You have showed some code, which runs once only. The output shows, that this function is called again afterwards, but we do not see, from where. But there is the point to modify your program.
The loop in the calling code might look like this:
Then modify this loop and insert a test inside the function, if an invalid input is chosen:
function proceed = circular
r = input("Enter the Radius (hit return to stop: ");
disp("Circumference is ");