Exam Reference Language

What Exam Reference Language is

Exam Reference Language (ERL) is OCR's own way of writing code. Every piece of code in the exam papers is printed in ERL, so you must be able to read it. When you write code, you can use ERL or Python.

ERL looks a lot like Python, but its keywords are different: blocks end with words such as endif and endwhile, rather than by indentation alone.

This sub-topic puts every construct from the specification's ERL section in one place, each beside its Python equivalent.

Operators that differ:

Meaning	ERL	Python
Remainder (modulo)	`MOD`	`%`
Whole-number division (quotient)	`DIV`	`//`
Power (exponent)	`^`	`**`
And, or, not	`AND`, `OR`, `NOT`	`and`, `or`, `not`

The others are the same in both: ==, !=, <, <=, >, >=, +, -, * and /. So 17 MOD 5 is 2, 17 DIV 5 is 3, and 2 ^ 3 is 8.

Key terms

Comments, variables, input and output

ERL

// Comments start with two slashes
const vat = 0.2
name = input("What is your name?")
price = 50
print("Hello " + name)
print(price * (1 + vat))

Python

# Comments start with a hash
VAT = 0.2
name = input("What is your name? ")
price = 50
print("Hello " + name)
print(price * (1 + VAT))

Assignment uses = in both.
A constant is made with const in ERL. Python has no constants, so programmers write the name in capitals to show it shouldn't change.
input() shows its message and returns what the user types, as a string; print() outputs.
A global variable, made with global in ERL, is shown under sub programs below.

Key terms

Casting

ERL

text = str(345)
whole = int("3")
decimal = float("4.52")
also_decimal = real("4.52")
answer = bool("True")

Python

text = str(345)
whole = int("3")
decimal = float("4.52")
also_decimal = float("4.52")
answer = "True" == "True"

ERL has real() as well as float(): they do the same. Python only has float().

Common mistake

In Python, bool() of any text that isn't empty is True, so bool("False") is True. Compare the text instead: answer == "True".

Count-controlled loops

ERL

for i = 0 to 4
    print(i)
next i

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

for i = 3 to 1 step -1
    print(i)
next i

Python

for i in range(0, 5):
    print(i)

for i in range(2, 11, 2):
    print(i)

for i in range(3, 0, -1):
    print(i)

The first loop prints 0 to 4, the second the even numbers from 2 to 10, and the third counts down 3, 2, 1. In ERL, next i marks the end of the loop.

Remember

ERL's for i = 0 to 4 includes 4. Python's range(0, 5) stops before 5, so its end number is always one past the last value.

Condition-controlled loops

A while loop checks its condition before each pass, so it may never run. A do … until loop runs once before checking, and stops when its condition becomes true.

ERL

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

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

Python

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

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

Python has no do … until. The same effect comes from running the loop's body once before a while loop, with the opposite condition.

Selection

ERL

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

day = input("Day?")
switch day:
    case "Sat":
        print("Weekend")
    case "Sun":
        print("Weekend")
    default:
        print("School day")
endswitch

Python

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

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

ERL writes then after each condition and ends with endif; Python uses a colon and indentation, and elif for elseif. ERL's switch checks one variable against several values, with default for anything else. In Python, the usual equivalent is if, elif and else.

String handling

ERL

word = "Programming"
print(word.length)
print(word.substring(3, 4))
print(word.left(3))
print(word.right(4))
print(word.upper)
print(word.lower)
print("Pro" + "gram")
print(ASC("B"))
print(CHR(100))

Python

word = "Programming"
print(len(word))
print(word[3:3 + 4])
print(word[:3])
print(word[-4:])
print(word.upper())
print(word.lower())
print("Pro" + "gram")
print(ord("B"))
print(chr(100))

These print 11, gram, Pro, ming, PROGRAMMING, programming, Program, 66 and d. Both count characters from 0. ERL's substring(3, 4) means start at 3 and take 4 characters; Python's slice [3:7] means start at 3 and stop before 7.

File handling

ERL

newFile("notes.txt")
file = open("notes.txt")
file.writeLine("First line")
file.writeLine("Second line")
file.close()

file = open("notes.txt")
while NOT file.endOfFile()
    print(file.readLine())
endwhile
file.close()

Python

file = open("notes.txt", "w")
file.write("First line\n")
file.write("Second line\n")
file.close()

file = open("notes.txt", "r")
line = file.readline()
while line != "":
    print(line.strip())
    line = file.readline()
file.close()

ERL makes a file with newFile(), and writeLine() always adds to the end of it. In Python, open() takes a mode: "w" makes a new file (or empties an old one), "a" adds to the end, and "r" reads.
Python's write() doesn't start a new line, so add \n.
ERL checks endOfFile(); in Python, readline() returns an empty string at the end.
Always close a file when you've finished with it.

Arrays

ERL

array scores[5]
array colours = ["Red", "Green", "Blue"]
scores[0] = 18
colours[2] = "Yellow"
print(colours[2])

array board[3, 3]
board[1, 2] = "X"
print(board[1, 2])

Python

scores = [0] * 5
colours = ["Red", "Green", "Blue"]
scores[0] = 18
colours[2] = "Yellow"
print(colours[2])

board = []
for row in range(3):
    board.append([""] * 3)
board[1][2] = "X"
print(board[1][2])

Both count from 0, so array scores[5] has positions 0 to 4. ERL arrays have a fixed length and hold one data type. Python uses lists, which can grow and mix types, but in the exam treat them like ERL arrays.

A 2D array is written board[1, 2] in ERL and board[1][2] in Python: row 1, column 2.

Sub programs

ERL

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

function cube(number)
    return number ^ 3
endfunction

greet("Sam")
answer = cube(2)
print(answer)

global score = 0
procedure add_point()
    score = score + 1
endprocedure
add_point()
print(score)

Python

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


def cube(number):
    return number ** 3


greet("Sam")
answer = cube(2)
print(answer)

score = 0
def add_point():
    global score
    score = score + 1
add_point()
print(score)

A procedure does a job; a function returns a value, which the program should store or use. Python writes both with def.

A global variable can be used anywhere in the program. In Python, a variable made outside every function is global, but a function must say global before it can change one.

Key terms

Random numbers

ERL

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

Python

import random

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

random(1, 6) gives a whole number from 1 to 6, including both. With real numbers, such as random(-1.0, 10.0), it gives a real number in that range. Python needs import random first, then uses random.randint() for whole numbers and random.uniform() for real ones.

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