Dynamic field names in MATLAB - Step-by-Step Execution

Concept Flow - Dynamic field names

Start

↓

Create struct

↓

Define field name as string

↓

Use dynamic field name to assign value

↓

Access or modify field using dynamic name

↓

End

This flow shows how to create a structure, define a field name as a string, and use that string to dynamically assign or access fields.

Execution Sample

MATLAB

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

This code creates a struct and uses a dynamic field name to assign and display a value.

Execution Table

Step	Action	Variable/Field	Value	Output
1	Assign string to fieldName	fieldName	'age'
2	Create empty struct	person	struct with no fields
3	Assign 30 to person.(fieldName)	person.age	30
4	Display person.age	person.age	30	30
5	End of code

💡 Code ends after displaying the value of the dynamic field.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
fieldName	undefined	'age'	'age'	'age'	'age'
person	undefined	undefined	empty struct	struct with field 'age' = 30	struct with field 'age' = 30

Key Moments - 2 Insights

Why do we use parentheses and a dot like person.(fieldName) instead of person.fieldName?

Using person.(fieldName) tells MATLAB to use the value of the variable fieldName as the field name. person.fieldName would look for a field literally named 'fieldName', which does not exist. See execution_table step 3.

What happens if the field name string changes after assignment?

Changing the string variable after assigning the field does not change the existing field. The field name is fixed at assignment. See variable_tracker for fieldName and person after step 3.

Visual Quiz - 3 Questions

Look at the execution table at step 3. What field does person have after assignment?

A 'person'

B 'fieldName'

C 'age'

D No fields

💡 Hint

Check the 'Variable/Field' column at step 3 in the execution_table.

At which step is the value 30 assigned to the struct field?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Action' and 'Value' columns in the execution_table.

If we change fieldName to 'height' after step 3, what happens to person.age?

A person.age changes to person.height

B person.age remains 30

C person.age is deleted

D person becomes empty

💡 Hint

Refer to key_moments about field name changes after assignment.

Concept Snapshot

Dynamic field names in MATLAB:
- Use a string variable as a field name with syntax: structVar.(fieldName)
- Allows flexible field assignment/access
- Field name is fixed at assignment time
- Useful for loops or variable field names
- Example: person.(fieldName) = value;

Full Transcript

This example shows how to use dynamic field names in MATLAB structures. First, we assign the string 'age' to the variable fieldName. Then, we create an empty struct called person. Using dynamic field name syntax person.(fieldName), we assign the value 30 to the field named 'age'. