1.3.1 Conventions, and when to use pseudocode | Programming and Object-Oriented Programming

Conventions used in this module

Purpose	Keyword and form
Start and end	BEGIN ... END
Input	READ name
Output	PRINT value
Assignment	total = total + i
Selection	IF condition THEN ... ELSE ... END IF
Pre-test loop	WHILE condition DO ... END WHILE
Counted loop	FOR i = 1 TO n DO ... END FOR
Post-test loop	REPEAT ... UNTIL condition
Procedure	PROCEDURE `name(parameters)` ... END PROCEDURE

Indent the body of every structure. Use meaningful variable names. Some books write assignment as total ← total + i; this module uses =.

Operators you can use

Kind	Operators	Example
Arithmetic	`+ - * /`	`area = length * width`
Whole-number division and remainder	`DIV`, `MOD`	`17 DIV 5` is 3, `17 MOD 5` is 2
Comparison	`= != < <= > >=`	`IF marks >= 50 THEN`
Logical	`AND OR NOT`	`IF age >= 18 AND hasID THEN`

MOD (the remainder) is very useful: n MOD 2 = 0 tests whether n is even, and n MOD d = 0 tests whether d divides n exactly.

The three loops compared

Loop	When the condition is tested	Smallest number of passes	Use it when
`FOR i = 1 TO n`	Before each pass (it counts)	0	You know how many times to repeat
`WHILE condition DO`	Before each pass	0	You repeat while something is true and might not need to run at all
`REPEAT ... UNTIL condition`	After each pass	1	The body must run at least once, for example asking for a password

Note the different meaning of the condition: WHILE repeats while the condition is true; REPEAT ... UNTIL repeats until the condition becomes true.

Rules for good pseudocode

One statement per line, with keywords in capitals.
Indent the body of every IF, loop and procedure, and close each one (END IF, END WHILE, END FOR).
Use meaningful names: totalMarks tells the reader much more than t.
Write what happens, not language details: READ marks, not Scanner sc = new Scanner(System.in).
Be precise enough that another programmer could turn it into code without asking you anything.

Flowchart or pseudocode?

Flowcharts show the overall flow visually and suit beginners and presentations. Pseudocode is faster to write, easier to edit and closer to real code. Professionals usually use pseudocode.

From pseudocode to Java

Because pseudocode is close to real code, each line translates almost directly:

Pseudocode	Java
`READ marks`	`int marks = input.nextInt();` (with a `Scanner` called `input`)
`PRINT total`	`System.out.println(total);`
`IF a > b THEN ... ELSE ... END IF`	`if (a > b) { ... } else { ... }`
`WHILE x != 0 DO ... END WHILE`	`while (x != 0) { ... }`
`FOR i = 1 TO n DO ... END FOR`	`for (int i = 1; i <= n; i++) { ... }`
`REPEAT ... UNTIL done`	`do { ... } while (!done);`
`n MOD 2`	`n % 2`
`a = b` (comparison in a condition)	`a == b`

Watch the last row: in pseudocode = is used both to assign and to compare, but Java uses = to assign and == to compare. Also note that REPEAT ... UNTIL becomes a do ... while loop with the condition reversed.

Remember

Pseudocode is for people, so clarity matters more than strict rules.
Indent and close every structure.
WHILE may run zero times; REPEAT ... UNTIL always runs at least once.