Pseudocode (Cambridge International AS Computer Science)

Input, process, output

What is an input?

• An input is data or information being entered/taken into a program before it is processed in the algorithm

• An input can come from a variety of sources, such as:

  • User- keyboard, mouse, controller, microphone
  • Sensors- temperature, pressure, movement

• Values are input using the INPUT command

What is a process?

• A process is a doing action performed in the algorithm that transforms inputs into the desired output. The central processing unit (CPU) executes the instructions that define the process

• An example would be:

  • Comparing two numbers
  • Calculating an average

What is an output?

• An output is the result of the processing in an algorithm and usually the way a user can see if an algorithm works as intended

• An output can take various forms, such as:

  • Numbers- result of calculations
  • Text
  • Images
  • Actions - triggering events

• Values are output using the OUTPUT command

More than one value can be output in the same command when separated by commas

Example 1 - Area of a shape

• A user wants to write a program to calculate the area of a shape

Example 2 - Average test score

• A teacher wants to calculate the average mark achieved on a test amongst students in a class
• The teacher needs to enter how many students in the class and for each students a score out of 50

Programming constructs

What is sequence?

• Sequence refers to lines of code which are run one line at a time
• The lines of code are run in the order that they written from the first line of code to the last line of code
• Sequence is crucial to the flow of a program, any instructions out of sequence can lead to unexpected behaviour or errors

Example

• A simple program to ask a user to input two numbers, number two is subtracted from number one and the result is outputted

What is selection?

• Selection is when the flow of a program is changed, depending on a set of conditions

• The outcome of this condition will then determine which lines or block of code is run next

• Selection is used for validation, calculation and making sense of a user's choices

• There are two ways to write selection statements:

  • if... then... else...
  • case...

What is iteration?

• Iteration means repeating a line or block of code using a loop
• It allows a program to perform a task multiple times until a condition is met

Types of iteration

Worked Example A teacher uses a paper-based system to store marks for a class test. The teacher requires a program to assign grades based on these results.

The program will output the grades together with the average mark.

Write a detailed description of the algorithm that will be needed.[6]

Final answer: Answer

Marks awarded for a description of each of the following steps of the algorithm:

1. Reference variables for Count of students and Total marks [1 mark]
2. Loop through all students (Count) [1 mark]
3. Input individual mark (in loop) [1 mark]
4. Compare mark with threshold / boundary values to determine grade (in loop) [1 mark]
5. Output the grade for a student (in loop) [1 mark]
6. Maintain a Total (and Count if required) (in loop) [1 mark]
7. Calculate average by dividing Total by Count and Output (after loop) [1 mark]