Why input data is validated

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Quick Questions

Shade in the A or B circle for each question.

1. Why does a program validate input data?

A To prove the data matches what the user meant to type

B To reject unreasonable or wrongly formed data before it is used

2. Who carries out validation?

A The program, automatically, when the data is input

B The user, by reading the data again after typing it

3. A program asks for an exam mark from 0 to 100 and a user types 450. What should validation do?

A Reject 450 and show an error message

B Accept 450 because it is a number

4. What happens to data that fails a validation check?

A It is rejected and the user is asked to enter it again

B It is used in the program anyway and corrected later

5. A program divides by a number the user types, and the user types 0. Which statement is true?

A Validation is not needed because the program will cope

B Validation can reject 0 so the program does not crash

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1IF Age < 0 OR Age > 120 THEN		AThe input is used straight away with no validation, so a negative or silly quantity gives a wrong total
2OUTPUT "Invalid input, try again"		BAverage ← (Mark1 + Mark2) / 2 // Mark2 typed as 800
3Validation keeps asking until the data passes the check, so bad data is never used		CREPEAT INPUT Mark UNTIL Mark >= 0 AND Mark <= 100
4Without validation, bad data gives wrong results, such as an average calculated from an impossible mark		DThe error message is shown but the user is not asked again, so the invalid data is still used
5INPUT Quantity Total ← Price * Quantity // Quantity is used without any check		EA validation check that rejects an unreasonable age before it is used
6IF Age < 0 THEN OUTPUT "Error" ENDIF // the program then carries on		FThe error message shown when data fails validation, before the user is asked again

Definitions

validation

Fill in the Blanks

Complete each line of code by filling in the missing parts.

IF Age < 0 __ Age > 120 THEN OUTPUT "Invalid age" ENDIF

REPEAT INPUT Mark UNTIL Mark >= 0 ___ Mark <= 100

IF Age > 120 THEN ______ "Age is too high" ENDIF

WHILE Score > 100 DO INPUT Score ________

IF ______(Password) < 8 THEN OUTPUT "Password too short" ENDIF

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A OUTPUT "Enter your age"

B OUTPUT Age

C WHILE Age < 0 OR Age > 120 DO

D OUTPUT "Invalid age"

E INPUT Age

F ENDWHILE

Question 2

A // Validation accepts bad data

B INPUT Height

C IF Height < 0 THEN

D OUTPUT "Enter 0 or more"

E ENDIF

Question 3

A INPUT Qty

B // Check the quantity is in range

C IF Qty < 1 AND Qty > 10 THEN

D OUTPUT "Enter 1 to 10"

E ENDIF

Question 4

A INPUT Name

B // Reject empty input

C WHILE Name <> "" DO

D OUTPUT "Name is required"

E INPUT Name

F ENDWHILE

Question 5

A INPUT Code

B IF LENGTH(Code) = 4 THEN

C OUTPUT "Need 4 characters"

D ENDIF

Answer Key

Why input data is validated

Section A: Quick Questions

B — To reject unreasonable or wrongly formed data before it is used

A — The program, automatically, when the data is input

A — Reject 450 and show an error message

A — It is rejected and the user is asked to enter it again

B — Validation can reject 0 so the program does not crash

Section B: Match the Pairs

IF Age < 0 OR Age > 120 THEN → A validation check that rejects an unreasonable age before it is used

OUTPUT "Invalid input, try again" → The error message shown when data fails validation, before the user is asked again

Validation keeps asking until the data passes the check, so bad data is never used → REPEAT INPUT Mark UNTIL Mark >= 0 AND Mark <= 100

Without validation, bad data gives wrong results, such as an average calculated from an impossible mark → Average ← (Mark1 + Mark2) / 2 // Mark2 typed as 800

INPUT Quantity Total ← Price * Quantity // Quantity is used without any check → The input is used straight away with no validation, so a negative or silly quantity gives a wrong total

IF Age < 0 THEN OUTPUT "Error" ENDIF // the program then carries on → The error message is shown but the user is not asked again, so the invalid data is still used

Section C: Definitions

validation — An automatic check by the program that input data is reasonable and in the right form. Bad data is rejected with an error message and the user is asked again, so it is never used.

Section D: Fill in the Blanks

IF Age < 0 OR Age > 120 THEN
    OUTPUT "Invalid age"
ENDIF

REPEAT
    INPUT Mark
UNTIL Mark >= 0 AND Mark <= 100

IF Age > 120 THEN
    OUTPUT "Age is too high"
ENDIF

WHILE Score > 100 DO
    INPUT Score
ENDWHILE

IF LENGTH(Password) < 8 THEN
    OUTPUT "Password too short"
ENDIF

Section E: Spot the Error

B — The age must be read in with INPUT so it can be validated. OUTPUT Age only displays a value and never gets the user's data. It should be INPUT Age.

A — Validation rejects data that is unreasonable or in the wrong form so it is never used. It does not accept bad data. The comment should say rejects.

C — No number can be below 1 AND above 10 at the same time, so the condition is never true and bad data is never caught. A range check needs OR: IF Qty < 1 OR Qty > 10 THEN.

C — A presence check must loop only when the name is empty. With <> the loop runs for valid names and skips empty ones. It should be WHILE Name = "" DO.

B — The error message must show only when the length is wrong. Using = 4 shows the error for valid codes and lets wrong lengths through. It should be IF LENGTH(Code) <> 4 THEN.