If-elseif-else statements in MATLAB - Step-by-Step Execution

Concept Flow - If-elseif-else statements

Start

↓

Check if condition1

|Yes↓

Execute block1

↓

End

No↓

Check if condition2

|Yes↓

Execute block2

↓

End

No↓

Execute else block

↓

End

The program checks conditions in order. It runs the first true block and skips the rest. If none are true, it runs the else block.

Execution Sample

MATLAB

x = 7;
if x < 5
    y = 10;
elseif x < 10
    y = 20;
else
    y = 30;
end

This code sets y based on the value of x using if-elseif-else.

Execution Table

Step	Condition Checked	Condition Result	Action Taken	Variable y Value
1	x < 5	7 < 5 is false	Skip if block	undefined
2	x < 10	7 < 10 is true	Execute elseif block: y = 20	20
3	Else block	Skipped because previous condition true	No action	20

💡 Execution stops after first true condition block runs (elseif block).

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
x	7	7	7	7
y	undefined	undefined	20	20

Key Moments - 2 Insights

Why does the else block not run even though it is last?

Because the elseif condition was true at step 2, the program runs that block and skips the else block, as shown in the execution_table row 3.

What happens if the first if condition is true?

If the first if condition is true, the program runs that block and skips all elseif and else blocks, stopping after step 1.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of y after step 2?

A 10

B 30

C 20

D undefined

💡 Hint

Check the 'Variable y Value' column in row 2 of the execution_table.

At which step does the program decide to skip the else block?

A Step 1

B Step 2

C Step 3

D Never

💡 Hint

Look at the 'Action Taken' column in step 2 where the elseif block runs.

If x was 3, how would the execution_table change at step 1?

A Condition would be true and y would be set to 10

B Condition would be false and y would be set to 20

C Condition would be false and y would be set to 30

D No change

💡 Hint

Check the condition 'x < 5' and what happens when it is true in the execution_table.

Concept Snapshot

If-elseif-else statements check conditions in order.
Syntax:
if condition1
  % code
elseif condition2
  % code
else
  % code
end
Only the first true condition block runs.
If none true, else runs.

Full Transcript

This visual execution shows how if-elseif-else statements work in MATLAB. The program checks the first condition. If false, it checks the next. When it finds a true condition, it runs that block and skips the rest. If none are true, it runs the else block. Variables change only in the executed block. This helps control program flow based on conditions.

Practice

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

easy

A. It creates a variable.

B. It repeats a block of code multiple times.

C. It defines a function.

D. It chooses one action based on conditions.

Solution

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

   The if-elseif-else structure lets the program check conditions one by one and run the code for the first true condition.

2. Step 2: Compare with other options

   Repeating code is done by loops, functions define reusable code, and variables store data, so these are not correct.

3. Final Answer:

   It chooses one action based on conditions. -> Option D

4. Quick Check:

   If-elseif-else = choose action [OK]

Hint: If you see conditions, think if-elseif-else [OK]

Common Mistakes:

• Confusing if-elseif-else with loops
• Thinking it defines functions
• Believing it creates variables

2. Which of the following is the correct syntax to start an if-elseif-else block in MATLAB?

easy

A. if x > 0
elseif x == 0
else
end

B. if x > 0 elseif x == 0 else end

C. if (x > 0) then elseif (x == 0) else end

D. if x > 0; elseif x == 0; else; end

Solution

1. Step 1: Check MATLAB syntax rules

   MATLAB requires each keyword (if, elseif, else, end) on its own line or separated by line breaks, without 'then'.

2. Step 2: Analyze each option

   if x > 0 elseif x == 0 else end uses line breaks correctly. if x > 0 elseif x == 0 else end misses line breaks. if (x > 0) then elseif (x == 0) else end uses 'then' which is invalid. if x > 0; elseif x == 0; else; end uses semicolons incorrectly in control flow.

