Sequential algorithms

Selection algorithms

includig binary and multiway

Repetition loops

Pre-test, post-test and for-next loops

all hail the mighty subroutine!

used to make a mainline simpler to read and follow!

mainlines and subroutines

use of modules in algorithm design

use of stubs to represent unfinished modules

Desk checking algorithms for errors

Use of algorithms

Algorithms can be used to do many thing, including:

Load an array and print its contents

The Syllabus support document (found here on NESA's website) has a standard algorithms to load an array and print its contents. These arrays are:

Load an array:

BEGIN LoadArray

Let i = 1

Read DataValue

WHILE DataValue < > “xxx”

Let Element (i) = DataValue

i = i + 1

Read DataValue

ENDWHILE

Let NumElements = i

Display “ There are” NumElements “ items loaded into the array”

END LoadArray

and to print it:

BEGIN PrintArrayContents

Let i = 1

REPEAT

Display Element (i)

i = i + 1

UNTIL i > = NumElements

END PrintArrayContents

Note that if the number of elements is not known, the sentinel value would be stored into the last element in the array, and printing would continue until the sentinel value of “xxx” is encountered.

source: Software Design and Development syllabus support document, updated 2013

Tasks: Can you:

• Create a flowchart of these algorithms?

• Create a testing table of these algorithms

add the contents of an array of numbers

The following arrays are printed in the syllabus support document demonstrating how an algorithm can be used to add the content of an array of numbers.

BEGIN SumArrayContents

Let i = 1

Let total = 0

REPEAT

Let total = Element (i) + total

i = i + 1

UNTIL i > NumElements

Display “ The sum of all of the elements in the array = ” total

END SumArrayContents

note that this is a post test loop. A pre-tested version, using a sentinal value of 999 can also be applied.

BEGIN SumArrayContents

Let i = 1

Let total = 0

WHILE Element (i) < > 999

Let total = Element (i) + total

i = i + 1

END WHILE

Display “There are ” i “ elements”

Display “The sum of all of the elements in the array = ” total

END SumArrayContents

Note that the sentinal is not added into the total.

source: Software Design and Development syllabus support document, updated 2013

Why use structured algorithms?

We use structured algorithms for:

ease of development

ease of understanding

ease of modification