While loops in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a while loop in MATLAB?

A while loop in MATLAB repeats a block of code as long as a given condition is true. It checks the condition before each repetition.

beginner

How do you write a basic while loop in MATLAB?

Use the syntax:

while condition
    % code to repeat
end

The loop runs while the condition is true.

beginner

What happens if the condition in a while loop is initially false?

The code inside the while loop does not run at all because the condition is checked before the first iteration.

intermediate

How can you avoid an infinite while loop in MATLAB?

Make sure the condition will eventually become false by changing variables inside the loop. Otherwise, the loop runs forever.

beginner

Example: What does this MATLAB code print?

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

It prints:
1
2
3
Because the loop runs while i is 1, 2, and 3, then stops when i becomes 4.

What does the while loop check before each repetition?

A The number of times it has run

B The condition to decide if it should run

C The last value printed

D The size of the array

A while loop checks the condition before each repetition to decide if it should continue running.

What happens if the condition in a while loop never becomes false?

A The loop stops immediately

B The program crashes

C The loop runs forever (infinite loop)

D The loop runs once

If the condition never becomes false, the while loop runs forever, causing an infinite loop.

Which keyword ends a while loop in MATLAB?

A end

B stop

C finish

D close

In MATLAB, the keyword 'end' is used to close a while loop block.

What will this code print?
i = 5;
while i < 3
disp(i)
i = i + 1;
end

A 1, 2

B 3, 4, 5

C 5

D Nothing

Since the condition i < 3 is false at the start (i=5), the loop does not run and prints nothing.

How can you make sure a while loop eventually stops?

A Change the condition variable inside the loop

B Use a for loop instead

C Add a break statement outside the loop

D Do not change any variables

To avoid infinite loops, you must change the variable in the condition inside the loop so it eventually becomes false.

Explain how a while loop works in MATLAB and give a simple example.

Think about how you repeat a task until something changes.

Describe how to prevent an infinite loop when using while loops.

What must happen inside the loop to stop it?

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