Assignment and compound assignment in PHP - Time & Space Complexity

Time Complexity: Assignment and compound assignment

O(n)

Understanding Time Complexity

Let's see how assignment and compound assignment affect the time it takes for a program to run.

We want to know how the number of steps changes when we assign or update values in variables.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

$sum = 0;
for ($i = 0; $i < $n; $i++) {
    $sum += $i;
}
echo $sum;
    

This code adds numbers from 0 up to n-1 using a compound assignment inside a loop.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The addition and compound assignment $sum += $i; inside the loop.
• How many times: This operation runs once for each number from 0 to n-1, so n times.

How Execution Grows With Input

As n grows, the number of additions grows the same way.

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

Pattern observation: The work grows directly with n; doubling n doubles the work.

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: "Compound assignment makes the loop run faster or slower than simple assignment."

[OK] Correct: Compound assignment is just a shorter way to write an operation plus assignment; it does not change how many times the loop runs or how the time grows.

Interview Connect

Understanding how simple assignments inside loops affect time helps you explain code efficiency clearly and confidently.

Self-Check

"What if we replaced the loop with a function that adds numbers recursively? How would the time complexity change?"

Practice

1. What does the += operator do in PHP?

easy

A. Assigns the right value to the left variable without any operation

B. Multiplies the left variable by the right value

C. Subtracts the right value from the left variable

D. Adds the right value to the left variable and assigns the result to the left variable

Solution

1. Step 1: Understand the += operator

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

2. Step 2: Assign the result back to the left variable

   It updates the left variable with the new sum.

3. Final Answer:

   Adds the right value to the left variable and assigns the result to the left variable -> Option D

4. Quick Check:

   += means add and assign [OK]

Hint: Think: add right to left and save in left [OK]

Common Mistakes:

• Confusing += with simple assignment =
• Thinking it subtracts or multiplies
• Believing it only adds without assignment

2. Which of the following is the correct syntax to multiply a variable $x by 5 and update it in PHP?

easy

A. $x *= 5;

B. $x =* 5;

C. $x = *5;

D. $x = 5*;

Solution

1. Step 1: Identify the correct compound assignment syntax

   The correct syntax for multiplying and assigning is $x *= 5; with no spaces between * and =.

2. Step 2: Check other options for syntax errors

   The other options have incorrect placement of operators or missing operands.

3. Final Answer:

   $x *= 5; -> Option A

4. Quick Check:

   Use *= for multiply and assign [OK]

Hint: Remember no space: variable *= value [OK]

Common Mistakes:

• Writing =* instead of *= operator
• Adding spaces between * and =
• Placing * or = incorrectly

3. What will be the output of the following PHP code?

$a = 10;
$a += 5;
$a -= 3;
echo $a;

medium

A. 10

B. 12

C. 8

D. 15

Solution

1. Step 1: Calculate after first operation

   Initially, $a = 10. After $a += 5;, $a becomes 10 + 5 = 15.

2. Step 2: Calculate after second operation

   Then $a -= 3; subtracts 3: 15 - 3 = 12.

3. Final Answer:

   12 -> Option B

4. Quick Check:

   10 + 5 - 3 = 12 [OK]

Hint: Perform operations step-by-step from top to bottom [OK]

Common Mistakes:

• Ignoring the order of operations
• Adding instead of subtracting in second step
• Confusing initial value with final

4. Identify the error in the following PHP code:

$count = 5;
$count +== 3;
echo $count;

medium

A. Variable name is invalid

B. Missing semicolon after first line

C. Using +== instead of +=

D. echo statement syntax error

Solution

1. Step 1: Check the compound assignment operator

   The operator +== is invalid in PHP. The correct operator to add and assign is +=.

2. Step 2: Verify other parts of code

   Semicolons are present, variable name is valid, and echo syntax is correct.

3. Final Answer:

   Using +== instead of += -> Option C

4. Quick Check:

   Compound assignment must be +=, not +== [OK]

Hint: Remember compound operators have one = only [OK]

Common Mistakes:

• Typing two equal signs in compound assignment
• Confusing comparison (==) with assignment (=)
• Assuming +== is valid operator

5. Given the code below, what will be the value of $result after execution?

$x = 4;
$y = 3;
$result = 10;
$result += $x * $y;
$result -= 5;
$result /= 2;

hard

A. 8.5

B. 6.5

C. 10

D. 9

Solution

1. Step 1: Calculate multiplication and addition

   $x * $y = 4 * 3 = 12. Then $result += 12 means 10 + 12 = 22.

2. Step 2: Subtract 5 from $result

   22 - 5 = 17.

3. Step 3: Divide $result by 2

   17 / 2 = 8.5.

4. Step 4: Check options carefully

   8.5 is 8.5, but note the question asks for final value of $result after all operations.

5. Final Answer:

   8.5 -> Option A

6. Quick Check:

   (10 + 4*3 - 5) / 2 = 8.5 [OK]

Hint: Follow order: multiply, add, subtract, divide [OK]

Common Mistakes:

• Ignoring operator precedence
• Doing operations in wrong order
• Dividing before subtracting