C programming minor issue with interactive menu when input a floating value

I finally could validate floating input. Thanks your advices so much! This is my new code. What else do you think an invalid input?

int menu(void)
{
    int option, parsed_inputs;
    char overcount_char;
    parsed_inputs = 1;

    printf("Text Encoder Service\n\n");
    printf("1.\tEnter name of input file (currently 'Secret.txt')\n");
    printf("2.\tEnter name of output file (currently not set)\n");
    printf("3.\tEnter number of characters data should be shifted (currently +7)\n");
    printf("4.\tEncode the text\n\n");
    printf("0.\tQuit\n\n");
    printf("Make your selection: ");

    parsed_inputs = scanf_s("%d%c", &option, &overcount_char);
    while( parsed_inputs > 1 )              /* number too large */
    {
        if((overcount_char != '\n') || (option < 0) || (option > 4))
        {
            fflush(stdin);                    /* clear bad data from buffer */
            printf("That selection isn't valid. Please try again.\n\n");
            printf("Your choice? ");
            scanf_s("%d%c", &option, &overcount_char);
        }
        else
            break;
    }
    return option;
}

An input of 1.1 leads to the following:

• The string is read into an internal buffer.
• It is then matched against the given format string.
• On the first non-match, the scanf() call is stopped and it returns the number of successfully scanned values.

Let's test it:

#include <stdio.h>
int main(int argc, char ** argv)
{
    while (1) {
        int option;
        int n = scanf(" %d", &option);
        printf("%d %d\n", n, option);
        if (n <= 0) break;
    }
}

This program reads one line.

Suppose I enter 123 132.

Then the following happens: * As the format string starts with a space, all leading whitespace is consumed. In this case, there is none. * Then the 123 is consumed and put into option. * As the format string is over now, parsing is stopped, n=1 and option=123. * In the next loop run, the same happens, giving n=1 and option=123.

But: Suppose I enter 123.321 or 123#321.

Then the following happens: * As the format string starts with a space, all leading whitespace is consumed. In this case, there is none. * Then the 123 is consumed and put into option. * As the format string is over now, parsing is stopped, n=1 and option=123. * In the next loop run, there is no whitespace to skip. .321 resp. #321 is tried to be matched to %d, but these are no valid ints. Thus, we get n=0 and option keeps its old value. * As no characters are consumed from the input stream (the one used is put back again), the same happens over and over again - that's why I put if (n <= 0) break;.

So you see that the behaviour has nothing to do with floating point, as it doesn't matter if we use . or # to "disturb".

We change our program to

#include <stdio.h>
int main(int argc, char ** argv)
{
    while (1) {
        int option; char c;
        int n = scanf("%d%c", &option, &c);
        printf("%d %d %d %c\n", n, option, c, c);
        if (n <= 0) break;
    }
}

and run it, inputting 4.235#6x7.

Then we get * n=2, option=4 and c='.' at the first run * n=2, option=235 and c='#' at the 2nd run * n=2, option=6 and c='x' at the 3rd run * n=2, option=7 and c='\n' (newline) at the 3rd run

and are prompted for further input.

This makes you open to the option

(parsed_inputs = scanf("%d%c", &option, &overcount_char)) < 1

and then check what overcount_char contains, whenever parsed_inputs is > 1.

I think you should put scanf() before the while loop and explicitly check "option" variable in the while loop. What happening is that, here scanf() will always return the value 1, because scanf() returns no. of arguments read successfully. Hence this while loop will run forever. For further information check this link->http://www.cplusplus.com/reference/cstdio/scanf/

Hope this helps!