Assignment and augmented assignment in Python - Time & Space Complexity

Time Complexity: Assignment and augmented assignment

O(n)

Understanding Time Complexity

Let's see how the time it takes to run code with assignment and augmented assignment changes as we work with bigger numbers.

We want to know how the number of steps grows when we do simple assignments or add-and-assign repeatedly.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

total = 0
for i in range(n):
    total += i

This code adds numbers from 0 up to n-1 into a total using augmented assignment.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The addition and assignment inside the loop (total += i).
• How many times: This happens once for each number from 0 to n-1, so n times.

How Execution Grows With Input

Each time we increase n, the loop runs more times, doing one addition and assignment each time.

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

Pattern observation: The number of steps grows directly with n; if n doubles, the work doubles.

Final Time Complexity

Time Complexity: O(n)

This means the time to finish grows in a straight line with the size of n.

Common Mistake

[X] Wrong: "Augmented assignment like total += i is faster and takes constant time regardless of n."

[OK] Correct: Even though augmented assignment looks simple, it happens inside a loop that runs n times, so total steps grow with n.

Interview Connect

Understanding how simple loops with assignments grow helps you explain how programs handle bigger data smoothly and clearly.

Self-Check

"What if we replaced the loop with a single assignment like total = n * (n - 1) // 2? How would the time complexity change?"

Practice

1.

What does the += operator do in Python?

easy

A. Adds the right value to the left variable and updates it

B. Assigns a new variable without changing the old one

C. Subtracts the right value from the left variable

D. Creates a new list from two variables

Solution

1. Step 1: Understand the meaning of +=

   The += operator adds the value on the right to the variable on the left.

2. Step 2: Recognize it updates the variable

   It changes the original variable by adding the new value to it.

3. Final Answer:

   Adds the right value to the left variable and updates it -> Option A

4. Quick Check:

   += means add and assign [OK]

Hint: Think: add then store back in same variable [OK]

Common Mistakes:

• Confusing += with simple assignment =
• Thinking += creates a new variable
• Mixing += with subtraction or multiplication

2.

Which of the following is the correct syntax to add 5 to variable x using augmented assignment?

x = 10
# Add 5 to x here

easy

A. x =+ 5

B. x += 5

C. x =+5

D. x = x +

Solution

1. Step 1: Identify correct augmented assignment syntax

   The correct syntax to add and assign is x += 5.

2. Step 2: Check other options for syntax errors

   Choices like x =+ 5 and x =+5 use incorrect operator placement; x = x + is incomplete.

3. Final Answer:

   x += 5 -> Option B

4. Quick Check:

   Use += for adding and assigning [OK]

Hint: Remember: operator before equals for augmented assignment [OK]

Common Mistakes:

• Writing =+ instead of +=
• Leaving out the value after +
• Using incomplete expressions

3.

What is the output of this code?

count = 3
count += 4
count -= 2
print(count)

medium

A. 9

B. 7

C. 3

D. 5

Solution

1. Step 1: Calculate count += 4

   Starting from 3, adding 4 gives 7.

2. Step 2: Calculate count -= 2

   Subtracting 2 from 7 gives 5.

3. Final Answer:

   5 -> Option D

4. Quick Check:

   3 + 4 - 2 = 5 [OK]

Hint: Follow each operation step-by-step [OK]

Common Mistakes:

• Adding and subtracting in wrong order
• Confusing += and -= effects
• Ignoring the initial value

4.

Find the error in this code snippet:

total = 10
total + = 5
print(total)

medium

A. TypeError because + cannot be used with int

B. NameError because total is not defined

C. SyntaxError due to space in augmented assignment

D. No error, prints 15

Solution

1. Step 1: Check augmented assignment syntax

   The operator += must not have spaces between + and =.

2. Step 2: Identify the error caused by space

   Writing + = causes a SyntaxError in Python.

3. Final Answer:

   SyntaxError due to space in augmented assignment -> Option C

4. Quick Check:

   No spaces allowed in += operator [OK]

Hint: No spaces between + and = in augmented assignment [OK]

Common Mistakes:

• Adding spaces inside augmented assignment operator
• Assuming it works like normal addition
• Ignoring syntax error messages

5.

You have a variable score = 50. You want to increase it by 10%, then subtract 5 using augmented assignments. Which code correctly does this?

score = 50
# Increase by 10%
# Subtract 5

hard

A. score *= 1.1
score -= 5

B. score += 10
score -= 5

C. score = score + 10%
score = score - 5

D. score += 0.1
score -= 5

Solution

1. Step 1: Increase score by 10%

   Multiplying by 1.1 increases the value by 10%, so score *= 1.1 is correct.

2. Step 2: Subtract 5 using augmented assignment

   Use score -= 5 to subtract 5 from the updated score.

3. Final Answer:

   score *= 1.1 score -= 5 -> Option A

4. Quick Check:

   Multiply by 1.1 then subtract 5 [OK]

Hint: Use *= for percent increase, then -= for subtraction [OK]

Common Mistakes:

• Adding 10 instead of 10%
• Using invalid syntax like 10%
• Not updating score before subtracting