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faults-and-errors

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Error types

What are the common types of error?

Designing algorithms is a skill that must be developed and when designing algorithms, mistakes and issues will occur
Trace tables can also help to find any kind of error in a program or algorithm
There are three main categories of errors that when designing algorithms a programmer must be able to identify & fix, they are:
- Syntax errors
- Logic errors
- Runtime errors

Syntax errors

What is a syntax error?

A syntax error is an error that breaks the grammatical rules of a programming language and stops it from running
Examples of syntax errors are:
- Typos and spelling errors
- Missing or extra brackets or quotes
- Misplaced or missing semicolons
- Invalid variable or function names
- Incorrect use of operators
- Incorrectly nested loops & blocks of code

Pseudocode – with syntax errors
FUNCTION generate_username(first_name STRING, last_name STRING) RETURNS STRING DECLARE username : STRING username = SUBSTRING(first_name, 1, 1) + SUBSTRING(last_name, 1, 1) + SUBSTRING(last_name, 1, 3) RETURN username ENDFUNCTION PROCEDURE main() DECLARE first_name : STRING DECLARE last_name : STRING DECLARE username : STRING OUTPUT "Enter your first name: " INPUT first_name OUTPUT "Enter your last name: " INPUT last_name username ← generate_username[first_name, last_name] OUTPUT "Here's a suggested username:" + username ENDPROCEDURE // Main program CALL Main
Pseudocode – without syntax errors
FUNCTION generate_username(first_name : STRING, last_name : STRING) RETURNS STRING DECLARE username : STRING username ← SUBSTRING(first_name, 1, 1) + SUBSTRING(last_name, 1, 1) + SUBSTRING(last_name, 1, 3) RETURN username ENDFUNCTION PROCEDURE main() DECLARE first_name : STRING DECLARE last_name : STRING DECLARE username : STRING OUTPUT "Enter your first name: " INPUT first_name OUTPUT "Enter your last name: " INPUT last_name username ← generate_username(first_name, last_name) OUTPUT "Here's a suggested username: ", username ENDPROCEDURE // Main program CALL main()
Syntax errors
`FUNCTION generate_username(first_name STRING...)` Missing colon : between parameter name and type `first_name : STRING and last_name : STRING` `username = SUBSTRING(...)` Uses = instead of correct assignment operator ← Should be `username ← ...` `username ← generate_username[first_name, last_name]` Incorrect use of square brackets [] instead of parentheses () for function call Should be `generate_username(first_name, last_name)` `CALL Main` Incorrect capitalisation and missing parentheses — procedure name is case-sensitive Should be `CALL main()`

Logic errors

What is a logic error?

A logic error is where incorrect code is used that causes the program to run, but produces an incorrect output or result
Logic errors can be difficult to identify by the person who wrote the program, so one method of finding them is to use ‘Trace Tables‘
Examples of logic errors are:
- Incorrect use of operators (< and >)
- Logical operator confusion (AND for OR)
- Looping one extra time
- Indexing arrays incorrectly (arrays indexing starts from 0)
- Using variables before they are assigned
- Infinite loops

Pseudocode
FUNCTION calculate_area(length : REAL, width : REAL) RETURNS REAL // Checks for valid dimensions and calculates area IF length > 0 OR width > 0 THEN DECLARE area : REAL area ← width - length RETURN area ELSE OUTPUT "Length and width must be positive values." ENDIF ENDFUNCTION PROCEDURE main() DECLARE length : REAL DECLARE width : REAL DECLARE area : REAL OUTPUT "Enter the length of the rectangle: " INPUT length OUTPUT "Enter the width of the rectangle: " INPUT width area ← calculate_area(length, width) OUTPUT "The area of the rectangle is approximately ", area, " square units." ENDPROCEDURE // Main program CALL main()

