How to display an error message for incorrect input?

Question

I am currently trying to make code for a calculator that will calculate the area of a circle, cube, or square, depending on which integer the user enters in the beginning, then prompt for the measurements, etc.
I want the user to choose between 1, 2, or 3.

My current code:

 #include <stdio.h>
 #include <math.h>

 int main(void){

     int shape;

     printf("\nArea calculation\n(1) Square\n(2) Cube \n(3) Circle \nPlease make a selection"); 
     scanf("%d", &shape);

     else{ 
         printf("\nIncorrect Choice");
     }

 return 0;
 }

My question is:

1. How can I make an error message pop up if they haven't entered that info?
2. How I can make it loop and ask the same question again if they enter something other than 1, 2,3.
3. How do I make it so if the user enters something line: 99 then the program shuts off?

Answer 1

You need to read and probably do a few c tutorials before you try to do this. These will get you started toward learning how to (1) print error output, (2) handle input, and (3) manage program control, which are the three things you seemed to ask about. There are a lot of ways to do this.

1. For error printing, look up stdout and stderr. A common strategy when learning might be using the function fprintf to write to standard error.

fprintf(stderr, "Error: incorrect value inputted. Please enter 1, 2, or 3.\n");

2. For input handling, you should google examples. Your scanf statement should end in a semicolon, not a colon. Statements in C end in a semicolon. Then you need some control flow to see what they entered and do something different based on that. A switch statement may make sense where, as here, you have a small number of options to deal with.

   /* put in a function so we can call it from main() multiple times. */
   int myFunction() {
    scanf("%d", &shape); 
    switch(shape) {
    case 1:
       /* do stuff */
       break;
    case 2:
       /* do stuff */
       break;
    case 99:
       return 99;
    default:
       fprintf(stderr, "Incorrect Choice");
    }
   }

3. Program Control. Finally, you want to be able to call this again if they fail. So put it in a function, and call that function from main.

   int main() {
       /* this empty while loop will call myFunction() forever, */
       /* or until it returns a 99 because someone quit. */
       while(myFunction() != 99) ;
   }
   

This is a bit inelegant but should get you started. Again, you really, really want to start looking at tutorials on learning the language.