Data Types and Collections

A program is only useful if it represents data correctly. A mark, a name, a password, a menu choice, and a truth value should not all be treated the same way. The data type determines what operations are valid and what mistakes are likely.

Values, Variables, and Assignment

A variable is a name that refers to a value.

student_name = "Aisha"
mark = 76
has_submitted = True

The assignment operator = stores a value under a name. It does not mean mathematical equality.

Caption: Assignment evaluates the expression on the right first, then stores the result under the name on the left.

mark = mark + 5

This means: take the current value of mark, add 5, then store the new value back into mark.

When tracing assignment, keep a small state table:

Statement	Value of mark after statement
mark = 76	76
mark = mark + 5	81

This habit is more reliable than trying to remember the values mentally.

Common Data Types

Conceptual type	Python example	Notes
integer	12	whole numbers, indices, counts
real	12.5	Python uses float for real-valued approximations
char	"A"	Python has no separate char type; use a one-character string
string	"S1234567A"	text; can contain digits that should not be calculated
Boolean	True	condition result or flag

Use type() when learning or debugging:

value = "45"
print(type(value))

This prints <class 'str'>, so arithmetic cannot be done safely until conversion.

Type Conversion

Python may convert values automatically in some mixed numeric expressions, but beginner code should usually make conversions explicit.

whole_number = int("42")
measurement = float("3.5")
label = str(108)

int() converts a suitable value to an integer, float() converts it to a real-valued approximation, and str() creates text. Conversion may fail when the source text is unsuitable, such as int("four").

Input Is a String

Python’s input() function always returns a string.

raw_age = input("Age: ")
age = int(raw_age)

If the user types 17, raw_age is "17", not 17. Convert only when the input is expected to behave like a number.

price = float(input("Price: "))
quantity = int(input("Quantity: "))
total = price * quantity
print(total)

Common bug:

age = input("Age: ")
print(age + 1)  # error: age is a string

Correct version:

age = int(input("Age: "))
print(age + 1)

Operators

Arithmetic operators include:

Operator	Meaning	Example
+	addition	2 + 3 gives 5
-	subtraction	7 - 3 gives 4
*	multiplication	2 * 3 gives 6
/	real division	20 / 8 gives 2.5
//	integer division	20 // 8 gives 2
%	remainder	20 % 8 gives 4
**	exponentiation	2 ** 3 gives 8

The % operator is often used to test divisibility:

if number % 2 == 0:
    print("even")

Characters and Strings

The syllabus distinguishes a character from a string. Python does not have a separate char type, so a character is represented by a string of length 1.

initial = "A"
name = "Aisha"

A string is a sequence of zero or more characters.

name = "computing"
print(name[0])
print(name[1:4])

Indexing starts at 0. For "computing", name[0] is "c".

Strings are immutable. This means an existing string cannot be changed in place.

word = "cat"
new_word = "b" + word[1:]

The original word remains "cat". The expression creates a new string "bat".

Useful string operations include:

message = "  Hello  "
clean = message.strip()
lowercase = clean.lower()
position = clean.find("ll")

Common methods include lower(), upper(), strip(), find(), count(), startswith(), and endswith(). String methods return new strings or other results; they do not change the original string directly.

ASCII and Unicode allow characters to be stored as numbers. Python provides:

code = ord("A")      # 65
character = chr(66)  # "B"

Use ord() when a program needs the code point of one character, and chr() to convert a valid code point back to a character.

Library Functions

The syllabus expects use of common library functions for input/output, strings, and mathematical operations. Some useful built-in functions are len(), abs(), round(), min(), and max().

values = [4, 9, 2]
print(len(values))
print(max(values))

Functions from a module must be imported before use:

import math
 
radius = 3
area = math.pi * radius ** 2

Lists as 1D Arrays

The syllabus refers to one-dimensional arrays. In Python teaching code, lists are normally used for this role.

scores = [72, 85, 68]
scores.append(91)
scores[0] = 75

Important ideas:

• index starts at 0;
• len(scores) gives the number of items;
• scores[-1] gives the last item;
• lists are mutable, so an item can be replaced.

Example:

scores = [72, 85, 68]
total = 0
 
for score in scores:
    total = total + score
 
average = total / len(scores)
print(average)

Nested Lists as 2D Arrays

A two-dimensional array can be represented as a list of lists.

Caption: In a 2D list, the first index selects the row; the second index selects the item within that row.

marks = [
    [82, 74, 91],
    [65, 88, 79],
]
 
print(marks[1][1])

This prints 88. The first index 1 selects the second row. The second index 1 selects the second item in that row.

Traversing Collections

A list can be processed by value:

for score in scores:
    print(score)

or by index:

for index in range(len(scores)):
    print(index, scores[index])

Use direct traversal when only the values are needed. Use index-based traversal when the position is also needed or when an item must be replaced.

For a 2D list, nested loops visit rows and columns:

for row in marks:
    for value in row:
        print(value)

Choosing the Right Representation

Use a number when the value will be calculated.

mark = 75

Use a string when the value is text or an identifier where the digits are not meant for arithmetic.

student_id = "S1234567A"

Use a Boolean when the value answers a yes/no question.

is_valid = mark >= 0 and mark <= 100

Use a list when there are many values with the same role.

temperatures = [28.1, 29.4, 30.2]

Common Bugs

• Forgetting that input() gives a string.
• Confusing = assignment with == comparison.
• Using an index that is outside the list.
• Forgetting that strings cannot be changed in place.
• Mixing integer division // and real division /.
• Storing numeric-looking identifiers as integers, which may remove leading zeroes.

Practice Trace

Trace this code:

values = [3, 5, 7]
values[1] = values[0] + values[2]
print(values)

After the assignment, values[1] becomes 3 + 7, so the list is [3, 10, 7].

Check Your Understanding

Try these before looking back at the explanations:

1. What is the type of the value returned by input()?
2. Why is "0123" usually stored as a string instead of an integer?
3. If scores = [4, 6, 8], what is scores[0] + scores[2]?
4. In grid[2][0], which index selects the row?

Answers:

1. str.
2. The leading zero is part of the identifier; converting to an integer would lose it.
3. 12.
4. The first index, 2, selects the row.

Related Notes

• Programming Fundamentals
• Control Flow and Functions