Exam Reference Language

Comparison operators

a == b
a != b
a < b
a <= b
a > b
a >= b

a == b
a != b
a < b
a <= b
a > b
a >= b

The same in both: equal to, not equal to, less than (or equal to), greater than (or equal to).

Arithmetic operators

total = a + b - c * d / e

total = a + b - c * d / e

Addition, subtraction, multiplication and division are the same in both.

Exponent (^)

area = side ^ 2

area = side ** 2

Python uses ** for powers; ^ means something else in Python.

Modulo (MOD)

remainder = 17 MOD 5

remainder = 17 % 5

The remainder after whole-number division: 17 MOD 5 is 2.

Quotient (DIV)

whole = 17 DIV 5

whole = 17 // 5

Whole-number division, ignoring the remainder: 17 DIV 5 is 3.

Boolean operators

if age >= 12 AND NOT member OR guest then
    print("Welcome")
endif

if age >= 12 and not member or guest:
    print("Welcome")

AND, OR and NOT are written in lower case in Python.

Comment

// Work out the total
total = price * quantity

# Work out the total
total = price * quantity

Assignment

score = 10
name = "Priya"

score = 10
name = "Priya"

Constant

const vat = 0.2

VAT = 0.2

Python has no constants: a name in capitals shows it shouldn't change.

Global variable

global high_score = 0

high_score = 0

def new_score(score):
    global high_score
    high_score = score

In Python, a variable made outside every function is global, but a function must say global before changing it.

Input

name = input("What is your name?")

name = input("What is your name? ")

Both return what the user types as a string.

Output

print("Hello " + name)

print("Hello " + name)

To a string: str()

text = str(42)

text = str(42)

To an integer: int()

age = int("15")

age = int("15")

To a real number: float() and real()

price = float("2.50")
price = real("2.50")

price = float("2.50")

ERL has real() as well as float(); Python only has float().

To a Boolean: bool()

paid = bool("True")

paid = "True" == "True"

In Python, bool() of any non-empty string is True, even bool("False"), so compare the text instead.

Count-controlled loop: for

for i = 1 to 5
    print(i)
next i

for i in range(1, 6):
    print(i)

ERL's to includes the end value; Python's range stops one before it.

Count-controlled loop with step

for i = 10 to 0 step -2
    print(i)
next i

for i in range(10, -1, -2):
    print(i)

Condition-controlled loop: while

answer = ""
while answer != "yes"
    answer = input("Ready?")
endwhile

answer = ""
while answer != "yes":
    answer = input("Ready? ")

Checks the condition before each pass, so it may never run.

Condition-controlled loop: do … until

do
    answer = input("Ready?")
until answer == "yes"

answer = input("Ready? ")
while answer != "yes":
    answer = input("Ready? ")

Runs once before checking. Python has no do … until: run the body once, then use while with the opposite condition.

if … then … elseif … else

if mark >= 70 then
    print("Distinction")
elseif mark >= 50 then
    print("Pass")
else
    print("Try again")
endif

if mark >= 70:
    print("Distinction")
elif mark >= 50:
    print("Pass")
else:
    print("Try again")

switch … case

switch day:
    case "Sat":
        print("Weekend")
    case "Sun":
        print("Weekend")
    default:
        print("School day")
endswitch

if day == "Sat":
    print("Weekend")
elif day == "Sun":
    print("Weekend")
else:
    print("School day")

Python's usual equivalent is if, elif and else.

Length: .length

size = word.length

size = len(word)

Substring: .substring(start, count)

part = word.substring(3, 4)

part = word[3:3 + 4]

ERL's substring takes a start and a number of characters; a Python slice takes a start and a position to stop before. Both count from 0.

Left: .left(count)

start = word.left(3)

start = word[:3]

Right: .right(count)

end = word.right(2)

end = word[-2:]

Concatenation: +

full = first + " " + last

full = first + " " + last

Upper and lower case: .upper and .lower

shout = word.upper
quiet = word.lower

shout = word.upper()
quiet = word.lower()

Python needs brackets after upper and lower.

Character codes: ASC() and CHR()

code = ASC("A")
letter = CHR(66)

code = ord("A")
letter = chr(66)

ASC is ord in Python; CHR is chr.

Open and close: open() and .close()

file = open("scores.txt")
file.close()

file = open("scores.txt", "r")
file.close()

Python's open() also takes a mode: "r" to read, "w" to write a new file, "a" to add to the end.

Read a line: .readLine()

line = file.readLine()

line = file.readline()

Write a line: .writeLine()

file.writeLine("Ada,18")

file.write("Ada,18\n")

ERL's writeLine adds to the end of the file. Python's write doesn't start a new line, so add \n.

End of file: .endOfFile()

while NOT file.endOfFile()
    print(file.readLine())
endwhile

line = file.readline()
while line != "":
    print(line.strip())
    line = file.readline()

In Python, readline() returns an empty string at the end of the file.

Create a file: newFile()

newFile("notes.txt")

file = open("notes.txt", "w")
file.close()

Declare an array

array scores[5]

scores = [0] * 5

Arrays count from 0, so scores has positions 0 to 4. ERL arrays have a fixed length and one data type.

Declare an array with values

array colours = ["Red", "Green", "Blue"]

colours = ["Red", "Green", "Blue"]

Declare a 2D array

array board[3, 3]

board = []
for row in range(3):
    board.append([""] * 3)

Assign to an array

scores[0] = 18
board[1, 2] = "X"

scores[0] = 18
board[1][2] = "X"

A 2D position is [row, column] in ERL and [row][column] in Python.

Procedure

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

def greet(name):
    print("Hello " + name)

Call a procedure

greet("Sam")

greet("Sam")

Function

function cube(number)
    return number ^ 3
endfunction

def cube(number):
    return number ** 3

A function returns a value; Python writes procedures and functions with def.

Call a function

answer = cube(2)
print(cube(3))

answer = cube(2)
print(cube(3))

Store or use the value a function returns.

Random numbers: random()

roll = random(1, 6)
amount = random(-1.0, 10.0)

import random

roll = random.randint(1, 6)
amount = random.uniform(-1.0, 10.0)

Both ends are included. Whole numbers give a whole number; real numbers give a real number.