gcse pseudocode ocr unit 5 algorithms computer ......ocr computer science j276 pseudocode unit 5...

GCSE OCR Computer Science J276

Pseudocode Unit 5 Algorithms

5

•  Define data types integer, real, Boolean, character, string

•  Define Boolean operators

•  Understand and use sequence, selection and iteration in an algorithm

•  Write algorithms in pseudocode involving sequence, selection and iteration

Objectives


Data types •  You will use the following data types in your

algorithms:

Data type Description Example

INTEGER a whole number 1475, 0, -5

REAL a number with a decimal point 56.75, 6.0, -2.456, 0.0

BOOLEAN Can only be TRUE or FALSE TRUE, FALSE

CHARACTER A single alphabetic or numeric character ‘a, ‘K’, ‘4’, ‘@’, ‘%’

STRING One or more characters enclosed in quote marks ‘Jo Hobson’, ‘123’


Boolean operators •  The following operators are used to compare two

values > greater than >= greater than or equal < less than <= less than or equal = equal (in Python this is written ==) <> not equal (in Python this is written !=)


Boolean expressions •  A Boolean expression evaluates to TRUE or FALSE

•  Which of the following expressions are TRUE? (i)  35 >= 5 * 7

(ii)  ‘A’ < ‘B’

(iii)  100 <= 10**2

(iv)  25 > 2 * 5

(v)  2 ^ 4 = 16

(vi)  n ^ 2 = n * 2


Boolean expressions •  A Boolean expression evaluates to TRUE or FALSE

•  Which of the following expressions are TRUE? (i)  35 >= 5 * 7 True

(ii)  ‘A’ < ‘B’ True

(iii)  100 <= 10**2 True

(iv)  25 > 2 * 5 True

(v)  2 ^ 4 = 16 True

(vi)  n ^ 2 = n * n True


Programming constructs •  There are three basic ways of controlling the order in

which program statements are carried out

•  You have already used examples of all of them:

•  Sequence

•  Selection

•  Iteration


Sequence •  The statements are executed in the order they are

written

mealCost=4.00drinkCost=2.00

total=mealCost+drinkcost


Selection •  An IF statement is a selection statement

•  The next statement to be executed depends on whether the condition being tested is true or false

hoursPerNight=(“Howmanyhoursanightdoyou sleep?”)

if(hoursPerNight<8)then

OUTPUT(“That’snotenough!”)else OUTPUT(“That’splenty!”)endif


The switch/case statement •  The switch/case statement is used if there are

several possible options to be tested switchoptionChosen: case1: print(“Youchoseoption1”) case2: print(“Youchoseoption2”) case3: print(“Youchoseoption2”)endswitch


Introducing pseudocode •  Pseudocode is a kind of structured English for

describing algorithms

•  It allows the designer to focus on the logic of the algorithm without being distracted by the syntax of the programming language

•  You will see pseudocode statements written in a consistent style in exam questions, but you can use alternative statements so long as the meaning is clear


Pseudocode for assignment •  To assign a value to a variable, you can write

statements such as total=0cost=adult*2+child*3counter=counter+1

•  How would you write a statement to subtract discount from markedPrice to give salePrice?


Pseudocode for input/output •  Most programs that you will write will ask the user to

enter some data, and then accept the user input and assign it to a variable

•  The pseudocode used in an exam will look like this: firstname=input(“Pleaseenteryourname”)

•  This is equivalent to two statements: print(“Pleaseenteryourname”)

firstname=input


Inputting data - examples Write a single statement to do each of the following:

•  Display a prompt “How old are you?” Accept an answer from the user Assign the answer to a variable called age

•  Display a prompt “Press Enter to continue” When the user presses Enter, continue to the next statement


Inputting data - Answer Write a single statement to do each of the following:

•  Display a prompt “How old are you?” Accept an answer from the user Assign the answer to a variable called age

age = input (“How old are you?”)

•  Display a prompt “Press Enter to continue” When the user presses Enter, continue to the next statement

x = input (“Press Enter to continue”)


Writing pseudocode •  Write pseudocode for a program which asks the user

to enter the cost of two items, adds the two costs and if the cost is greater than £10.00, displays a message “Sorry, too much”. Otherwise it displays the change due from £10.00


Pseudocode solution item1=input(“Pleaseenterpriceoffirstitem:”)item2=input(“Pleaseenterpriceofseconditem:”)total=item1+item2iftotal>10thenprint(“Sorry,toomuch”)

elsechange=10–item1-item2print(“Changefrom£10.00is£”,change)

endif


Flowchart or pseudocode? •  Write pseudocode corresponding to this flowchart:

Input mark

Yes

No

Yes

No

print “Merit”

print “Pass”

print “Fail”

mark >=70?

mark >=50?


Iteration •  Iteration means repetition

•  There are three types of iteration statement in most, but not all, imperative programming languages such as Pascal, Visual Basic or Python

•  for … next

•  while … endwhile

•  do … until

•  Some languages (e.g. Python) do not have the do … until statement


for … next loop •  Use this when you want to execute the loop a

specific number of times total = 0 for counter from 1 to 7

maxTemperature = input(“enter max temperature: ”) add maxTemperature to total

next counter averageWeeksTemp = total / 7 print (“This week’s average is ”, averageweekstemp)


while … end while •  use this when you want to execute the loop while a

certain condition is true. emailaddress=input(“Enteremailaddress:”)whileemailaddressdoesnotcontain“@”

print(“invalidaddress–pleasere-enter”) emailaddress=input(“Enteremailaddress:”)endwhileprint(“thankyou”)

•  the condition is tested at the beginning of the loop •  how many times will the loop be executed if the user

enters [email protected]?


Flowchart while … end while emailAddress=input(Pleaseenteremailaddress”)whileemailAddressdoesnotcontain“@”emailAddress=input(“Invalid

address–pleasere-enter”)endwhileprint(“Thankyou”)

Input emailAddress

print “Invalid address – please

re-enter”

emailAddress contains @?

Input emailAddress

No

Yes


do … until •  Use this when you want to execute the loop until a

certain condition is true

•  The condition is tested at the end of the loop

•  Can you rewrite this algorithm using a do loop instead of a while loop? emailAddress=input(Pleaseenteremailaddress”)whileemailAddressdoesnotcontain“@”emailAddress=input(“Invalidaddress–please re-enter”)endwhileprint(“Thankyou”)


Using do … until •  The condition is not tested until the end of the loop

(it is always executed at least once), so you need an if statement as well as the do loop doemailAddress=input(Pleaseenteremailaddress”)ifemailaddressdoesnotcontain“@”then emailAddress=input(“Invalidaddress– pleasere-enter”)endif

untilemailAddresscontains“@”


Worksheet 5 •  Complete Questions 1 - 4 on the worksheet

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