Solid user input check - Newbie Learning C

Question

So I'm learning C and got this exercise to do with functions. Not gonna ask how to do it. So I created this function:

int menu(void) {
char user;

do {
    printf("Choise: ");
    user = getchar();

if (user == '1') {
    /* code and no return here */
}
else if (user == '2') {
   /* code */
    return 2;
}
else if (user == '3') {
    /* code */
    return 3;
}
} while (user != '3');

Got others controls flows like isdigit(user) and !isdigit(user). The problem here is that if the user input "fw" (for example), the program prints 2 times "Choise: " or more if there's more input of course. I tried several others controls flows and changing the variable user to an array and force user[0] == ... && user[1] == '\n' but to no avail.

What I'm trying to do is, if the user don't enter one of the 3 options the program just stop reading after the 1st input and waits for another input. Already checked several questions at StackOverflow but it doesn't answer to my question :/

I'm all ears to advises! Thank in advance!

Answer 1

The underlying cause here is that getchar() in C gets one single character; it's not like, for example, input() in Python, which gets an entire string of as many characters as you like. The usual technique in C to get a string consisting of more than one character is with pointers: You declare a variable, for instance, as char* a; To obtain user input, you use scanf() and store the input under that pointer address: scanf(&a); Technically, a is the pointer pointing at the memory address of the first character in your string, but the compiler stores the individual characters of the string in a contiguous block of memory until a null character is reached to mark the end of the string.

To avoid the risk of seg faults, you might want to reserve as much memory as you need to hold the longest string you think you'll need: a = malloc((sizeof(char)*n);, with n being the number of characters you want to set memory aside for.

Answer 2

Apologies for not posting in a while, I've been travelling. I thought about the problem again, and I think the root of the problem is that getchar() will return the character cast into an integer, namely, the ASCII value of the character. So if the user keys in "1", you can't run an if statement that tests if (user == '1') (C doesn't natively support direct string comparison). Instead, you should test if (user == 49) - 49 is the ASCII value of the digit "1" ("2" is 50, "3" is 51). If you write your if statements and the loop condition accordingly, that should work.