Programming Fundamentals

Variables and constants

A variable is a named location in memory that holds a value which can change while the program runs.

A constant is a named value that is fixed and cannot be changed once set.

score = 0            # variable, will change
lives = 3            # variable
VAT_RATE = 0.20      # constant by convention in Python
PI = 3.14159         # constant

Why use constants

• The value appears once, so changing it means editing one line rather than hunting through the program
• The name explains what the number means, so VAT_RATE is far clearer than 0.2
• The value cannot be changed accidentally, which prevents a whole class of bugs

The three constructs

Sequence

Instructions are carried out one after another, in order.

name = input("What is your name? ")
greeting = "Hello " + name
print(greeting)

Selection

The program chooses between different paths based on a condition.

mark = int(input("Enter the mark: "))

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

Note the order. If the first condition were mark >= 50, then a mark of 80 would print "Pass" and never reach the distinction branch. Order the conditions from most restrictive to least.

Iteration

Instructions are repeated.

Count controlled loops repeat a known number of times.

for i in range(5):
    print("Line", i)

Condition controlled loops repeat until a condition changes.

password = ""
while password != "opensesame":
    password = input("Enter the password: ")
print("Access granted")

Nested constructs

Constructs can be placed inside each other.

for row in range(3):
    for col in range(3):
        if row == col:
            print("X", end=" ")
        else:
            print(".", end=" ")
    print()

The inner loop completes entirely for each single iteration of the outer loop, so this runs nine times in total.

Operators

Arithmetic

MOD is extremely useful. number % 2 == 0 tests whether a number is even. seconds % 60 gives the seconds part of a time.

Comparison

Boolean

• AND is true only when both conditions are true
• OR is true when at least one is true
• NOT reverses a condition

if age >= 13 and age <= 19:
    print("Teenager")

if day == "Saturday" or day == "Sunday":
    print("Weekend")

if not logged_in:
    print("Please sign in")

Writing code the exam can mark

Paper 2 asks you to write code in Python, in OCR reference language, or in another high level language. Whichever you choose, a few habits make a large difference.

Habits that earn marks

1. Indent consistently. In Python, indentation is the structure. In reference language it makes your logic readable.
2. Use meaningful variable names. numberOfGuesses beats n.
3. Convert input types. input() always returns a string, so use int() or float() when you need a number.
4. Comment anything non obvious, briefly.
5. Close every structure. In reference language, every if needs an endif and every while needs an endwhile.
6. Read the question for the exact requirements. If it says validate the input, a program without validation cannot get full marks however elegant it is.

A complete example

# Ask for 5 marks, then report the average and the highest

TOTAL_STUDENTS = 5
marks = []

for i in range(TOTAL_STUDENTS):
    valid = False
    while not valid:
        entry = input("Enter mark " + str(i + 1) + ": ")
        if entry.isdigit() and 0 <= int(entry) <= 100:
            valid = True
        else:
            print("Please enter a whole number between 0 and 100.")
    marks.append(int(entry))

average = sum(marks) / TOTAL_STUDENTS
highest = max(marks)

print("Average:", round(average, 1))
print("Highest:", highest)

Notice: a constant for the fixed value, validation before use, meaningful names, and clear output.

Key terms you must be able to define

Variable

A named location in memory holding a value that can change while the program runs.

Constant

A named value that is fixed and cannot be changed once it has been set.

Sequence

Instructions carried out one after another in the order written.

Selection

Choosing between different paths through a program based on a condition.

Iteration

Repeating a block of instructions, either a set number of times or until a condition is met.

Count controlled loop

A loop that repeats a known, fixed number of times.

Condition controlled loop

A loop that repeats until a condition becomes true or false.

MOD

The modulus operator, which returns the remainder after division.

DIV

Integer division, which returns the whole number part of a division.

Nested loop

A loop placed inside another loop.

Flashcards

Say the definition out loud before you flip the card. Retrieving it from memory is what builds recall, reading it again is not.

Reach the top grade on this topic

Programming marks are won by completeness, not cleverness.

Answer exactly what is asked. If the question requires input, validation, a calculation and an output, the mark scheme has a mark for each. A brilliant program missing the validation loses that mark regardless.

Choose the right loop. A known number of repetitions means a for loop. An unknown number, such as repeating until the user types quit, means a while loop. Choosing wrongly is often penalised.

Order your conditions correctly. With overlapping ranges, put the most restrictive first. if mark >= 70 must come before elif mark >= 50.

Handle the input type. In Python, input() returns a string. Comparing that string with a number will never behave as intended.

• Write a validated input loop from memory
• Trace a nested loop and state exactly how many times the inner body runs
• Use MOD to test for even numbers and to extract digits
• Explain the difference between a variable and a constant, with a reason for using each

Mistakes that cost marks here