Validation and authentication
Validation checks
Validation checks that input is sensible before a program uses it.
It cannot check that the data is true: a valid age can still be the wrong age.
- Range check: a number is between two limits, such as a month from 1 to 12.
- Length check: text has an allowed number of characters.
- Presence check: something has been entered at all.
- Format check: text follows a pattern, such as two letters then three digits.
- Type check: the input is the right kind of data, such as a whole number.
A range check as a function
function valid_month(month)
return month >= 1 AND month <= 12
endfunction
print(valid_month(12))
print(valid_month(13))SUBROUTINE valid_month(month)
RETURN month ≥ 1 AND month ≤ 12
ENDSUBROUTINE
OUTPUT valid_month(12)
OUTPUT valid_month(13)def valid_month(month):
return month >= 1 and month <= 12
print(valid_month(12))
print(valid_month(13))This prints True, then False. Both limits are allowed, so the comparisons are >= and <=.
Keep asking until it is valid
A validation loop reads the input, then repeats while the input is not acceptable.
The first read is before the loop, so the condition has something to check.
Re-ask until the number is from 1 to 10
n = int(input())
while n < 1 OR n > 10
print("Out of range")
n = int(input())
endwhile
print("Accepted")n ← STRING_TO_INT(USERINPUT)
WHILE n < 1 OR n > 10
OUTPUT "Out of range"
n ← STRING_TO_INT(USERINPUT)
ENDWHILE
OUTPUT "Accepted"n = int(input())
while n < 1 or n > 10:
print("Out of range")
n = int(input())
print("Accepted")The examples here call input() with no prompt message. In the questions, do the same, so the output is only the lines the task asks for.
Test a check with three kinds of data:
- Normal data: a typical value that should be accepted, such as 5.
- Boundary data: the values on each limit and just outside them, such as 0, 1, 10 and 11.
- Erroneous data: values that should be rejected, such as 25.
Authentication
Authentication checks that a user is who they say they are, usually with a username and a password.
A good routine limits the number of attempts, so a password cannot be guessed by trying again and again.
Three attempts at a password
attempts = 0
ok = false
while attempts < 3 AND NOT ok
word = input()
if word == "Tiger42" then
ok = true
else
print("Incorrect")
attempts = attempts + 1
endif
endwhile
if ok then
print("Welcome")
else
print("Locked")
endifattempts ← 0
ok ← False
WHILE attempts < 3 AND NOT ok
word ← USERINPUT
IF word = "Tiger42" THEN
ok ← True
ELSE
OUTPUT "Incorrect"
attempts ← attempts + 1
ENDIF
ENDWHILE
IF ok THEN
OUTPUT "Welcome"
ELSE
OUTPUT "Locked"
ENDIFattempts = 0
ok = False
while attempts < 3 and not ok:
word = input()
if word == "Tiger42":
ok = True
else:
print("Incorrect")
attempts = attempts + 1
if ok:
print("Welcome")
else:
print("Locked")The counter limits the attempts and the flag stops the loop as soon as the password is right. When both a username and a password are needed, both must match: join the two comparisons with AND.
Practise: While loops and Testing on Brainlag Code.
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