:20

answered Apr 2, 2012 at 21:14

Emmanuel N

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Comments

ouah · Accepted Answer · 2012-04-02 21:23:52Z

0

Use strtol function to convert your string to a long. strtol can do error checking to check you really got a long. Then check the number is between 1000 and 9999 (for positive 4-digits decimal integers).

answered Apr 2, 2012 at 21:23

ouah

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