Robust programs and testing

Defensive design

A robust program copes with whatever users type.

• Validation checks that input is sensible before it is used: range (from 1 to 12), length (at least 8 characters), presence (not left empty), format (looks like a postcode), type (a whole number).
• Authentication checks that a user is who they claim to be, for example a username and password.
• Maintainability: comments, meaningful names, indentation and subroutines make code easier to fix later.

n = int(input("Enter a month number"))
while n < 1 OR n > 12
    print("Invalid")
    n = int(input("Enter a month number"))
endwhile
print("Accepted " + str(n))

n ← STRING_TO_INT(USERINPUT)
WHILE n < 1 OR n > 12
    OUTPUT "Invalid"
    n ← STRING_TO_INT(USERINPUT)
ENDWHILE
OUTPUT "Accepted " + INT_TO_STRING(n)

n = int(input("Enter a month number"))
while n < 1 or n > 12:
    print("Invalid")
    n = int(input("Enter a month number"))
print("Accepted " + str(n))

Suppose the user types 15, then 0, then 7. 15 is more than 12, so the loop prints Invalid and asks again. 0 is less than 1, so it prints Invalid again.

7 is in range, so the condition is false, the loop ends, and the program prints Accepted 7. Invalid was output 2 times: once for each value that was out of range.