Describing an Algorithm

Objectives

• Understand the logical steps involved in solving a problem using algorithms
• Know the difference between Structured English and Pseudocode
• Know how to express an algorithms using pseudocode
• Be able to read and comprehend algorithms represented in the form of flowcharts
• Understand that a computer program is an implementation of an algorithm; an algorithm is not a computer program

In the previous section we were working on an algorithm to find the largest value in a list of numbers: [23, 8, -76, 49, 65, -2]. We set out the steps using Structured English:

1. Start with the first number in the list and consider it as the current maximum.
2. Iterate through the remaining numbers in the list.
3. For each number, compare it with the current maximum.
4. If the current number is greater than the current maximum, update the current maximum to be that number.
5. Continue this process until all numbers in the list have been considered.
6. The final value of the current maximum is the maximum number in the list.

Structured English is a high-level description language that uses natural language constructs to represent the logic of a process or algorithm.

It is some step removed from the code used for a programming language. It is intended for humans to read who may not be familiar with the syntax or forms used in a formal programming language such as Python.

For GCSE Computer Science we need to be familiar with a more stylised form known as pseudocode and how flowcharts can also be used to describe an algorithm. Both AQA and OCR have their own versions of pseudocode and require students to express their answers to any questions requiring an algorithm using their version of pseudocode.

Note

For questions that require pseudocode answers, marks will still be awarded if the pseudocode is incorrect but the meaning of the algorithm remains sufficiently clear.

Pseudocode

Pseudocode is similar to Structured English in that they both provide high-level, human-readable representations of algorithms or processes without specifying the exact syntax of a particular programming language. However, there are some distinctions between the two:

Structured English	Pseudocode
It often resembles a more natural language, using sentences and phrases to describe the steps of the algorithm in a way that is closer to everyday language.	It tends to have a structure that is more similar to code, using keywords and simple statements that are a bit closer to actual programming syntax
It is generally less formal and more human-readable. It is often used in the early stages of designing algorithms, providing a conceptual understanding of the process.	It can be a bit more formal and may include constructs that resemble programming language syntax. Pseudocode is often used as a transitional step between algorithm design and actual code implementation.
It is often intended for a broader audience, including stakeholders and individuals who may not have programming expertise.	It is typically geared more towards programmers or those with some familiarity with programming concepts.

The finding the max algorithm becomes:

values <- [23, 8, -76, 49, 65, -2]
current_maximum <- first value in the list   # or current_maximum <- values[0]
number_of_values <- LEN(values)              # LEN() returns length of the list
FOR i <- 1 TO number_of_values
    current_value <- values[i]
    IF current_value > current_maximum THEN
        current_maximum <- current_value
    ENDIF
ENDFOR
OUTPUT current_maximum

The pseudocode used above follows the form used by AQA.

The OCR version would be:

values = [23, 8, -76, 49, 65, -2]
current_maximum = first value in the list   # or current_maximum <- values[0]
number_of_values = LEN(values)              # LEN() returns length of the list
for i = 1 to number_of_values
    current_value <- values[i]
    if current_value > current_maximum then
        current_maximum <- current_value
    endif
next i
print current_maximum

In both cases note how the pseudocode differs from the Structured English version. It look more like programming code but stops short of being an actual programming language. As programmers we can recognise assignment, a loop and a section statement, it may not take someone not familiar with programming that long to work out the purpose of this algorithm.

The pseudocode should express unambiguously the steps required to solve the given problem. We should be able to take the pseudocode and convert into the programming language of our choice: Python, C# etc..

Flowcharts

Another form we can use is a visual form, a diagram to represent the steps known as a flowchart.

The flowchart uses various shapes and arrows to illustrate the steps and the flow of control within that process. It is a diagrammatic tool that provides a clear and systematic way to represent the logical sequence of actions or decisions in a system.

They serve the same purpose as pseudocode and can be easier for non-programmers to follow and understand.

The flowchart uses a number of symbols to represent the steps in an algorithm and their sequence:

The flowchart for the find the maximum algorithm might be:

Flowcharts are a valuable tool for many situations. It is required for the GCSE that you both know how to create a flowchart and to read the algorithm from the given flowchart. However, they can be time consuming to construct and for more complex algorithms the diagrams quickly become large and fiddly to deal with. They do not handle detailed code well but are OK for high level view of an algorithm and useful for sketching ideas.

Questions

1. What is the key difference between Structured English and Pseudocode?

Pseudocode is less formal and more human-readable.

Structured English resembles everyday language, while Pseudocode resembles programming syntax.

Structured English is for programmers only.

Pseudocode can be understood by anyone without programming experience.

Structured English uses more natural language, while Pseudocode is closer to programming syntax.

Submit

2. What does the following pseudocode do?

values <- [23, 8, -76, 49, 65, -2]
current_maximum <- first value in the list
FOR each value in the list
    IF value > current_maximum THEN
        current_maximum <- value
ENDFOR
OUTPUT current_maximum

Finds the sum of all the values in the list.

Finds the minimum value in the list.

Finds the maximum value in the list.

Calculates the average of the list values.

The pseudocode iterates through the list and updates the current maximum if a larger value is found.

Submit

3. Which of the following best describes a flowchart?

A visual representation of an algorithm using shapes and arrows.

A way to write code in a simpler language.

A table showing possible values of variables.

A method for writing pseudocode in a more complex form.

Flowcharts use shapes and arrows to diagrammatically represent the steps of an algorithm.

Submit

4. Why might Structured English be used instead of pseudocode?

It is faster to write.

It is more formal and precise.

It is intended for broader audiences, including non-programmers.

It looks more like actual programming code.

Structured English is designed to be more readable and accessible to non-programmers.

Submit

5. Which pseudocode statement correctly initializes a list in AQA format?

list <- [1, 2, 3]

list = {1, 2, 3}

list = [1, 2, 3]

list := [1, 2, 3]

The AQA pseudocode format uses '<-' for assignment.

Submit

6. Which flowchart symbol represents a decision point (e.g., IF-THEN statement)?

A rectangle

A diamond

A circle

An arrow

The diamond shape is used to represent decisions in a flowchart.

Submit

7. In the following pseudocode, what will be the output if the list contains [5, 12, 7]?

values <- [5, 12, 7]
current_maximum <- first value in the list
FOR i <- 1 TO LEN(values)
    current_value <- values[i]
    IF current_value > current_maximum THEN
        current_maximum <- current_value
ENDFOR
OUTPUT current_maximum

5

7

12

The code will not run.

The pseudocode finds the maximum value in the list, which is 12.

Submit

8. Why are flowcharts considered easier for non-programmers to follow?

They use shapes instead of programming constructs.

They are written in plain English.

They include detailed code explanations.

They are only used for simple algorithms.

Flowcharts represent logical steps visually, making them easier for non-programmers to understand.

Submit

9. Which of the following is a characteristic of pseudocode?

It is written in the syntax of a specific programming language.

It is human-readable but close to actual code syntax.

It includes detailed comments explaining each step.

It is only used in Python programming.

Pseudocode is designed to be human-readable while resembling programming constructs.

Submit

10. What is the purpose of the LEN() function in the pseudocode?

values <- [23, 8, -76, 49, 65, -2]
number_of_values <- LEN(values)

It returns the total number of values in the list.

It returns the maximum value in the list.

It returns the sum of all the values in the list.

It sorts the values in the list.

The LEN() function returns the length of the list, i.e., the number of elements in it.

Submit