Variable assignment in Python - Time & Space Complexity

Time Complexity: Variable assignment in Python

O(n)

Understanding Time Complexity

Let's see how fast variable assignment happens in Python.

We want to know how the time to assign a value changes as we assign more variables.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

for i in range(n):
    x = i

This code assigns the value of i to x repeatedly in a loop.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Variable assignment x = i inside the loop.
• How many times: Exactly n times, once per loop cycle.

How Execution Grows With Input

Each time we increase n, the number of assignments grows the same amount.

Input Size (n)	Approx. Operations
10	10 assignments
100	100 assignments
1000	1000 assignments

Pattern observation: The number of assignments grows directly with n.

Final Time Complexity

Time Complexity: O(n)

This means the time to finish grows in a straight line as you assign more variables.

Common Mistake

[X] Wrong: "Variable assignment is always instant and does not depend on how many times it runs."

[OK] Correct: While one assignment is very fast, doing it many times adds up, so total time grows with the number of assignments.

Interview Connect

Understanding how simple steps add up helps you explain code speed clearly and confidently.

Self-Check

"What if we assigned a list of size n to x inside the loop? How would the time complexity change?"

Practice

1. What does the following Python statement do?
age = 25

easy

A. Prints the value 25

B. Checks if age is equal to 25

C. Stores the value 25 in the variable named age

D. Deletes the variable age

Solution

1. Step 1: Understand the assignment operator

   The = sign in Python assigns the value on the right to the variable on the left.

2. Step 2: Analyze the statement age = 25

   This means the number 25 is stored inside the variable named age.

3. Final Answer:

   Stores the value 25 in the variable named age -> Option C

4. Quick Check:

   Assignment means storing value [OK]

Hint: Remember: '=' means store value, not compare [OK]

Common Mistakes:

• Confusing '=' with '==' for comparison
• Thinking '=' prints or deletes variables
• Assuming variable names are values

2. Which of the following is the correct way to assign the value 10 to a variable named count in Python?

easy

A. count == 10

B. count := 10

C. 10 = count

D. count = 10

Solution

1. Step 1: Identify the assignment operator

   In Python, the single equals sign = is used to assign values to variables.

2. Step 2: Check each option

   count == 10 uses == which is a comparison, not assignment. 10 = count tries to assign to a value, which is invalid. count := 10 uses walrus operator := which is valid in Python 3.8+, but not standard for simple assignment. count = 10 correctly assigns 10 to count.

3. Final Answer:

   count = 10 -> Option D

4. Quick Check:

   Use '=' for assignment [OK]

Hint: Use single '=' to assign values, not '==' [OK]

Common Mistakes:

• Using '==' instead of '=' for assignment
• Trying to assign value to a number
• Confusing walrus operator with assignment

3. What will be the output of this code?

name = "Alice"
age = 30
name = "Bob"
print(name, age)

medium

A. Alice 30

B. Bob 30

C. Alice 0

D. Bob 0

Solution

1. Step 1: Trace variable assignments

   Initially, name is set to "Alice" and age to 30. Then name is reassigned to "Bob".

2. Step 2: Understand print output

   The print statement outputs the current values of name and age. Since name was changed to "Bob", and age remains 30, the output is "Bob 30".

3. Final Answer:

   Bob 30 -> Option B

4. Quick Check:

   Last assignment wins [OK]

Hint: Last value assigned to variable is what prints [OK]

Common Mistakes:

• Thinking first assignment stays unchanged
• Confusing variable names or values
• Assuming age changes without assignment

4. Identify the error in this code snippet:

5number = 10
print(5number)

medium

A. Variable names cannot start with a number

B. Missing quotes around 5number

C. Assignment operator is wrong

D. print statement syntax error

Solution

1. Step 1: Check variable naming rules

   In Python, variable names must start with a letter or underscore, not a number.

2. Step 2: Analyze the code

   The variable name 5number starts with a digit, which is invalid syntax and causes an error.

3. Final Answer:

   Variable names cannot start with a number -> Option A

4. Quick Check:

   Variable names start with letter or underscore [OK]

Hint: Variable names cannot begin with digits [OK]

Common Mistakes:

• Starting variable names with numbers
• Confusing strings with variable names
• Assuming print syntax is wrong

5. You want to swap the values of two variables a and b in Python. Which of the following is the correct way to do it?

hard

A. a, b = b, a

B. a = b
b = a

C. temp = a
b = temp
a = b

D. a = a + b
b = a - b
a = a - b

Solution

1. Step 1: Understand swapping variables

   Swapping means exchanging values so that a gets b's value and b gets a's value.

2. Step 2: Analyze each option

   a = b b = a assigns b to a, then a to b, losing original a. temp = a b = temp a = b incorrectly assigns b before a. a = a + b b = a - b a = a - b uses arithmetic swap which works but is less clear and can cause errors with non-numbers. a, b = b, a uses Python's tuple unpacking to swap values correctly and clearly.

3. Final Answer:

   a, b = b, a -> Option A

4. Quick Check:

   Use tuple unpacking for swap [OK]

Hint: Swap with a, b = b, a for clean code [OK]

Common Mistakes:

• Overwriting variables before swapping
• Using temporary variable incorrectly
• Using arithmetic swap with non-numbers