C Input only decimals

Question

I'm trying to restrict the input to only numerics in a simple C application however I get a warning. I suspect it's to do with the following segment. Is version one the proper way of doing things? Thanks.

    for(i=0;i< strlen(string); i++)
{
    if( string[i]<"0" || string[i]>"9")
        valid= FALSE;
}

OR

    for(i=0;i< strlen(string); i++)
{
    if( string[i]<'0' || string[i]>'9')
        valid= FALSE;
}

rest of my code is

    void main()

{
    char number[4];
    printf("Please enter a decimal number: ");
    gets(number);   
    if (numeric(number))
        printf("'%s' is a valid number\n", number);
    else
        printf("'%s' is an invalid number\n", number);
}

To clarify I have to do the check in the fashion given according to the exercise I'm doing. Thanks.

The second one is (more) right. – Seth Carnegie Nov 05 '11 at 19:17 — Seth Carnegie, Nov 05 '11 at 19:17
So... you _don't_ want to check for a decimal point? – Matt Ball Nov 05 '11 at 19:19 — Matt Ball, Nov 05 '11 at 19:19

score 1 · Accepted Answer · answered Nov 05 '11 at 19:18

1

Version one is entirely wrong. It does pointer comparisons, because string literals decay to pointers and characters are promoted to int.

Version two is close, though you should really use isdigit from <ctype.h>.

Btw., are you aware that i < strlen(string) in a loop termination condition turns linear-time algorithms into quadratic-time ones?

answered Nov 05 '11 at 19:18

Fred Foo

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Thanks the exercise I was given we have to use this approach. But I can't seem to locate my error – John Lane Nov 05 '11 at 19:21
1

@MarioPen: well, does using the second option (or `isdigit`) solve the problem? – Fred Foo Nov 05 '11 at 19:27

score 0 · Answer 2 · answered Nov 05 '11 at 19:23

0

Can't you just use if (scanf ("%d", &n)==1) or call strtol ?

answered Nov 05 '11 at 19:23

Basile Starynkevitch

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`strtol` would work, but `sscanf` wouldn't be equivalent. For example, his code says `1a` is invalid, but `sscanf` would still convert the 1 to a number, so it would return 1. – Jerry Coffin Nov 05 '11 at 19:47
I wrote `scanf` on purpose, assuming he is reading from `stdin` – Basile Starynkevitch Nov 05 '11 at 20:41

C Input only decimals

2 Answers2