Control Structures, Pseudocode and Trace Tables

Documenting Algorithms

Pseudocode is our main tool for documenting algorithms.

Flowcharts are equivalent to pseudocode and are also common, but are not part of the VCE courses. Flowcharts are easier for non-programmers to read as they are inherently two-dimensional, so branches (conditionals, if statements) and loops (iterations) are actually represented by a branching or looping of the flowchart.

Pseudocode flattens out the flowchart into a sequence of lines of something like "real" code. The hardest part of the translation from flowchart to pseudocode is the choice loop structure to use, as flowcharts are more flexible in this area.

Pseudocode

Assigning values to variables is done with an arrow, which serves to show the flow of data, e.g., count ← 5

Control Structures have clear beginning and ending keywords and their code blocks should be clearly indented.

Control Structures

Algorithms are built out of a sequence of statements to control input and output, assign variables to results, control flow. There are four types of flow in algorithms:

Sequence
- is just the idea that code runs "top-to-bottom", one statement after another.
Selection or Branching
1. Binary Selection (IF … THEN … ELSE … ENDIF)
2. Multiway Selection (CASE … OF … OTHERWISE … ENDCASE)
Iteration or Loops
1. Pre-test loop (WHILE … DO … ENDWHILE)
2. Post-test loop (REPEAT … UNTIL ... )
3. Count controlled loop (FOR … TO … NEXT)
Functions or Procedures (aka Submodules and Methods)
- Reusable sections of code that are defined and then called by name to execute

See the Pseudocode to Python dictionary and Python Snippets pages to compare the above pseudocode to Python.

Basic algorithms

Adding two numbers

Standard Algorithms

Trace Tables

AKA Desk Checks or Dry Runs.

This is a method to check an algorithms by running them by hand using test data (how to choose a suitable set of test data is discussed in the next section). If the pseudocode does not produce the expected output for the test then the trace will help you locate the error.

The idea is you track all of the variables and output in a table, working line-by-line through the code and updating the variables when they are assigned a new value. It helps to use a finger or a 2nd pen to point to the line currently being evaluated.

Here is an animated trace table example from 101computing.net.

Note that their psuedocode uses a slightly different syntax from the VCE "standards".

number ← 3

OUTPUT number

FOR count FROM 1 TO 3 STEP 1

      number ← number + 5

      OUTPUT number

NEXT count

OUTPUT "?"

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