Pseudocode (Cambridge (CIE) A Level Computer Science) : Revision Note

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

INPUT <identifier>

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

OUTPUT <value(s)>

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

OUTPUT "First name: ", Fname, "Surname: ", Sname

Example 1 - Area of a shape

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

// Calculate the area of a rectangle

INPUT Length
INPUT Width

Area ← Length * Width

OUTPUT "The area is ", Area

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

// Calculate the average test score for a class

DECLARE TotalScore : INTEGER
DECLARE Score : INTEGER
DECLARE NumberOfStudents : INTEGER
DECLARE Average : REAL

TotalScore ← 0

INPUT NumberOfStudents

FOR StudentIndex ← 1 TO NumberOfStudents
    OUTPUT "Enter score for student ", StudentIndex
    INPUT Score
    TotalScore ← TotalScore + Score
NEXT StudentIndex

Average ← TotalScore / NumberOfStudents

OUTPUT "The average score is ", Average

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...

INPUT Age
IF Age >= 18 THEN
   OUTPUT "You can vote"
ELSE
   OUTPUT "You cannot vote"
ENDIF

INPUT Direction
CASE OF Direction
    "N" : OUTPUT "North"
    "S" : OUTPUT "South"
    "E" : OUTPUT "East"
    "W" : OUTPUT "West"
    OTHERWISE : OUTPUT "Invalid direction"
ENDCASE

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

// Count-controlled
FOR i ← 1 TO 5
    OUTPUT i
NEXT i

// Post-condition
REPEAT
    INPUT Password
UNTIL Password = "Secret123"

// Pre-condition
WHILE Score < 100
    Score ← Score + 10
ENDWHILE