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Identify Errors In Algorithms Ocr

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Exam code:J277

Designing algorithms is a skill that must be developed and when designing algorithms, mistakes will be made
There are two main types of errors that when designing algorithms a programmer must be able to identify & fix, they are:
- Syntax errors
- Logic 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

Examples

Syntax Errors	Corrected
`age = input("Enter age)` `favNum == input("Enter favourite number")` `print age + favNum)` `print (age x favNum)`	`age = input("Enter age") # Missing "` `favNum = input("Enter favourite number")` `# Only one equal sign` `print (age + favNum) # Missing bracket` `print (age * favNum) # Multiply symbol is *`
`num1 = imput("Enter the first number")` `num2 = input(Enter the second number)` `if num1 > num2 then` `print(num1 + " is larger")` `elseif num2 > num1 then` `Print(num2 + " is larger")` `else` `print("The numbers are the same")` `endif`	`num1 = input("Enter the first number") # Misspelt word` `num2 = input("Enter the second number") # Missing quotes` `if num1 > num2 then` `print(num1 + " is larger") # Block not indented` `elseif num2 > num1 then` `print(num2 + " is larger") # Lowercase p` `else` `print("The numbers are the same")` `endif`

Syntax Errors

Corrected

age = input("Enter age)

favNum == input("Enter favourite number")

print age + favNum)

print (age x favNum)

age = input("Enter age") # Missing "

favNum = input("Enter favourite number")
# Only one equal sign

print (age + favNum) # Missing bracket

print (age * favNum) # Multiply symbol is *

num1 = imput("Enter the first number")

num2 = input(Enter the second number)

if num1 > num2 then

print(num1 + " is larger")

elseif num2 > num1 then

Print(num2 + " is larger")

else

print("The numbers are the same")

endif

num1 = input("Enter the first number") # Misspelt word

num2 = input("Enter the second number") # Missing quotes

if num1 > num2 then

print(num1 + " is larger") # Block not indented

elseif num2 > num1 then

print(num2 + " is larger") # Lowercase p

else

print("The numbers are the same")

endif

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

Example

An algorithm is written to take as input the number of miles travelled. The algorithm works out how much this will cost, with each mile costing £0.30 in petrol. If this is greater than £10.00 then it is reduced by 10%.

Logic errors	Corrected
`miles = input("Enter the number of miles)` `cost = 0.3` `if cost = 10 then` `cost = cost * 0.1` `endif` `print(cost)`	`miles = input("Enter the number of miles")` `cost = miles * 0.3` `if cost > 10 then` `cost = cost * 0.9` `endif` `print(cost)`
Commentary
The cost was set to 0.3 (30p) instead of correctly calculating the cost of the trip by applying the formula, *miles 0.3 The cost should only be reduced if the cost is greater than 10, in the original algorithm it only checked if the cost was equal to 10 To calculate a discount of 10%, either calculate what 10% is and subtract it from the original or multiply the full cost by 0.9**. In the original algorithm it calculates what 10% is and sets the cost to equal it.

Logic errors

Corrected

miles = input("Enter the number of miles)

cost = 0.3

if cost = 10 then

cost = cost * 0.1

endif

print(cost)

miles = input("Enter the number of miles")

cost = miles * 0.3

if cost > 10 then

cost = cost * 0.9

endif

print(cost)

Commentary

The cost was set to 0.3 (30p) instead of correctly calculating the cost of the trip by applying the formula, miles * 0.3
The cost should only be reduced if the cost is greater than 10, in the original algorithm it only checked if the cost was equal to 10
To calculate a discount of 10%, either calculate what 10% is and subtract it from the original or multiply the full cost by 0.9. In the original algorithm it calculates what 10% is and sets the cost to equal it.

Worked Example

Nine players take part in a competition, their scores are stored in an array. The array is named scores and the index represents the player

Array scores

Index	0	1	2	3	4	5	6	7	8
Score	7	9	2	11	8	4	13	10	5

The following program counts the total score of all the players

for x = 1 to 8

total = 0

total = total + scores[x]

next x

print(total)

When tested, the program is found to contain two logic errors.

Describe how the program can be refined to remove these logic errors [2]

How to answer this question

A common logic error if an algorithm contains a loop is checking it loops the correct amount of times, how many times should this algorithm loop?

Answer

For loop changed to include 0
total = 0 moved to before loop starts

Guidance

Moving total outside the loop is not enough, it could be moved after the loop which would still be a logic error)
Corrected code accepted

total = 0

for x = 0 to 8

total = total + scores[x]

next x

print(total)

Computer Science GCES OCR

Identify Errors In Algorithms Ocr

Syntax Errors

What is a syntax error?

Examples

Logic Errors

What is a logic error?

Example

Worked Example

Responses