Pseudocode

FUNCTION calculate_area(length : REAL, width : REAL) RETURNS REAL // Checks for valid dimensions and calculates area IF length > 0 OR width > 0 THEN DECLARE area : REAL area ← width - length RETURN area ELSE OUTPUT "Length and width must be positive values." ENDIF
ENDFUNCTION PROCEDURE main() DECLARE length : REAL DECLARE width : REAL DECLARE area : REAL OUTPUT "Enter the length of the rectangle: " INPUT length OUTPUT "Enter the width of the rectangle: " INPUT width area ← calculate_area(length, width) OUTPUT "The area of the rectangle is approximately ", area, " square units."
ENDPROCEDURE // Main program
CALL main()

Logic errors

Line	Error	Correction
`IF length > 0 OR width > 0 THEN`	The check allows one value to be negative, which is not valid	Should be `IF length > 0 AND width > 0 THEN`
`area ← width - length`	Incorrect calculation – subtracts instead of multiplying to find the area	Should be `area ← length * width`

Runtime errors

What is a runtime error?

A runtime error is where an error causes a program to crash
Examples of runtime errors are:
- Dividing a number by 0
- Index out of the range of an array
- Unable to read or write a drive

Pseudocode
FUNCTION calculate_area(length : REAL, width : REAL) RETURNS REAL // Calculates area of a rectangle after input validation IF length < 0 OR width < 0 THEN OUTPUT "Length and width must be positive values." ENDIF DECLARE area : REAL area ← length * width RETURN area ENDFUNCTION PROCEDURE main() DECLARE length : REAL DECLARE width : REAL DECLARE area : REAL OUTPUT "Enter the length of the rectangle: " INPUT length OUTPUT "Enter the width of the rectangle: " INPUT width area ← calculate_area(length, width) OUTPUT "The area of the rectangle is approximately ", area, " square units." ENDPROCEDURE // Main program CALL main()

Pseudocode

FUNCTION calculate_area(length : REAL, width : REAL) RETURNS REAL // Calculates area of a rectangle after input validation IF length < 0 OR width < 0 THEN OUTPUT "Length and width must be positive values." ENDIF DECLARE area : REAL area ← length * width RETURN area
ENDFUNCTION PROCEDURE main() DECLARE length : REAL DECLARE width : REAL DECLARE area : REAL OUTPUT "Enter the length of the rectangle: " INPUT length OUTPUT "Enter the width of the rectangle: " INPUT width area ← calculate_area(length, width) OUTPUT "The area of the rectangle is approximately ", area, " square units."
ENDPROCEDURE // Main program
CALL main()

Runtime error

Line	Error	Correction
`area ← calculate_area(length)`	Only one parameter is passed instead of two – will crash at runtime	Should be `area ← calculate_area(length, width)`

Worked Example

Each pseudocode statement in the following table may contain an error due to the incorrect use of the function or operator.

Describe the error in each case, or write ‘NO ERROR’ if the statement contains no error.

You can assume that none of the variables referenced are of an incorrect type.[5]

Statement	Error
`Result ← 2 & 4`
`SubString ← MID("pseudocode", 4, 1)`
`IF x = 3 OR 4 THEN`
`Result ← Status AND INT(x/2)`
`Message ← "Done" + LENGTH(MyString)`

Answer

Statement	Error
`Result ← 2 & 4`	Should be arithmetic operator (not &) OR 2 and 4 should be CHAR / STRING
`SubString ← MID("pseudocode", 4, 1)`	NO ERROR
`IF x = 3 OR 4 THEN`	Not Boolean values / incorrect operator OR Condition incorrect
`Result ← Status AND INT(x/2)`	`INT(X/2)` doesn’t evaluate to a Boolean value / incorrect operator
`Message ← "Done" + LENGTH(MyString)`	Can’t add string to number / “Done” is not a number

Computer-science_A-level_Cie

faults-and-errors

Error types

What are the common types of error?

Syntax errors

What is a syntax error?

Logic errors

What is a logic error?

Logic errors

Runtime errors

What is a runtime error?

Runtime error

Worked Example

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