Programming Fundamentals

A variable is a named storage location whose value can change while the program runs. A constant is a named value that is set once and never changes during execution.

score = 0          # variable — changes as the game progresses
MAX_SCORE = 100    # constant — fixed upper limit

An assignment statement stores a value in a variable. The variable name goes on the left; the value or expression goes on the right.

x = 5
x = x + 1      # x is now 6 — the old value of x is read, 1 is added, result stored back
name = "Alice"

Constants make programs easier to maintain — if the value needs to change (e.g. changing the maximum score from 100 to 50), you update it in one place rather than hunting through the code.

Feature	Variable	Constant
Value can change at runtime	Yes	No
Typical naming convention	camelCase or snake_case	ALL_CAPS
Example uses	Score, user input, running total	Maximum lives, tax rate, screen size

Sequence is the most basic programming construct: instructions are executed one after another, in the order they are written.

Input reads data from the user (keyboard, file, sensor). Output displays or returns data.

name = input("Enter your name: ")   # reads a string from the keyboard
age = int(input("Enter your age: ")) # reads and converts to integer
print("Hello, " + name)             # outputs a string
print("You are", age, "years old")  # outputs name and age

Worked example — program that calculates the area of a rectangle in sequence:

length = float(input("Enter length: "))
width = float(input("Enter width: "))
area = length * width
print("Area =", area)

Each line executes in order: read length, read width, calculate, display. Remove or reorder any line and the program breaks. Sequence is the foundation on which selection and iteration are built.

Selection lets a program choose between different paths depending on a condition. The condition evaluates to either True or False.

IF … ELSE is the core selection structure:

score = int(input("Enter score: "))
if score >= 50:
    print("Pass")
else:
    print("Fail")

IF … ELIF … ELSE handles multiple branches:

if score >= 70:
    grade = "A"
elif score >= 50:
    grade = "B"
else:
    grade = "C"
print("Grade:", grade)

Conditions are checked top to bottom. The first branch whose condition is True executes; all others are skipped.

In pseudocode (OCR style):

IF score >= 70 THEN
    grade = "A"
ELSE IF score >= 50 THEN
    grade = "B"
ELSE
    grade = "C"
END IF

Iteration (looping) repeats a block of code. A count-controlled loop runs a fixed number of times, determined by a counter variable.

for i in range(1, 6):     # i takes values 1, 2, 3, 4, 5
    print(i)

In pseudocode:

FOR i = 1 TO 5
    OUTPUT i
END FOR

Worked example — print the 7 times table:

for i in range(1, 13):
    print(7, "×", i, "=", 7 * i)

Output: 7 × 1 = 7, 7 × 2 = 14, … 7 × 12 = 84.

Count-controlled loops are used when the number of repetitions is known before the loop starts. Use them for: processing every item in a list, repeating a fixed number of times, building tables.

range(1, 6) generates values 1, 2, 3, 4, 5 — the upper bound is excluded. range(0, 5) gives 0, 1, 2, 3, 4.

A condition-controlled loop repeats while (or until) a condition is true. The number of repetitions is not known in advance.

WHILE loop — checks the condition before each iteration:

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

In pseudocode:

password = ""
WHILE password != "secret"
    password = INPUT("Enter password: ")
END WHILE
OUTPUT "Access granted"

Worked example — sum numbers until the user enters 0:

total = 0
number = int(input("Enter number (0 to stop): "))
while number != 0:
    total = total + number
    number = int(input("Enter number (0 to stop): "))
print("Total:", total)

If the condition is False on the first check, a WHILE loop body executes zero times. Use a WHILE loop when the number of repetitions depends on user input or data values.

OCR J277 requires knowledge of three categories of operator.

Arithmetic operators — perform calculations:

Operator	Meaning	Example	Result
+	Addition	7 + 3	10
-	Subtraction	7 - 3	4
*	Multiplication	7 * 3	21
/	Division	7 / 2	3.5
MOD	Remainder (modulo)	7 MOD 2	1
DIV	Integer quotient	7 DIV 2	3
^	Exponentiation	2 ^ 8	256

Comparison operators — produce a Boolean result:

== (equal), != (not equal), <, <=, >, >=

Boolean operators — combine Boolean expressions:

Operator	Result is True when...	Example
AND	Both conditions are True	age >= 18 AND hasID == True
OR	At least one condition is True	grade == "A" OR grade == "B"
NOT	The condition is False	NOT gameOver

MOD and DIV are commonly tested. 17 MOD 5 = 2 (remainder); 17 DIV 5 = 3 (whole quotient).

1. Using = instead of == in conditions

= is assignment: x = 5 stores 5 in x.
== is comparison: x == 5 checks whether x equals 5.
Using = in an IF condition is a syntax error in most languages.

2. Confusing MOD and DIV

MOD gives the remainder: 17 MOD 5 = 2 (17 = 3×5 + 2).
DIV gives the integer quotient: 17 DIV 5 = 3 (the whole number part of 17÷5).

3. Off-by-one errors in FOR loops

FOR i = 1 TO 5 runs with i = 1, 2, 3, 4, 5 (five iterations). In Python, range(1, 5) gives 1, 2, 3, 4 (four iterations). Match the loop bounds carefully to the required repetition count.

4. WHILE loop condition never becoming False

A condition-controlled loop must have a route to termination. If the variable tested in the condition is never changed inside the loop, the loop runs forever (infinite loop).

Mistake	Correction
if score = 50	if score == 50
"MOD gives the whole number part"	MOD gives the remainder; DIV gives the whole quotient
WHILE loop that never ends	Ensure the variable in the condition is updated inside the loop