Declaring Variables and Constants
Variables are assigned using the = operator e.g. x = 3. |
Local variables |
Variables declared inside a function or procedure are local to that subroutine. |
Global variables |
Variables in the main program can be made global with the keyword global. E.g. GLOBAL userid = 123. |
Constants |
The values of constants do not change throughout the program.
E.g. CONST Vat = 20. |
Data Types
Integer |
VAR age as INTEGER |
Whole numbers only |
0, 6, 10293, -999 |
Real or Float |
VAR price as REAL |
Numbers that have a decimal point |
0.15, -5.87, 100.0 |
Char |
VAR letter as CHAR |
A single letter, number, symbol |
"A", "k", "5", "-", "$" |
String |
VAR name as STRING |
Used to represent text, it is a collection of characters |
"FsTmQ2", "$money$" |
Boolean |
VAR numFound as BOOLEAN |
Could take one of two values, usually TRUE or FALSE |
True/False, 1/0, Yes/No |
Casting Variables
You can change the data type of a variable by using casting. |
Converting integer 3 to string. |
str(3) returns "3" |
Converting string "3" to integer. |
int("3") returns 3 |
Converting string "3.14" to float. |
float("3.14") returns 3.14 |
String Handling
Finding the length of a string |
VAR name as STRING
name = INPUT("Enter your name")
PRINT("Your name has" + name.length + "characters") |
Getting a substring |
stringname.subString(startingPosition, numberOfCharacters)
NB The string will start with the 0th character.
Example:
someText = "Computer Science"
PRINT(someText.length)
PRINT(someText.substring(3,3))
Will display:
16
put |
Extracting a specific chatacter from a string |
name[i]
Example:
name = "Paloma"
name[3] returns "o" |
Converting to uppercase |
name.UPPER() |
Converting to lowercase |
name.LOWER() |
Taking inputs from user
Inputs taken from a user need to be stored in a variable. |
Example: |
VAR name as STRING
name = INPUT("Enter your name") |
Outputting to screen
Outputting a string |
PRINT("Hello") |
Outputting a variable set by you |
word = ("Hello")
PRINT(word) |
Outputting a variable entered by the user |
VAR name as STRING
name = INPUT("What is your name?")
PRINT("Hello" + name) |
1-Dimensional Arrays
Declaring an array |
ARRAY names[5] |
Initialising an array - filling it up with values |
names[0] = "Ahmad"
names[1] = "Ben"
names[2] = "Catherine"
names[3] = "Dana"
names[4] = "Elijah" |
Displaying a specific item from an array |
PRINT(names[3])
will display "Dana" |
Displaying ALL items in an array - method 1 |
FOR i = 0 to 5
PRINT(names[i])
NEXT i |
Displaying ALL items in an array - method 2 |
ARRAY names[5]
names[0] = "Ahmad"
names[1] = "Ben"
names[2] = "Catherine"
names[3] = "Dana"
names[4] = "Elijah"
PRINT(names) |
Dynamically inserting values in an array |
E.g. Ask the user to enter 5 names
FOR i = 0 to 5
names[i] = INPUT("Enter name:")
NEXT i |
Performing calculations on one Array element |
E.g. Increase element 2 of ARRAY age by 10: age[2] = age[2] + 10 |
Performing calculations on Array elements |
E.g. Increase ALL the values in ARRAY ages by 2:
FOR i = 0 to 4
age[i] = age[i] + 2
NEXT i |
2-Dimensional Arrays
Note: |
Refer to CGP Page 50 |
Declaring a 2D array |
A 2D array is built as ARRAY(row, column)
ARRAY score[4,5]
builds an array of 4 rows, 5 columns.
This can be interpreted as 4 Tests, 5 Students |
Initialising a 2D array - filling it up with values |
score[0,0] = "15"
Sets score 15 to Test 0, Student 0 |
Displaying a specific item from a 2D array |
PRINT(score[1,3])
will display 14 |
Dynamically inserting values in an array |
E.g. Ask the user to enter all the scores
FOR i = 0 to 3
FOR j = 0 to 4
score[i,j] = INPUT("Enter score for Test " + i + " Student " + j + ": ")
NEXT j
NEXT i |
Sub Programs - Functions
Functions take at least one parameter and they must always return a value. |
Example: Write a function to join two strings together with a space between them and show it working on the strings "computer" and "science". |
FUNCTION join_strings(x as STRING, y as STRING) as STRING
RETURN x + " " + y
ENDFUNCTION |
Calling the function from the main program:
subject = join_strings("computer", "science")
PRINT(subject) |
Sub Programs - Procedures
Procedures don't have to take parameters...
