Converting between types in MATLAB

Introduction

Sometimes you need to change data from one form to another so your program can work correctly. This is called converting between types.

When you want to change a number stored as text into a real number to do math.

When you need to turn a decimal number into a whole number.

When you want to convert a number into text to show it on the screen.

When you have data in one format but a function needs it in another format.

When you want to save memory by changing a large number type to a smaller one.

Syntax

MATLAB

newVariable = typeName(oldVariable);

Replace typeName with the type you want, like double, int32, or char.

This creates a new variable with the converted value.

Examples

Convert text '123' to the number 123.

MATLAB

x = '123';
y = str2double(x);

Convert decimal 3.7 to whole number 3 by cutting off the decimal part.

MATLAB

a = 3.7;
b = int32(a);

Convert number 65 to character 'A' using ASCII code.

MATLAB

num = 65;
charVal = char(num);

Sample Program

This program shows how to convert a string to a number, then to an integer, and also how to convert a number to a character.

MATLAB

strNum = '45.67';
num = str2double(strNum);
intNum = int32(num);
charVal = char(65);

fprintf('Original string: %s\n', strNum);
fprintf('Converted to double: %.2f\n', num);
fprintf('Converted to int32: %d\n', intNum);
fprintf('Number 65 as char: %c\n', charVal);

Important Notes

When converting from string to number, use str2double instead of just double.

Converting from decimal to integer cuts off the decimal part, it does not round.

Converting numbers to characters uses ASCII codes, so 65 becomes 'A'.

Summary

Converting types helps your program use data correctly.

Use functions like str2double, int32, and char to convert data.

Be careful: converting decimals to integers cuts off decimals, it does not round.

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