combining fgets/scanf and switch to read a character

I would like to create a kind of menu. I will enter a character and depending on what character some text will be printed. The problem for me lies in the default option in the code bellow. I want it to recognize everything that is not 1, 2, 3 or q. Not just characters, but strings as well. If the input is "sf<ssad", I do not want it to print "Invalid input" 7 times, only once. Additionally, if I enter "qf<<dsc", even though the first character is valid, I want to count the whole input as invalid since it is more than one character. Similarly, an input like "1sd<j" should not print the result "hello". Instead it should print it as "Invalid input".

I want a reliable code that preferably does not include scanf and is easy on the eyes. I tried the following:

#include <stdio.h>
int main(void) {
char cmd;

do {
    printf("Enter 1 to print hello, enter 2 to print world, enter 3 to print hello world and enter q to quit\n");
    printf("Enter: ");
    scanf(" %c", &cmd);

    switch(cmd) {
       case '1' : printf("hello\n"); break;
       case '2' : printf("world\n"); break;
       case '3' : printf("hello world\n"); break;
       case 'q' : printf("Exiting...\n"); break;
       default : printf("Invalid input!\n"); break; 
    }
} while(cmd!='q');

return 0;
}

This is not sufficient since if I enter "sf<ssad", "Invalid input" will be printed 7 times. I tried some other versions but they had similar problems. One idea I have is to use fgets, and then take the first character and then use atoi, although I am not sure what it would look like. Thanks in advance.

Call fgets(), check for error, and then set cmd to the first character of the line. Then the rest of your code is the same.

char line[100];
char cmd;
do {
    printf("Enter 1 to print hello, enter 2 to print world, enter 3 to print hello world and enter q to quit\n");
    printf("Enter: ");
    char *res = fgets(line, sizeof line, stdin);
    if (!res) {
        break;
    }

    cmd = line[0];
    switch(cmd) {
       case '1' : printf("hello\n"); break;
       case '2' : printf("world\n"); break;
       case '3' : printf("hello world\n"); break;
       case 'q' : printf("Exiting...\n"); break;
       default : printf("Invalid input!\n"); break; 
    }

} while (cmd != 'q');

> If the input is "sf<ssad", I do not want it to print "Invalid input" 7 times, only once.

In that case, you should read an entire line of input at once, instead of one character at once. For this, I recommend that you use the function fgets instead of scanf.

Here is an example:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
void get_line_from_user( const char prompt[], char buffer[], int buffer_size );
int main( void )
{
bool quit = false;
while ( !quit )
{
char line[200];
//print menu
printf(
"Menu:\n"
"Enter 1 to print hello,\n"
"enter 2 to print world,\n"
"enter 3 to print hello world,\n"
"or enter q to quit\n"
);
//get one line of input from the user
get_line_from_user( "Enter choice: ", line, sizeof line );
//verify that user entered exactly one character
if ( line[0] == '\0' || line[1] != '\0' )
{
printf( "Error: Please only enter a single character!\n\n" );
continue;
}
switch( line[0] )
{
case '1':
printf( "hello\n" );
break;
case '2':
printf( "world\n" );
break;
case '3':
printf( "hello world\n" );
break;
case 'q':
printf( "Exiting...\n" );
quit = true;
break;
default:
printf("Invalid input!\n");
}
//add spacing
printf( "\n" );
}
return EXIT_SUCCESS;
}
//This function will read exactly one line of input from the
//user. It will remove the newline character, if it exists. If
//the line is too long to fit in the buffer, then the function
//will automatically reprompt the user for input. On failure,
//the function will never return, but will print an error
//message and call "exit" instead.
void get_line_from_user( const char prompt[], char buffer[], int buffer_size )
{
for (;;) //infinite loop, equivalent to while(1)
{
char *p;
//prompt user for input
fputs( prompt, stdout );
//attempt to read one line of input
if ( fgets( buffer, buffer_size, stdin ) == NULL )
{
printf( "Error reading from input!\n" );
exit( EXIT_FAILURE );
}
//attempt to find newline character
p = strchr( buffer, '\n' );
//make sure that entire line was read in (i.e. that
//the buffer was not too small to store the entire line)
if ( p == NULL )
{
int c;
//a missing newline character is ok if the next
//character is a newline character or if we have
//reached end-of-file (for example if the input is
//being piped from a file or if the user enters
//end-of-file in the terminal itself)
if ( (c=getchar()) != '\n' && !feof(stdin) )
{
if ( c == EOF )
{
printf( "Error reading from input!\n" );
exit( EXIT_FAILURE );
}
printf( "Input was too long to fit in buffer!\n" );
//discard remainder of line
do
{
c = getchar();
if ( c == EOF )
{
//this error message will be printed if either
//a stream error or an unexpected end-of-file
//is encountered
printf( "Error reading from input!\n" );
exit( EXIT_FAILURE );
}
} while ( c != '\n' );
//reprompt user for input by restarting loop
continue;
}
}
else
{
//remove newline character by overwriting it with
//null character
*p = '\0';
}
//input was ok, so break out of loop
break;
}
}

This program has the following behavior:

Menu:
Enter 1 to print hello,
enter 2 to print world,
enter 3 to print hello world,
or enter q to quit
Enter choice: sf<ssad
Error: Please only enter a single character!
Menu:
Enter 1 to print hello,
enter 2 to print world,
enter 3 to print hello world,
or enter q to quit
Enter choice: qsf<dsf
Error: Please only enter a single character!
Menu:
Enter 1 to print hello,
enter 2 to print world,
enter 3 to print hello world,
or enter q to quit
Enter choice: 1
hello
Menu:
Enter 1 to print hello,
enter 2 to print world,
enter 3 to print hello world,
or enter q to quit
Enter choice: 2
world
Menu:
Enter 1 to print hello,
enter 2 to print world,
enter 3 to print hello world,
or enter q to quit
Enter choice: 3
hello world
Menu:
Enter 1 to print hello,
enter 2 to print world,
enter 3 to print hello world,
or enter q to quit
Enter choice: q
Exiting...