8.1 Programming concepts

8.1.1 Variables and constants

A variable is a named data store whose value may change while a program is running. A constant is a named data store whose value should stay fixed during execution. Both should have meaningful names so another programmer can understand their purpose.

Declaring data stores

Some languages require an explicit declaration, where a data type is stated. Other languages infer the type from the value assigned. The source compares pseudocode, Python, Visual Basic and Java:

8.1.2 Basic data types

Data types tell the computer what kind of value is being stored and what operations are sensible for that value. The five basic types required here are:

A number stored as a string is text, so it cannot be used directly in arithmetic until it is converted to a numeric type.

8.1.3 Input and output

Programs need input statements to receive data and output statements to display results. A useful program should tell the user what to enter and should label its output clearly.

Input and prompts

Keyboard input often arrives as text, so numeric input may need to be converted to an integer or real value. This conversion is often called casting.

Output with a message

Complete idea: volume of a cylinder

The source uses a cylinder program to bring constants, input, arithmetic and output together. The logic is the same in any language:

CONSTANT PI ← 3.142
INPUT Radius
INPUT Length
Volume ← Radius * Radius * Length * PI
OUTPUT "Volume of the cylinder is ", Volume

Python, Visual Basic and Java use different syntax, but the sequence of operations remains the same.

8.1.4 Basic concepts

This large subtopic brings together six ideas: sequence, selection, iteration, totalling and counting, string handling, and operators.

8.1.4(a) Sequence

Sequence means carrying out statements in the correct order. Changing the order can change the answer or cause extra values to be processed.

The textbook demonstrates this with a marks algorithm using 999 as a sentinel. In the incorrect sequence, the sentinel is added to the total and counted, so the total, average and count are all wrong. The corrected structure adds the previous valid mark before taking the next input and calculates the average after the loop:

Total ← 0
Mark ← 0
Counter ← -1
OUTPUT "Enter marks, 999 to finish"
REPEAT
  Total ← Total + Mark
  INPUT Mark
  Counter ← Counter + 1
UNTIL Mark = 999
OUTPUT "The total mark is ", Total
Average ← Total / Counter
OUTPUT "The average mark is ", Average
OUTPUT "The number of marks is ", Counter

With test data 25, 27, 23, 999, the corrected algorithm gives a total of 75, average 25 and count 3.

8.1.4(b) Selection

Selection chooses different paths depending on a condition. The main structures are IF and a multiple-choice structure such as CASE.

8.1.4(c) Iteration

Iteration repeats statements. The three loop categories are:

Python provides for and while; Visual Basic provides For...Next, While...End While and Do...Loop Until; Java provides for, while and do...while.

8.1.4(d) Totalling and counting

A running total adds each new value to an accumulated total. A counter increases (or decreases) to record how many times something occurs.

TotalWeight ← TotalWeight + Weight
NumberOfItems ← NumberOfItems + 1

In Java, NumberOfItems++; is a shorter way to add one to a counter.

8.1.4(e) String handling

A string stores text. The first character position may be numbered from zero or one depending on the language. You need to know four string operations:

8.1.4(f) Arithmetic, logical and Boolean operators

Arithmetic operators perform calculations; logical comparison operators compare values; Boolean operators combine or reverse conditions.

8.1.5 Use of nested statements

Nesting means placing one selection or iteration structure inside another. For example, an IF can be placed inside a loop, or one loop can be placed inside another.

The textbook worked example uses three nested loops to process marks:

The source uses constants for the number of tests, subjects and students so the values can be reduced during testing. Each level has its own totals and maximum/minimum values, and results from an inner level are passed outward to build the next level.

FOR Student ← 1 TO ClassSize
  // reset student values
  FOR Subject ← 1 TO NumberOfSubjects
    // reset subject values
    FOR Test ← 1 TO NumberOfTests
      INPUT Mark
      // update subject high, low and total
    NEXT Test
    // calculate subject average and update student values
  NEXT Subject
  // calculate student average and update class values
NEXT Student
// calculate class average

8.1.6 Procedures and functions

A repeated group of statements can be placed in a subroutine, defined once and called whenever needed. The two types required here are procedures and functions.

Procedures without and with parameters

PROCEDURE Stars
  OUTPUT "************"
ENDPROCEDURE
CALL Stars

PROCEDURE Stars(Number : INTEGER)
  FOR Counter ← 1 TO Number
    OUTPUT "*"
  NEXT Counter
ENDPROCEDURE
CALL Stars(7)

An argument is the value supplied in the call; a parameter is the variable in the definition that receives that value. For this course, procedure/function examples use no more than two parameters.

Functions

FUNCTION Celsius(Temperature : REAL) RETURNS REAL
  RETURN (Temperature - 32) / 1.8
ENDFUNCTION

MyTemp ← Celsius(MyTemp)

Different languages use different terminology: Python often describes procedures as void functions and value-returning routines as functions; Visual Basic uses Sub and Function; Java uses methods, with or without a return value.

Local and global variables

A global variable has scope across the whole program. A local variable can only be used inside the procedure/function or block where it was declared. A local variable can even have the same name as a global variable without referring to the same storage location.

8.1.7 Library routines

Programming environments provide library routines: pre-written, tested functions and procedures for common tasks. Some languages require a library to be imported before its routines can be used.

The four routines highlighted for this course are:

8.1.8 Creating a maintainable program

A program may be changed years after it is first written, possibly by a different programmer. Good code should therefore be understandable without relying on the original programmer's memory.

A maintainable program should:

• use meaningful names for variables, constants, arrays, procedures and functions;
• divide the solution into modules for separate tasks, using procedures and functions;
• include useful comments that explain the purpose of sections of code.