How to make getchar() read a negative number? - c

Question

I am writing a program that can calculate the areas of a square, cube, and circle. The program needs to present an error message and allow the user to enter a new choice if they enter something not included in the menu. My problem is that if they type anything includes my menu options then the program still executes. (i.e. -1, 23, 344) I was wondering how to get it to ignore anything after the first character or to read the whole string. Or if there is something better than getchar(). I'm open to any solutions! Thank you!
#include <stdio.h>
#include <stdlib.h>
int main(void) {
int choice;
int lengthsq;
int areasq;
int lengthcube;
int areacube;
int radius;
double circlearea;
printf("Area Calculation\n");
printf("(1) Square\n");
printf("(2) Cube\n");
printf("(3) Circle\n");
fputs("Please make a selction: ", stdout);
while((choice = getchar()) != '\n')
switch (choice) {
case '1':
printf("\nPlease enter the length: ");
scanf("%d", &lengthsq);
while(lengthsq <= 0){
printf("Error! Please enter a positive number: ");
scanf("%d", &lengthsq);
}
areasq = lengthsq * lengthsq;
printf("The area of the square is %d.", areasq);
return 0;
case '2':
printf("\nPlease enter the length: ");
scanf("%d", &lengthcube);
while (lengthcube <= 0) {
printf("Error! Please enter a positive number: ");
scanf("%d", &lengthcube);
}
areacube = 6 * lengthcube * lengthcube;
printf("The surface area of the cube is %d.\n", areacube);
return 0;
case '3':
printf("\nPlease enter the radius: ");
scanf("%d", &radius);
while(radius <= 0){
printf("Error! Pleae enter a postive number: ");
scanf("%d", &radius);
}
circlearea = 3.14159 * radius * radius;
printf("The area of the circle is %.2f.\n", circlearea);
return 0;
case '\n':
case '\t':
case ' ':
break;
default:
printf("\nInvalid choice entered.\n");
fputs("Enter a new choice: ", stdout);
break;
}
}

Answer 1

You could add another switch case for the dash, which would toggle some kind of negative flag and then read a number as you're already doing. If you do not like introducing such a flag, then the best option would be using fgets, which returns the entire input line. But that has the downside that you need to parse the input. I.e. do some string manipulation, which may be slightly more complex than a simple flag parameter.
On the other hand, from the code you attached, I deduct that the only valid input consists of mere numbers (integers). You could just read an integer then with scanf.