Y9 Cs Unit 2: Algorithms

Algorithm

An unambiguous, finite, sequence of steps designed to solve a problem or complete a task.

Condition-controlled Repetition

A kind of loop repeated until a condition is met - used when the number of times around the loop is not known before the loop is started.

Count-controlled Repetition

A kind of loop repeated until a count is met - used when the number of times around the loop is known before the loop is started.

Dry Run

the process of executing the algorithm outside the machine, either in the programmer's head or using pencil and paper.

Finite

Having a start and an end - an important property of an algorithm

Flowchart

Graphical method of representing an algorithm using shapes and arrows.

Initialisation

the process of assigning the first (initial) value to a variable.

Input

data sent to a computer for processing.

Loop

a sequence of program statements that is repeated over and over again.

Output

data that was processed by and sent out from a computer.

Pseudocode

A method of describing an algorithm using a structure that is close to a real programming language

Repetition

Algorithmic construct in which a group of statements is executed repeatedly.

Selection

Algorithmic construct in which a Boolean condition is used to decide which instructions (if any) will be executed.

Sequencing

Programming construct in which a set of instructions is executed in the order in which they appear.

Trace Table

A method used to manually work out what variable values should be throughout the code. Each column represents a variable or output and each row represents a change in value of that variable.

Unambiguous

Clear and precise - an important part of each step in an algorithm

Validation

to check that the data entering a program, by any means, is fit for processing.

Variables

names associated with containers which programmers use to hold data. The contents can be changed during execution.

algorithms must be… (4)

1. unambiguous

2. finite

3. sequenced

4. solves problem / completes task

algorithms can be written in…

1. plain english

2. code / programming language

3. flow chart

flowchart symbols and their functions (4)

1. oval - start / end

2. rectangle - processes (eg: maths / assignment)

3. parallelogram - inputs / outputs

4. diamond - yes / no decision (condition) used for selection and repetition

boolean condition V arithmetic expressions

boolean conditions are for comparisons with the answer of yes or no

arithmetic expressions are calculations with a number answer

trace tables are read from … to … , … to …

trace tables contain columns for … and an … column to track the changes of these things in an … during a … …

top

bottom

left

right

variables

output

algorithm

dry run

pre condition loops / top test iteration

A loop where the condition is checked BEFORE the loop runs.

post condition loops / bottom test iteration

A loop where the condition is checked AFTER the loop runs, so the loop always runs at least once.

GAP FILL: general approach to validation (5)

• create a (1) loop control variable called (2) and set it to (3)

• start a (4) which executes all the time when (2) is still (3)

• inside the loop we get the (5)

• ...and we check the (5)

• ...setting (2) to (6) if it is acceptable or printing an error message if not.

boolean

valid

false

loop

input

true

HARDER GAP FILL: general approach to validation (5)

• create a (1) called (2) and set it to (3)

• start a loop which (4)

• inside the loop we get the (5)

• ...and we check the (5)

• if (2) is (3) we… (6)

• if (2) is True we… (7)

boolean loop control variable

valid

false

executes all the time while valid is still False

input

print an error message

leave it be

count controlled loops in flowcharts don’t have a while function, instead … is used which starts at … or … and is changed at the end of the loop with the expression …

a count variable

0

1

count = count + 1