Checking if input is a single character and meets validation in C

Question

I'm new to the C language and have to write a program that asks the user to type the letter "t" in uppercase or lowercase and then press enter. The program should inform you if you entered the letter "t" or other data. The case of a user's input like "tre" should be treated as invalid. I've tried something like this:

#include <stdio.h>

int main(void) {
    char input;
    printf("Please type letter \"t\" in upper or lower case:\n");
    input = getchar();
    printf("%c", input);
    if (input != 't')
    {
        printf("\nWrong input");
        return 0;
    }
    else {
        printf("\nYou have entered: \"%c\"", input);
    }
}

The issue is that if the user's input is something like "tasd" or "tre" or anything that starts with the letter "t", only the first letter is read and the if statement is treated as true. I've tried to research the answer but I couldn't find anything useful.

Hint: read a string instead of a char, and then verify that it is a one character string containing `t`/`T`. — wohlstad, Oct 17 '22 at 13:13
siuggest using the maccros in `ctype.h` such as toupper() to make your code easier to read and debug since the code needs to check for both upper and lower case 't' — user3629249, Oct 20 '22 at 00:52

score 2 · Answer 1 · answered Oct 17 '22 at 13:18

2

Check that the next character is an end of line '\n'

#include <stdio.h>

int main(void)
{
    char input;
    char nextchar;
    printf("Please type letter \"t\" in upper or lower case:\n");
    input = getchar();
    nextchar = getchar();
    printf("%c", input);
    if (input != 't' || nextchar!='\n')
    {
        printf("\nWrong input");
        return 0;
    }
    else
    {
        printf("\nYou have entered: \"%c\"", input);
    }
}

answered Oct 17 '22 at 13:18

CGi03

452
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8

2

Your posted code will incorrectly reject the input `'T'` followed by `'\n'`. It will only accept `'t'` followed by `'\n'`. – Andreas Wenzel Oct 17 '22 at 14:22
1

Yes, I only posted code that answers his question and not the exercise. – CGi03 Oct 17 '22 at 14:40

Vlad from Moscow · Answer 2 · 2022-10-17T14:00:40.253

For starters the variable input should have the type int.

int input;

Instead of reading one character

input = getchar();

you could use for example

#include <ctype.h>
#include <stdio.h>

//...

int valid = ( input = getchar() ) != EOF && toupper( ( unsigned char )input ) == 'T';

if ( valid )
{
    int tmp;
    while ( ( tmp = getchar() ) != '\n' && isblank( ( unsigned char )tmp ) );
    valid = tmp == '\n';
}

if ( valid )
{
    printf( "\nYou have entered: \"%c\"\n", input );
}
else
{
    puts( "\nWrong input" );
}

That is the used can enter the character 't' and when several spaces. Such input should be considered as valid.

Andreas Wenzel · Answer 3 · 2022-10-17T14:12:47.840

According to the information provided in your question, you do not want to validate only the first character of input, but rather an entire line of input.

In order to read an entire line of input as a string, I recommend that you use the function fgets. Afterwards, you can examine the string to determine whether the input is valid or not.

Here is an example:

#include <stdio.h>
#include <stdlib.h>

int main( void )
{
    char line[200];

    //prompt user for input
    printf( "Please type letter \"t\" in upper or lower case: " );

    //attempt to read one line of input
    if ( fgets( line, sizeof line, stdin ) == NULL )
    {
        fprintf( stderr, "Error reading input!\n" );
        exit( EXIT_FAILURE );
    }

    //verify that input string is valid
    if ( ( line[0] == 't' || line[0] == 'T' ) && line[1] == '\n' )
        printf( "Input is valid. You entered \"%c\".\n", line[0] );
    else
        printf( "Input is invalid!\n" );
}

This program has the following behavior:

Please type letter "t" in upper or lower case: tre
Input is invalid!

Please type letter "t" in upper or lower case: t
Input is valid. You entered "t".

Please type letter "t" in upper or lower case: T
Input is valid. You entered "T".

score 0 · Answer 4 · answered Oct 17 '22 at 13:17

0

You only read first char. You can solve that with a for loop till your char is equal to \n.

answered Oct 17 '22 at 13:17

scheinwerfer

36
7

score 0 · Answer 5 · answered Oct 17 '22 at 13:19

0

Instead of reading a single char, read an entire string. put validations around the read string

Size of input should be 1 char.
The char should be 't'/'T'

answered Oct 17 '22 at 13:19

Kraken

23,393
37
102
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Checking if input is a single character and meets validation in C

5 Answers5