3. Final Answer:

   if x > 0 elseif x == 0 else end -> Option A

4. Quick Check:

   MATLAB if-elseif-else needs line breaks [OK]

Hint: Use line breaks, no 'then' in MATLAB if statements [OK]

Common Mistakes:

• Using 'then' keyword
• Putting all keywords on one line without breaks
• Using semicolons after if/elseif/else

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

score = 75;
if score >= 90
    grade = 'A';
elseif score >= 80
    grade = 'B';
elseif score >= 70
    grade = 'C';
else
    grade = 'F';
end
disp(grade)

medium

A. C

B. B

C. A

D. F

Solution

1. Step 1: Check conditions in order

   score is 75, first condition score >= 90 is false, second score >= 80 is false, third score >= 70 is true.

2. Step 2: Assign grade and display

   Since third condition is true, grade = 'C' is assigned and displayed.

3. Final Answer:

   C -> Option A

4. Quick Check:

   75 >= 70 -> grade C [OK]

Hint: Check conditions top to bottom, first true wins [OK]

Common Mistakes:

• Choosing grade B or A incorrectly
• Ignoring order of conditions
• Forgetting to use end to close if block

4. Identify the error in this MATLAB code snippet:

value = 10;
if value > 5
    disp('Greater than 5')
elseif value < 5
    disp('Less than 5')
else
    disp('Equal to 5')

medium

A. Missing semicolon after disp statements

B. Missing end to close the if block

C. Incorrect use of elseif instead of else if

D. Using single quotes instead of double quotes

Solution

1. Step 1: Check if block completeness

   The if-elseif-else block must end with an end keyword in MATLAB to close it properly.

2. Step 2: Verify other syntax elements

   Semicolons after disp are optional, elseif is correct keyword, and single quotes are correct for strings in MATLAB.

3. Final Answer:

   Missing end to close the if block -> Option B

4. Quick Check:

   if blocks need end [OK]

Hint: Always close if with end [OK]

Common Mistakes:

• Forgetting end keyword
• Confusing elseif with else if
• Thinking semicolons are mandatory after disp

5. You want to write a MATLAB function that returns:
- 'Positive' if input > 0
- 'Zero' if input == 0
- 'Negative' if input < 0
Which if-elseif-else structure correctly implements this?

hard

A. if x > 0
result = 'Positive';
else if x == 0
result = 'Zero';
else
result = 'Negative';
end

B. if x >= 0
result = 'Positive';
elseif x == 0
result = 'Zero';
else
result = 'Negative';
end

C. if x > 0
result = 'Positive';
elseif x == 0
result = 'Zero';
else
result = 'Negative';
end

D. if x > 0
result = 'Positive';
elseif x > 0
result = 'Zero';
else
result = 'Negative';
end

Solution

1. Step 1: Check condition order and correctness

   if x > 0 result = 'Positive'; elseif x == 0 result = 'Zero'; else result = 'Negative'; end correctly checks x > 0 first, then x == 0, else negative. This covers all cases without overlap.

2. Step 2: Analyze other options for errors

   if x >= 0 result = 'Positive'; elseif x == 0 result = 'Zero'; else result = 'Negative'; end wrongly treats x == 0 as Positive. if x > 0 result = 'Positive'; else if x == 0 result = 'Zero'; else result = 'Negative'; end uses invalid 'else if' instead of 'elseif'. if x > 0 result = 'Positive'; elseif x > 0 result = 'Zero'; else result = 'Negative'; end repeats x > 0 condition twice, missing zero case.

3. Final Answer:

   if x > 0 result = 'Positive'; elseif x == 0 result = 'Zero'; else result = 'Negative'; end -> Option C

4. Quick Check:

   Check conditions in order, use elseif [OK]

Hint: Use elseif, not else if; order conditions carefully [OK]

Common Mistakes:

• Using else if instead of elseif
• Overlapping conditions
• Wrong order causing incorrect results