While loops in MATLAB - Step-by-Step Execution

Concept Flow - While loops

Initialize variable

↓

Check condition

|Yes↓

Execute loop body

↓

Update variable

Back to Check↓

Exit loop

↩Back to Check

The loop starts by initializing a variable, then checks a condition. If true, it runs the loop body, updates the variable, and repeats. If false, it exits.

Execution Sample

MATLAB

count = 1;
while count <= 3
    disp(count);
    count = count + 1;
end

This code prints numbers 1 to 3 using a while loop that runs while count is less than or equal to 3.

Execution Table

Step	count	Condition (count <= 3)	Action	Output
1	1	true	disp(1); count = 2;	1
2	2	true	disp(2); count = 3;	2
3	3	true	disp(3); count = 4;	3
4	4	false	Exit loop

💡 count reaches 4, condition 4 <= 3 is false, loop stops

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
count	1	2	3	4	4

Key Moments - 2 Insights

Why does the loop stop when count is 4 even though the loop body changes count?

The condition is checked before each loop iteration (see execution_table step 4). When count becomes 4, the condition count <= 3 is false, so the loop exits before running the body again.

What happens if we forget to update count inside the loop?

The condition will always be true (count stays 1), causing an infinite loop. The execution_table shows count increasing each step, which is necessary to eventually stop.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of count at step 3?

A 2

B 3

C 4

D 1

💡 Hint

Check the 'count' column in execution_table row for step 3.

At which step does the condition become false and the loop stops?

A Step 3

B Step 2

C Step 4

D Step 1

💡 Hint

Look at the 'Condition' column in execution_table to find when it is false.

If we change the initial count to 4, what will happen to the loop execution?

A The loop will not run at all

B The loop will run once

C The loop will run infinitely

D The loop will run twice

💡 Hint

Refer to variable_tracker start value and condition check in execution_table step 1.

Concept Snapshot

while condition
    % code to repeat
    update variables
end

- The condition is checked before each loop run.
- Loop runs only if condition is true.
- Update variables inside to avoid infinite loops.

Full Transcript

This visual trace shows how a while loop works in MATLAB. We start with count = 1. The loop checks if count <= 3. Since it is true, it prints count and increases it by 1. This repeats until count becomes 4. At that point, the condition is false, so the loop stops. The variable tracker shows count changing from 1 to 4 step by step. Key points: the condition is checked before running the loop body, and updating the variable inside the loop is essential to avoid infinite loops.

Practice

1. What does a while loop do in MATLAB?

easy

A. Repeats a block of code as long as a condition is true

B. Executes code only once

C. Runs code a fixed number of times

D. Stops the program immediately

Solution

1. Step 1: Understand the purpose of a while loop

   A while loop runs repeatedly while its condition remains true.

2. Step 2: Compare options with definition

   Only Repeats a block of code as long as a condition is true correctly describes this behavior.

3. Final Answer:

   Repeats a block of code as long as a condition is true -> Option A

4. Quick Check:

   While loop = repeat while true [OK]

Hint: While loops run while condition is true, not fixed times [OK]

Common Mistakes:

• Thinking while loops run a fixed number of times
• Confusing while with if statement
• Believing while loops run only once

2. Which of the following is the correct syntax for a while loop in MATLAB?

easy

A. while condition
statements
end

B. while (condition) {
statements
}

C. while condition:
statements
end

D. while condition do
statements
end

Solution

1. Step 1: Recall MATLAB while loop syntax

   MATLAB uses 'while condition' followed by statements and ends with 'end'.

2. Step 2: Check each option

   while condition statements end matches MATLAB syntax; others use syntax from other languages.

3. Final Answer:

   while condition\n statements\nend -> Option A

4. Quick Check:

   MATLAB uses 'end' to close loops [OK]

Hint: MATLAB while loops end with 'end', no braces [OK]

Common Mistakes:

• Using braces {} instead of 'end'
• Adding colons after condition
• Using 'do' keyword

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

i = 1;
while i <= 3
  disp(i)
  i = i + 1;
end

medium

A. 1 2 3 4

B. 1 2 3

C. 1 2

D. Error: infinite loop

Solution

1. Step 1: Analyze loop condition and increments

   The loop runs while i is less than or equal to 3, starting at 1 and increasing by 1 each time.

2. Step 2: Trace output values

   Values displayed are 1, then 2, then 3. When i becomes 4, condition fails and loop stops.

3. Final Answer:

   1 2 3 -> Option B

4. Quick Check:

   Loop prints 1 to 3 [OK]

Hint: Increment loop variable to avoid infinite loop [OK]

Common Mistakes:

• Forgetting to increment i causing infinite loop
• Expecting output to include 4
• Confusing loop condition with < instead of <=

4. Identify the error in this MATLAB code snippet:

count = 5;
while count > 0
  disp(count)
end

medium

A. disp function cannot be used inside while

B. Incorrect while syntax

C. Missing increment or decrement inside the loop

D. Variable 'count' not initialized

Solution

1. Step 1: Check loop condition and body

   The loop condition is 'count > 0', but inside the loop count is never changed.

2. Step 2: Understand effect of missing update

   Without changing count, the condition stays true forever, causing an infinite loop.

3. Final Answer:

   Missing increment or decrement inside the loop -> Option C

4. Quick Check:

   Update loop variable to avoid infinite loop [OK]

Hint: Always update loop variable inside while loop [OK]

Common Mistakes:

• Forgetting to change loop variable
• Assuming disp causes error
• Thinking syntax is wrong

5. Write a MATLAB while loop that sums numbers from 1 to 10 and stores the result in total. Which code correctly does this?

hard

A. total = 0;
i = 1;
while i <= 10
total = total + i;
i = i - 1;
end

B. total = 0;
i = 1;
while i < 10
total = total + i;
end

C. total = 0;
for i = 1:10
total = total + i;
end

D. total = 0;
i = 1;
while i <= 10
total = total + i;
i = i + 1;
end

Solution

1. Step 1: Understand the goal and loop condition

   We want to add numbers 1 through 10 using a while loop that stops when i > 10.

2. Step 2: Check each option for correctness

   total = 0; i = 1; while i <= 10 total = total + i; i = i + 1; end initializes total and i, adds i to total, increments i, and stops at 10 correctly. total = 0; i = 1; while i < 10 total = total + i; end misses increment causing infinite loop. total = 0; for i = 1:10 total = total + i; end uses for loop, not while. total = 0; i = 1; while i <= 10 total = total + i; i = i - 1; end decrements i causing infinite loop.

3. Final Answer:

   total = 0;\ni = 1;\nwhile i <= 10\n total = total + i;\n i = i + 1;\nend -> Option D

4. Quick Check:

   Increment i to sum 1 to 10 [OK]

Hint: Increment loop variable to reach end condition [OK]

Common Mistakes:

• Forgetting to increment i
• Using for loop instead of while
• Decrementing i causing infinite loop