Converting between types in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of converting numeric array to logical

What is the output of this MATLAB code?

a = [0 1 2 -3 0];
b = logical(a);
disp(b);

MATLAB

a = [0 1 2 -3 0];
b = logical(a);
disp(b);

A [0 0 0 0 0]

B [false true true true false]

C [false 1 1 1 false]

D [0 1 1 1 0]

💡 Hint

Logical conversion treats zero as false and nonzero as true.

In MATLAB, logical conversion changes zeros to false and any nonzero number to true. Displaying a logical array shows 'false' and 'true'.

❓ Predict Output

intermediate

Result of char to double conversion

What is the output of this MATLAB code?

c = 'MATLAB';
d = double(c);
disp(d);

MATLAB

c = 'MATLAB';
d = double(c);
disp(d);

A [77 65 84 76 65 66]

B ['M' 'A' 'T' 'L' 'A' 'B']

C [109 97 116 108 97 98]

D [0 0 0 0 0 0]

💡 Hint

double converts characters to their ASCII numeric codes.

The double function converts each character to its ASCII code. 'M' is 77, 'A' is 65, etc.

❓ Predict Output

advanced

Output of converting numeric to string and back

What is the value of variable y after running this MATLAB code?

x = 123.45;
s = string(x);
y = double(s);

MATLAB

x = 123.45;
s = string(x);
y = double(s);

A NaN

B Error: Cannot convert string to double directly

C [49 50 51 46 52 53]

D 123.45

💡 Hint

double applied to a string array returns NaN if string is not numeric array.

The string '123.45' cannot be directly converted to a numeric double array by double(). Instead, double(string) returns NaN. To convert string to number, use str2double.

❓ Predict Output

advanced

Effect of converting logical array to double

What is the output of this MATLAB code?

l = [true false true];
d = double(l);
disp(d);

MATLAB

l = [true false true];
d = double(l);
disp(d);

A [2 1 2]

B [true false true]

C [0 0 0]

D [1 0 1]

💡 Hint

Logical true converts to 1, false to 0 when cast to double.

Converting logical true to double gives 1, false gives 0. So the output is [1 0 1].

❓ Predict Output

expert

Output of converting cell array of mixed types to numeric array

What error or output does this MATLAB code produce?

c = {1, '2', 3};
a = cell2mat(c);

MATLAB

c = {1, '2', 3};
a = cell2mat(c);

A [1 2 3]

B [1 '2' 3]

C Error: cell2mat cannot convert cell array with mixed types

D [1 0 3]

💡 Hint

cell2mat requires all cells to be numeric arrays of compatible sizes.

cell2mat fails if the cell array contains mixed types like numeric and char. It throws an error.

Practice

1. Which MATLAB function converts a string like '123.45' into a numeric double value?

easy

A. char

B. int32

C. str2double

D. num2str

Solution

1. Step 1: Understand the input and desired output

   The input is a string representing a number, and we want a numeric double value.

2. Step 2: Identify the correct conversion function

   str2double converts strings to double precision numbers, while int32 converts numbers to integers, char converts numbers to characters, and num2str converts numbers to strings.

3. Final Answer:

   str2double -> Option C

4. Quick Check:

   String to double = str2double [OK]

Hint: Use str2double for string to number conversion [OK]

Common Mistakes:

• Using int32 on strings causes errors
• Using char converts numbers to characters, not numbers
• Using num2str converts numbers to strings, not the other way

2. Which of the following is the correct syntax to convert the number 3.7 to an integer in MATLAB?

easy

A. int32(3.7)

B. int(3.7)

C. integer(3.7)

D. toInt(3.7)

Solution

1. Step 1: Recall MATLAB integer conversion functions

   MATLAB uses functions like int32, int16, etc., to convert numbers to integers.

2. Step 2: Check syntax correctness

   int32(3.7) is valid syntax; the others are not MATLAB functions.

3. Final Answer:

   int32(3.7) -> Option A

4. Quick Check:

   Correct integer conversion = int32(3.7) [OK]

Hint: Use int32() to convert decimals to integers [OK]

Common Mistakes:

• Using non-existent functions like int() or integer()
• Forgetting parentheses in function calls
• Expecting rounding instead of truncation

3. What is the output of the following MATLAB code?

val = int32(5.9);
disp(val);

medium

A. 5

B. Error

C. 5.9

D. 6

Solution

1. Step 1: Understand int32 conversion behavior

   Converting a decimal number to int32 truncates the decimal part, it does not round.

2. Step 2: Apply truncation to 5.9

   Truncating 5.9 results in 5, so val becomes 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   int32 truncates decimals = 5 [OK]

Hint: int32 cuts decimals, does not round [OK]

Common Mistakes:

• Assuming int32 rounds up
• Expecting output to be 5.9 (no conversion)
• Thinking it causes an error

4. The following MATLAB code throws an error. What is the problem?

num = '123';
val = int32(num);

medium

A. Variable name 'num' is reserved

B. int32 cannot convert strings directly

C. Missing semicolon after assignment

D. int32 only works with integers, not strings

Solution

1. Step 1: Analyze the input type

   The variable num is a string, not a numeric value.

2. Step 2: Check int32 input requirements

   int32 expects numeric input, not strings. To convert a string to a number, use str2double first.

3. Final Answer:

   int32 cannot convert strings directly -> Option B

4. Quick Check:

   int32 needs numeric input [OK]

Hint: Convert string to number before int32 [OK]

Common Mistakes:

• Trying to convert string directly with int32
• Ignoring type mismatch errors
• Assuming int32 rounds strings

5. You have a cell array of strings representing numbers: {'10.5', '20.1', '30.9'}. Which MATLAB code correctly converts this to an integer array by truncating decimals?

hard

A. intArray = str2double(int32(cellArray));

B. intArray = int32(cellArray);

C. intArray = int32(str2num(cellArray));

D. intArray = int32(cellfun(@str2double, cellArray));

Solution

1. Step 1: Understand cell array conversion

   Each string in the cell array must be converted individually to a number.

2. Step 2: Use cellfun with str2double and then int32

   cellfun(@str2double, cellArray) converts each string to double, then int32 truncates decimals for the whole array.

3. Step 3: Check other options

   intArray = str2double(int32(cellArray)); tries to convert the cell array to int32 first, which fails. intArray = int32(cellArray); tries int32 on strings directly, which errors. intArray = int32(str2num(cellArray)); uses str2num incorrectly on cell array.

4. Final Answer:

   intArray = int32(cellfun(@str2double, cellArray)); -> Option D

5. Quick Check:

   Use cellfun + str2double + int32 [OK]

Hint: Use cellfun with str2double before int32 [OK]

Common Mistakes:

• Trying to convert whole cell array at once
• Using int32 directly on strings
• Confusing str2num with str2double on cell arrays