How to get a input with prompt using scanf?

Question

I am using c and I am a newbie.

I want to get a input (1 or 2 or 3) and I will give the user suggest;

printf("please do it\n");
printf("1. \n");
printf("2. \n");
printf("3. \n");
char opt;
scanf("%c"&opt")；

if opt is not 1 or 2 or 3 then

printf("error\n");
printf("please re do it");

and all is in a while(true) loop until user enter the enter(new line charactor) to exit;

and how to do it?

I tried to create a function.

void get_order(char opt){
    switch(opt){
        case '1':break;
        case '2':break;
        case '3':break;
        default:
        printf("error\n");
        printf("please re do it"):
        char option;
        scanf("%c",&option);
        get_order(option);
    }
}

but it not work. thank you.

You state that this is in a while(true) loop, but there is no such loop in your code. You should provide a complete example (including a main and all included files). See stackoverflow.com/help/minimal-reproducible-example — William Pursell
– William Pursell, Commented Dec 20, 2021 at 14:23
Also read sekrit.de/webdocs/c/beginners-guide-away-from-scanf.html — William Pursell
– William Pursell, Commented Dec 20, 2021 at 14:23
That is not a good approach, your code is using recursion, it will consume memory more and more while the user don't enter the correct input. Use a loop instead. — riquefr
– riquefr, Commented Dec 20, 2021 at 14:29
If your compiler lets scanf("%c"&opt"); through, turn on more warnings or change to a better compiler. — Ted Lyngmo
– Ted Lyngmo, Commented Dec 20, 2021 at 14:31
Related question: Validate the type of input in a do-while loop C In my answer to that question, I provided a function get_int_from_user. That function may be useful for you, in the sense that it keeps reprompting the user for input until the user enters a valid number. However, the part about verifying that this number is between 1 and 3 would have to be added by you. — Andreas Wenzel
– Andreas Wenzel, Commented Dec 20, 2021 at 14:40

riquefr · Accepted Answer · 2021-12-20 14:41:13Z

2

That is not a good approach, your code is using recursion, it will consume memory more and more while the user don't enter the correct input. Use a loop instead. Your code should look like this:

#include <stdio.h>

int main() {
    printf("please do it\n");
    printf("1. \n");
    printf("2. \n");
    printf("3. \n");
    char opt;
    scanf("%c", &opt); //correct scanf
    scanf("%*c"); //consume the line break
    while(!(opt == '1' || opt == '2' || opt == '3')) {
        printf("error\n");
        printf("please re do it\n");
        scanf("%c", &opt); //correct scanf
        scanf("%*c"); //consume the line break
    }   
    return 0;
}

answered Dec 20, 2021 at 14:41

riquefr

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6 Comments

Andreas Wenzel Over a year ago

The line scanf("%*c"); will not consume the newline character if the user entered more than one character of input.

John Bode Over a year ago

Better to use scanf( " %c", &opt ) to skip over any leading whitespace. Of course, better to not use scanf at all for this.

user666 Over a year ago

what should I use if not use scanf?@JohnBode

riquefr Over a year ago

You can use other types of input handlers, but they are a bit harder to use. Like getline, gets and some others.

Jabberwocky Over a year ago

@riquefr you should not recommend gets.

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