Appendix 5d (OCR A-Level Computer Science): Revision Notes

Languages and Boolean Logic Guide

Variables

Variables are assigned using the = operator.

x = 3
name = "Bob"

A variable is declared the first time a value is assigned. It assumes the data type of the value it is given. Variables declared inside a function or procedure are local to that subroutine.

Variables in the main programme can be made global with the keyword global.

global userid = 123

Casting

Variables can be typecast using the int, str, and float functions.

str(3) returns "3"
int("3") returns 3
float("3.14") returns 3.14

Outputting to Screen

print(string)

print("hello")

Taking Input from User

variable = input(prompt to user)

name = input("Please enter your name")

Iteration

Count Controlled

for i = 0 to 7
 print("Hello")
next i

Will print "Hello" 8 times (0–7 inclusive).

Condition Controlled

While Loop:

while answer != "computer"
 answer = input("What is the password?")
endwhile

Do Until Loop:

do
 answer = input("What is the password?")
until answer == "computer"

Logical Operators

AND, OR, NOT

while x <= 5 AND flag == false

Comparison Operators

Arithmetic Operators

Selection

If/Else

if entry == "a" then
 print("You selected A")
elseif entry == "b" then
 print("You selected B")
else
 print("Unrecognised selection")
endif

Switch/Case

switch entry:
 case "A":
  print("You selected A")
 case "B":
  print("You selected B")
 default:
  print("Unrecognised selection")
endswitch

String Handling

Length of a String:

stringname.length

Substring:

stringname.substring(startingPosition, numberOfCharacters)

(Strings start at the 0th character.)

someText = "Computer Science"

print(someText.length)
print(someText.substring(3, 3))

Subroutines

function triple(number)
 return number * 3
endfunction

// Called from main programme
y = triple(7)

procedure greeting(name)
 print("Hello " + name)
endprocedure

// Called from main programme
greeting("Hamish")

Values are passed by value unless stated otherwise. Use byVal and byRef for clarity if required.

Arrays

Arrays are 0-based and declared using the array keyword.

array names[5]
names[0] = "Ahmad"
names[1] = "Ben"
names[2] = "Catherine"
names[3] = "Dana"
names[4] = "Elijah"

print(names[3])

array board[8, 8]
board[0, 0] = "rook"

Reading and Writing Files

Reading

myFile = openRead("sample.txt")
x = myFile.readLine()
myFile.close()

Writing

myFile = openWrite("sample.txt")
myFile.writeLine("Hello World")
myFile.close()

Comments

Comments are denoted by //.

print("Hello World") // This is a comment