Programming Constructs

Programs are designed using common building blocks. These building blocks, known as programming constructs (or programming concepts), form the basis for all programs.

There are three basic building blocks to consider:

sequence is the order in which instructions occur and are processed
selection determines which path a program takes when it is running
iteration is the repeated execution of a section of code when a program is running

Sequence

Sequence is the order in which programming statements are executed. Programming statements usually run one after another in order, unless one of the other programming constructs is used. The sequence of a program is extremely important as once these are translated, carrying out instructions in the wrong order leads to a program performing incorrectly.

The following example code will execute each line in order/sequence

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print("Let's introduce ourselves.")
print("My name is Eliza. What is your name?")
name = input()
print("Hello " + name + ", nice to meet you.")

Sequence in a Flowchart
Sequence in a flowchart

Selection

IF statements

Selection is a programming construct where a section of code is run only if a condition is met. In programming, there are often occasions when a decision needs to be made. Selection is the process of making a decision. The result of the decision can either be TRUE or FALSE, this determines which path the program will take next.

The following code uses selection to test the condition varHeight > 1.6, if this condition is TRUE then “Tall enough to enter ride” is printed.

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if varHeight > 1.6:
	print("Tall enough to enter ride")

The following code uses selection to test the condition username == “Callum”, if this condition is TRUE then “Access granted” is printed. If the condition is FASLE we can use else: to execute a different line of code.

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if username == "Callum":
	print("Access granted")
else:
	print("Unknown username")

Selection in a Flowchart
In flowcharts we us a diamond shape to represent a question or test condition that can be TRUE or FALSE.
Selection in a flowchart

Iteration or Loop

Programs often need to repeat certain steps while or until a condition has been met. This process is known as iteration.

Iteration or repetition is often referred to as looping, since the program ‘loops’ back to an earlier line of code.

Iteration allows programmers to simplify a program and make it more efficient. Instead of writing out the same lines of code again and again(mistages happen), a programmer can write a section of code once, and ask the program to execute the same line repeatedly until no longer needed.