Re-execute program based on user input in C - c

Question

Hi i'm trying to learn programming in C on my own and have managed to make a verry, verry simple program that calculates the surface of a circle based on user input.
However the program runs only one time and then closes it.
This was initially the intention because it is only for learning but i want to expand on this program to increase my skills/knowledge and hope someone can point me in the right direction.
What i want to do now is instead of terminating the program after running it once; i would like to offer the user a choise to either stop the program or to continue it and to calculate a new circle.
I understand that it has to be done with an if else statment with the getchar function but i have some issues wrapping my mind around it on how to put it in a program flow. I hope someone can give me some directions on how to tackle this problem or can point me to some documentation that explains this properly.
Currently i have this:
int main(void){
float diameter;
double straal;
double oppervlakte;
char ch;
printf("Type de diameter van de cirkel:\t");
scanf("%g", &diameter);
printf("\n");
straal = diameter / 2;
oppervlakte = PI * (straal * straal);
printf("De straal =\t%g \n\n", straal );
printf("De oppervlakte =\t%f \n\n" , oppervlakte);
printf("Druk enter om af te sluiten.");
scanf("%c",&ch);
getchar();
return 0;
}
and im trying to accomplish something like this(below) but i can't get it to work properly (i get the warning that the label "diameter" is not defined while trying to compile it.)
#include <stdio.h>
#define PI 3.14
int main(void){
float diameter;
double straal;
double oppervlakte;
char ch;
printf("Type de diameter van de cirkel:\t");
scanf("%g", &diameter);
printf("\n");
straal = diameter / 2;
oppervlakte = PI * (straal * straal);
printf("De straal =\t%g \n\n", straal );
printf("De oppervlakte =\t%f \n\n" , oppervlakte);
printf("Druk 'd' om door te gaan of druk enter om af te sluiten.");
if(getchar() == 'd')
{
goto diameter; /* tried to use main(void) here but that also doesnt work */
}
else{
scanf("%c",&ch);
getchar();
}
return 0;
}
i do understand that goto is not the best practise to use but in this case it seemed the easyest way to solve this issue. (and the program is not that complex ofc). However if im wrong in this please let me also know.

Answer 1

Option 1: (likely the best choice): use a do..while loop. Place a do { above your primary code block, and add a } while (<repeat condition>); at the end. The program will run through the code once, check the repeat condition (which will be "did the user enter yes"), and if so repeat, otherwise not.
Option 2: recursively call main(). You said you "tried that", but I'm not sure if you tried it by attempting to use a goto or not. Just use the line main() to call the function again. Note that if you do this too many times you can end up with a stack overflow, because the computer keeps track of each call. It takes a lot to have it be a problem, but with enough repeats it can happen.

Answer 2

You can do something like:
while(true) //this is an endless loop
{
//display a menu like
1. calc area
2. [anything else if you want to add in future]
.
.
.
0. exit
//take user input (e.g 1 for calculating the area)
switch(user input)
{
case 1:
//code to calculate area
break;
case 2:
//code for anything else
break
case 0:
exit(0); //this will terminate the program
}
}
If you follow this pattern, you can add more options to your program in future. You just need to add a case in your switch statement and include that operation in your menu. You can search for menu driven c program to get more details. Try reading about while loop and switch... case statements.

Answer 3

I actually managed to make work in both ways.
Thanks for the tips and suggestions.