Dynamic field names in MATLAB

Introduction

Dynamic field names let you create or access parts of a structure using names stored in variables. This helps when you don't know the field names before running the program.

When you want to store data with names that come from user input.

When you need to loop over a list of names to create or read fields in a structure.

When you want to build a structure with fields based on changing conditions.

When you want to write flexible code that works with different sets of field names.

Syntax

MATLAB

value = structName.(fieldNameVariable);  % Access field
structName.(fieldNameVariable) = value;  % Assign field

The field name is inside parentheses and single quotes are not used around the variable.

The variable fieldNameVariable must be a string or character vector.

Examples

Creates a field 'age' in the structure person and sets it to 30.

MATLAB

fieldName = 'age';
person.(fieldName) = 30;

Reads the value of the field 'score' from the structure data.

MATLAB

field = 'score';
value = data.(field);

Creates fields 'height' and 'weight' in stats with values 10 and 20.

MATLAB

fields = {'height', 'weight'};
for i = 1:length(fields)
    stats.(fields{i}) = i * 10;
end

Sample Program

This program creates a structure person and adds fields 'name' and 'age' using dynamic field names. Then it shows the structure.

MATLAB

person = struct();
fieldName = 'name';
person.(fieldName) = 'Alice';
fieldName = 'age';
person.(fieldName) = 28;

% Display the structure
disp(person);

Important Notes

Dynamic field names are useful but can make code harder to read if overused.

Make sure the field name variable contains valid field names (no spaces or special characters).

Use dynamic field names to write flexible and reusable code.

Summary

Dynamic field names let you use variables to name structure fields.

They help when field names are not fixed before running the program.

Use the syntax structName.(fieldNameVariable) to get or set fields.

Practice

1. What is the main purpose of using dynamic field names in MATLAB structures?

easy

A. To use variables as field names in structures

B. To create arrays with dynamic sizes

C. To perform matrix multiplication

D. To define functions inside structures

Solution

1. Step 1: Understand dynamic field names

   Dynamic field names allow using a variable to specify the name of a field in a structure.

2. Step 2: Identify the purpose

   This helps when field names are not fixed and need to be set or accessed dynamically during program execution.

3. Final Answer:

   To use variables as field names in structures -> Option A

4. Quick Check:

   Dynamic field names = variable field names [OK]

Hint: Dynamic field names use variables for field keys [OK]

Common Mistakes:

• Confusing dynamic fields with arrays
• Thinking dynamic fields define functions
• Mixing dynamic fields with matrix operations

2. Which of the following is the correct syntax to assign a value to a dynamic field name fname in a structure S?

easy

A. S.(fname) = 10;

B. S.fname = 10;

C. S{fname} = 10;

D. S[fname] = 10;

Solution

1. Step 1: Recall MATLAB syntax for dynamic fields

   Dynamic field names use parentheses and dot notation: structure.(variableName).

2. Step 2: Check each option

   S.(fname) = 10; uses S.(fname) which is correct syntax. Options A, C, and D are invalid for dynamic field names.

3. Final Answer:

   S.(fname) = 10; -> Option A

4. Quick Check:

   Use dot and parentheses for dynamic fields [OK]

Hint: Use S.(variable) to access dynamic fields [OK]

Common Mistakes:

• Using dot without parentheses for variable field
• Using curly braces or square brackets instead of dot
• Confusing cell or array indexing with struct fields

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

field = 'age';
S.(field) = 25;
disp(S.age);

medium

A. age

B. Empty output

C. Error: Undefined field 'age'

D. 25

Solution

1. Step 1: Assign dynamic field

   The variable field holds 'age'. Using S.(field) = 25; creates field 'age' with value 25 in structure S.

2. Step 2: Display the field value

   disp(S.age); prints the value of field 'age', which is 25.

3. Final Answer:

   25 -> Option D

4. Quick Check:

   Dynamic field 'age' = 25 [OK]

Hint: Dynamic field names create fields with variable keys [OK]

Common Mistakes:

• Expecting field name to be literal 'field'
• Confusing variable name with field name
• Not using parentheses for dynamic field assignment

4. Identify the error in the following MATLAB code snippet:

fname = 'score';
S.fname = 100;
disp(S.score);

medium

A. Variable fname is not defined

B. Missing semicolon after assignment

C. Using dot notation with variable name instead of dynamic field

D. disp function cannot display structure fields

Solution

1. Step 1: Analyze field assignment

   The code uses S.fname = 100; which creates a field literally named 'fname', not 'score'.

2. Step 2: Check display statement

   disp(S.score); tries to display field 'score' which does not exist, causing an error.

3. Final Answer:

   Using dot notation with variable name instead of dynamic field -> Option C

4. Quick Check:

   Use S.(fname) for dynamic fields [OK]

Hint: Use parentheses for dynamic field names to avoid errors [OK]

Common Mistakes:

• Using dot notation without parentheses for variable fields
• Assuming variable name becomes field name automatically
• Ignoring error from missing field access

5. Given a list of field names fields = {'name', 'age', 'score'} and values vals = {'Alice', 30, 95}, which code correctly creates a structure S with these dynamic fields and values?

hard

A. for i = 1:length(fields)
S{fields{i}} = vals{i};
end

B. for i = 1:length(fields)
S.(fields{i}) = vals{i};
end

C. for i = 1:length(fields)
S.fields{i} = vals{i};
end

D. for i = 1:length(fields)
S.(vals{i}) = fields{i};
end

Solution

1. Step 1: Understand dynamic assignment in loop

   We want to assign each field name from fields as a field in S with corresponding value from vals.

2. Step 2: Check each option

   for i = 1:length(fields) S.(fields{i}) = vals{i}; end uses S.(fields{i}) = vals{i}; which correctly assigns dynamic fields. Options A, B, and D use incorrect syntax or swap keys and values.

3. Final Answer:

   for i = 1:length(fields) S.(fields{i}) = vals{i}; end -> Option B

4. Quick Check:

   Loop with S.(fields{i}) = vals{i} assigns dynamic fields [OK]

Hint: Use S.(fields{i}) = vals{i} in loop for dynamic fields [OK]

Common Mistakes:

• Using dot notation without parentheses inside loop
• Confusing cell array indexing with struct field access
• Swapping keys and values in assignment