PROCEDURE welcome()
PRINT("Hello and welcome.")
PRINT("Let's learn about procedures.")
ENDPROCEDURE |
...but sometimes they will.
PROCEDURE betterwelcome(name as STRING)
PRINT("Hello" + name + "and welcome.")
PRINT("Let's learn about procedures.")
ENDPROCEDURE |
Procedures are called by typing their name... |
...and giving an argument if necessary |
welcome() |
betterwelcome("Pablo") |
Will display:
Hello and welcome.
Let's Learn about procedures. |
Will display:
Hello Pablo and welcome.
Let's Learn about procedures. |
Note that procedures DO NOT return a value
File Handling - Writing to a file
Adding a line of text to a file |
myFile = openWrite("sample.txt")
myFile.writeline("Hello World")
myFile.close() |
File Handling - Reading from a file
Reading and outputting a single line from the text file (see further details in CGP Pg 51) |
myFile = openRead("sample.txt")
x = myFile.readLine()
myFile.close() |
Reading and outputting the whole contents of a text file |
myFile = openRead("sample.txt")
while NOT myFile.endOfFile()
PRINT(myFile.readLine())
ENDWHILE
myFile.close() |
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Comparison operators
== |
Equal to |
!= |
Not equal to |
< |
Less than |
<= |
Less than or equal to |
> |
Greater than |
>= |
Greater than or equal to |
Arithmetic operators
+ |
Addition
e.g. x=6+5 gives 11 |
- |
Subtraction
e.g. x=6-5 gives 1 |
* |
Multiplication
e.g. x=12*2 gives 24 |
/ |
Division
e.g. x=12/2 gives 6 |
MOD |
Modulus
e.g. 12MOD5 gives 2 |
DIV |
Quotient
e.g. 17DIV5 gives 3 |
^ |
Exponentiation
e.g. 3^4 gives 81 |
Boolean operators
AND |
If two or more statements are true. |
OR |
If either statement is true. |
NOT |
To reverse the logical results of a statement. |
Selection - if/else
Selection involves making decisions based on a comparison. Comparison operators are used, sometimes with boolean operators. |
IF entry == "A" THEN
PRINT("You selected A")
ELSEIF entry == "B" THEN
PRINT("You selected B")
ELSE:
PRINT("Unrecognised selection")
ENDIF |
Selection - switch/case
Selection involves making decisions based on a comparison. Comparison operators are used, sometimes with boolean operators. |
SWITCH entry:
CASE "A":
PRINT("You selected A")
CASE "B":
PRINT("You selected B")
DEFAULT:
PRINT("Unrecognised selection")
ENDSWITCH |
Iteration - For Loop
FOR loops will repeat the code inside them a fixed number of times. The number of times that the code repeats will depend on an initial value, end value, and the step count. |
Example:
FOR i = 0 to 7
PRINT("Hello")
NEXT i
Will print hello 8 times (0-7 inclusive). |
Iteration - Repeat Loop
This loop is controlled by a condition at the end of the loop. Keep going until the condition is TRUE (i.e. while it is false). Always runs the code inside it at least once. You get an infinite loop if the condition is never true. |
Example: Write an algorithm that a supermarket self-scan machine could use to check if enough money has been fed into it and output the right amount of change. |
VAR total as INTEGER
total = 0
VAR cost, coin, change as INTEGER
cost = total cost in pence
REPEAT
coin = INPUT("Value of coin")
total = total + coin
UNTIL total >= cost
change = total - cost
OUTPUT change |
Iteration - While Loop
This loop is controlled by a condition at the start of the loop. Keep going while the condition is TRUE (i.e. until it is false). Never runs the code inside if condition is initially false. You get an infinite loop if the condition is always true. |
Example: Write an algorithm that a supermarket self-scan machine could use to check if enough money has been fed into it and output the right amount of change. |
VAR total as INTEGER
total = 0
VAR cost, coin, change as INTEGER
cost = total cost in pence
WHILE total < cost
coin = INPUT("Value of coin")
total = total + coin
ENDWHILE
change = total - cost
OUTPUT change |
Iteration - Do While Loop
This loop is controlled by a condition at the end of the loop. Keep going while the condition is TRUE (i.e. until it is false). Always runs the code inside it at least once. You get an infinite loop if the condition is always true. |
Example: Write an algorithm that a supermarket self-scan machine could use to check if enough money has been fed into it and output the right amount of change. |
VAR total as INTEGER
total = 0
VAR cost, coin, change as INTEGER
cost = total cost in pence
DO
coin = INPUT("Value of coin")
total = total + coin
WHILE total < cost
OUTPUT change |
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