Do-while loop execution model in PHP - Time & Space Complexity

Time Complexity: Do-while loop execution model

O(n)

Understanding Time Complexity

We want to understand how the time a do-while loop takes changes as the input size grows.

Specifically, how many times does the loop run when the input gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

    $i = 0;
    do {
        echo $i . "\n";
        $i++;
    } while ($i < $n);
    

This code prints numbers from 0 up to one less than n using a do-while loop.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The do-while loop that prints and increments the counter.
• How many times: It runs exactly n times, once for each number from 0 to n-1.

How Execution Grows With Input

As n grows, the number of loop runs grows the same way.

Input Size (n)	Approx. Operations
10	10 loop runs
100	100 loop runs
1000	1000 loop runs

Pattern observation: The number of operations grows directly with n, so doubling n doubles the work.

Final Time Complexity

Time Complexity: O(n)

This means the time taken grows in a straight line with the input size.

Common Mistake

[X] Wrong: "The do-while loop runs one more time than n because it checks after the first run."

[OK] Correct: The do-while loop runs exactly n times here, not more or less, because the condition is checked after each iteration and the loop stops when $i is no longer less than n.

Interview Connect

Understanding how loops like do-while behave helps you explain how your code scales and shows you can think about performance clearly.

Self-Check

"What if we changed the loop to run while $i <= $n instead of $i < $n? How would the time complexity change?"

Practice

1. What is the main difference between a do-while loop and a while loop in PHP?

easy

A. while loop always executes the code block at least once before checking the condition.

B. do-while loop executes the code block at least once before checking the condition.

C. do-while loop never checks the condition.

D. while loop executes the code block only if the condition is false.

Solution

1. Step 1: Understand the execution order of do-while loop

   A do-while loop runs the code block first, then checks the condition to decide if it should repeat.

2. Step 2: Compare with while loop behavior

   A while loop checks the condition first before running the code block, so it may not run at all if the condition is false initially.

3. Final Answer:

   do-while loop executes the code block at least once before checking the condition. -> Option B

4. Quick Check:

   do-while runs code once before condition check [OK]

Hint: Remember: do-while runs code first, then checks condition [OK]

Common Mistakes:

• Thinking while loop runs code before condition check
• Believing do-while skips condition check
• Confusing execution order of loops

2. Which of the following is the correct syntax for a do-while loop in PHP?

easy

A. do { /* code */ } while (condition);

B. do (condition) { /* code */ };

C. while (condition) { /* code */ } do;

D. do { /* code */ } until (condition);

Solution

1. Step 1: Recall PHP do-while syntax

   The correct syntax is: do { /* code */ } while (condition); with a semicolon after the while condition.

2. Step 2: Identify incorrect options

   Options A, C, and D use wrong keywords or misplaced parentheses, which are invalid in PHP.

3. Final Answer:

   do { /* code */ } while (condition); -> Option A

4. Quick Check:

   Correct do-while syntax ends with semicolon [OK]

Hint: Syntax: do { } while(condition); ends with semicolon [OK]

Common Mistakes:

• Omitting semicolon after while condition
• Using 'until' instead of 'while'
• Placing condition before code block

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

$i = 3;
do {
    echo $i . ' ';
    $i--;
} while ($i > 0);

medium

A. 3 2 1

B. 2 1 0

C. 3 2 1 0

D. 1 2 3

Solution

1. Step 1: Trace the loop iterations

   Initial value: $i = 3. Loop prints $i then decrements it. Iterations:
   1) print 3, $i=2
   2) print 2, $i=1
   3) print 1, $i=0

2. Step 2: Check loop condition after each iteration

   Loop continues while $i > 0. After printing 1, $i becomes 0, condition fails, loop stops.

3. Final Answer:

   3 2 1 -> Option A

4. Quick Check:

   Loop prints 3,2,1 then stops [OK]

Hint: do-while runs once before condition; count down from 3 [OK]

Common Mistakes:

• Including 0 in output
• Starting output from 1 instead of 3
• Confusing pre- and post-decrement

4. Identify the error in this PHP do-while loop:

$count = 1;
do {
    echo $count;
    $count++
} while ($count <= 3);

medium

A. Using do instead of while

B. Incorrect loop condition syntax

C. Missing semicolon after $count++

D. Variable $count not initialized

Solution

1. Step 1: Check each line for syntax errors

   The line $count++ is missing a semicolon at the end, which is required in PHP.

2. Step 2: Verify other parts of the loop

   The loop condition and variable initialization are correct. The use of do is proper for a do-while loop.

3. Final Answer:

   Missing semicolon after $count++ -> Option C

4. Quick Check:

   Every statement must end with semicolon [OK]

Hint: Check for missing semicolons inside loop [OK]

Common Mistakes:

• Forgetting semicolon after statements
• Misplacing loop condition parentheses
• Confusing do-while with while syntax

5. You want to read user input at least once and continue reading while the input is not empty. Which PHP do-while loop correctly implements this?

do {
    $input = trim(fgets(STDIN));
    // process input
} ???;

hard

A. while ($input !== '');

B. while ($input);

C. while ($input == '');

D. while (!empty($input));

Solution

1. Step 1: Understand the input condition

   We want to continue reading while input is not empty. The PHP function empty() returns true if input is empty.

2. Step 2: Choose the correct condition

   Using while (!empty($input)); means loop continues while input has some content, stopping when empty.

3. Final Answer:

   while (!empty($input)); -> Option D

4. Quick Check:

   Loop continues while input is not empty [OK]

Hint: Use while(!empty($input)) to loop until input is empty [OK]

Common Mistakes:

• Using equality to empty string incorrectly
• Using condition that stops loop too early
• Confusing empty() with isset()