## 6.2: Reading Algorithms

## What is an algorithm?

An algorithm is a set of instructions, presented in a logical sequence.

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In an exam you may be asked to read and understand an algorithm that has been written. To prove your understanding you may be asked to respond by actions such as listing the outputs of the algorithm or identifying an error within it.

Example Algorithm:

Start NewProgram

number is integer

maxvalue is integer

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input maxvalue

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for i = 1 to maxvalue

output (i * i)

???????

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output 'program finished'

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End NewProgram

Example Questions:

1. List the outputs produced by the algorithm if the 'maxvalue' input is 5.

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2. State the code that has been replaced by '???????' and what the code's purpose is.

Example Answers:

1.

Outputs:

1

4

9

16

25

program finished

2.

Missing Code: next i

Purpose: Moves the loop to the next iteration.

## Previous Exam Questions to try

2014 Exam Algorithm:

set counter = 0

output "Before the loop"

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repeat

set counter = counter + 1

output "Count is" counter

until counter = 3

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output "Loop has ended"

2015 Exam Algorithm:

Total is integer

Mean is real

i is integer

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startmainprog

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set Total = 0

for i = 1 to 5

set Total = Total + i

output "Total is", Total

next i

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set Mean = Total / 5

output "Mean is", Mean

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endmainprog

2014 Exam Question:

Write down all the outputs produced by the algorithm.

2016 Exam Algorithm 1:

M is integer

P is integer

i is integer

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startmainprog

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input M

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for i = 1 to 4

set P = i * M

output P

endfor

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endmainprog

2015 Exam Question:

2016 Exam Question 1:

Write down all the outputs produced by the algorithm when the value of M input is 3.

Write down all the outputs in the correct order produced by the algorithm.

2016 Exam Algorithm 2:

Total is ? {stores the total of the numbers input}

Mean is ? {stores the mean of the numbers input}

Count is ? {stores the loop control value}

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startmainprog

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set Total = 0 {initialise variables}

set Count = 0

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repeat

set Count = Count + 1

set Total = Total + Count

until Count = 20

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output "The total is ", Total

set Mean = Total / 20

output "mean is ", Mean

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endmainprog

2017 Exam Algorithm 1:

HighNum is integer

Total is integer

i is integer

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startmainprog

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input HighNum

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set Total = 0

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if HighNum < 1 then

output "Number not valid"

else

for i = 1 to HighNum

set Total = Total + i

output Total

endfor

endif

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endmainprog

2016 Exam Question 2:

State, giving a reason for each, the most suitable data type for the variables below:

a. Mean

b. Count

2017 Exam Question 1:

Write down all the outputs in the correct order produced by the algorithm for the input below:

a. Input is 0

b. Input is 4

2017 Exam Algorithm 2:

IF (X > 5) AND (Y < 7) THEN

OUTPUT true

ELSE

OUTPUT false

ENDIF

2017 Exam Question 2:

Write down the output for the following values of X and Y:

a. X = 2 and Y = 4

b. X = 5 and Y = 4

c. X = 6 and Y = 6

2019 Exam Algorithm:

Start addProc

number is integer

a is integer

total is integer

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if i = 1 to 5

output "Please enter next number"

input number

a = a + number

next i

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total = a

output "The total = ", total

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End addProc

2019 Exam Question:

1. Identify the error in this algorithm and suggest a suitable change to this code to address the error.

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2. Suggest a suitable change to address the inefficient use of memory in this algorithm.

6.2 - Reading Algorithms:

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Answer each of the previous exam questions on this page.