If-else execution flow in PHP - Time & Space Complexity

Time Complexity: If-else execution flow

O(1)

Understanding Time Complexity

We want to understand how the time it takes to run if-else statements changes as the input changes.

Specifically, we ask: does adding more conditions or inputs make the program slower?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function checkNumber($num) {
    if ($num > 0) {
        return "Positive";
    } elseif ($num < 0) {
        return "Negative";
    } else {
        return "Zero";
    }
}
    

This code checks if a number is positive, negative, or zero and returns a string accordingly.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Single if-else checks (no loops or recursion)
• How many times: Each condition is checked once per function call

How Execution Grows With Input

The time to run this code stays about the same no matter what number you give it.

Input Size (n)	Approx. Operations
10	3 checks
100	3 checks
1000	3 checks

Pattern observation: The number of checks does not grow with input size; it stays constant.

Final Time Complexity

Time Complexity: O(1)

This means the time to run the code stays the same no matter how big the input number is.

Common Mistake

[X] Wrong: "If I give a bigger number, the code will take longer because it has to check more things."

[OK] Correct: The code only checks fixed conditions once, so bigger numbers don't add more checks.

Interview Connect

Understanding that simple if-else checks run in constant time helps you explain how decisions in code affect speed.

Self-Check

"What if we added a loop that runs the if-else check multiple times? How would the time complexity change?"

Practice

1. What does an if-else statement do in PHP?

easy

A. It repeats code multiple times.

B. It chooses one block of code to run based on a condition.

C. It runs all blocks of code regardless of conditions.

D. It defines a function.

Solution

1. Step 1: Understand the purpose of if-else

   An if-else statement checks a condition and runs code based on whether the condition is true or false.

2. Step 2: Compare options with definition

   Only "It chooses one block of code to run based on a condition." correctly describes this behavior; others describe loops or functions.

3. Final Answer:

   It chooses one block of code to run based on a condition. -> Option B

4. Quick Check:

   If-else = choose one path [OK]

Hint: If-else picks one path based on true or false [OK]

Common Mistakes:

• Thinking if-else runs all code blocks
• Confusing if-else with loops
• Believing if-else defines functions

2. Which of the following is the correct syntax for an if-else statement in PHP?

easy

A. if ($x > 10) { echo 'Yes' } else { echo 'No' }

B. if $x > 10 then echo 'Yes' else echo 'No'

C. if ($x > 10) echo 'Yes' else echo 'No'

D. if ($x > 10) { echo 'Yes'; } else { echo 'No'; }

Solution

1. Step 1: Check PHP if-else syntax rules

   PHP requires parentheses around the condition and braces around the code blocks.

2. Step 2: Identify correct syntax

   if ($x > 10) { echo 'Yes'; } else { echo 'No'; } uses parentheses and braces correctly. if $x > 10 then echo 'Yes' else echo 'No' uses invalid keywords. if ($x > 10) echo 'Yes' else echo 'No' misses braces which is allowed but needs semicolons. if ($x > 10) { echo 'Yes' } else { echo 'No' } misses semicolons inside braces.

3. Final Answer:

   if ($x > 10) { echo 'Yes'; } else { echo 'No'; } -> Option D

4. Quick Check:

   Correct syntax = if ($x > 10) { echo 'Yes'; } else { echo 'No'; } [OK]

Hint: Use parentheses and braces with semicolons in PHP if-else [OK]

Common Mistakes:

• Omitting parentheses around condition
• Missing semicolons after statements
• Using invalid keywords like 'then'

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

$score = 75;
if ($score >= 60) {
    echo "Pass";
} else {
    echo "Fail";
}

medium

A. Pass

B. Fail

C. 60

D. No output

Solution

1. Step 1: Evaluate the condition $score >= 60

   The variable $score is 75, which is greater than or equal to 60, so the condition is true.

2. Step 2: Determine which block runs

   Since the condition is true, the code inside the if block runs, printing "Pass".

3. Final Answer:

   Pass -> Option A

4. Quick Check:

   75 >= 60 is true, so output = Pass [OK]

Hint: Check if condition is true to pick if or else output [OK]

Common Mistakes:

• Confusing true and false branches
• Ignoring comparison operators
• Expecting numeric output instead of string

4. Find the error in this PHP code snippet:

if ($age > 18)
    echo "Adult";
else
    echo "Minor"

medium

A. Missing parentheses around condition

B. Missing braces around if and else blocks

C. Missing semicolon after echo "Minor"

D. Incorrect comparison operator

Solution

1. Step 1: Check syntax for echo statements

   Each echo statement must end with a semicolon in PHP.

2. Step 2: Identify missing semicolon

   The echo "Minor" line is missing a semicolon at the end, causing a syntax error.

3. Final Answer:

   Missing semicolon after echo "Minor" -> Option C

4. Quick Check:

   Every statement needs semicolon [OK]

Hint: Always end echo statements with semicolon [OK]

Common Mistakes:

• Forgetting semicolon after last statement
• Thinking braces are always required
• Misreading comparison operators

5. You want to write a PHP program that prints "Good morning" if the hour is less than 12, "Good afternoon" if the hour is between 12 and 17 (inclusive), and "Good evening" otherwise. Which code correctly implements this using if-else statements?

hard

A. if ($hour < 12) {
echo "Good morning";
} else if ($hour <= 17) {
echo "Good afternoon";
} else {
echo "Good evening";
}

B. if ($hour < 12) {
echo "Good morning";
} else if ($hour > 12 && $hour < 17) {
echo "Good afternoon";
} else {
echo "Good evening";
}

C. if ($hour <= 12) {
echo "Good morning";
} else if ($hour >= 12 && $hour <= 17) {
echo "Good afternoon";
} else {
echo "Good evening";
}

D. if ($hour < 12) {
echo "Good morning";
} else if ($hour >= 12 && $hour < 17) {
echo "Good afternoon";
} else {
echo "Good evening";
}

Solution

1. Step 1: Analyze the conditions for each greeting

   "Good morning" if hour < 12, "Good afternoon" if hour between 12 and 17 inclusive, else "Good evening".

2. Step 2: Check each option's condition ranges

   if ($hour < 12) { echo "Good morning"; } else if ($hour <= 17) { echo "Good afternoon"; } else { echo "Good evening"; } correctly uses $hour < 12, then $hour <= 17 for afternoon, covering 12 to 17 inclusive. Others either exclude 12 or 17 or overlap incorrectly.

3. Final Answer:

   if ($hour < 12) { echo "Good morning"; } else if ($hour <= 17) { echo "Good afternoon"; } else { echo "Good evening"; } -> Option A

4. Quick Check:

   Correct ranges cover all hours without gaps [OK]

Hint: Check boundary conditions carefully in if-else chains [OK]

Common Mistakes:

• Using wrong comparison operators for boundaries
• Overlapping or missing hour ranges
• Confusing inclusive and exclusive conditions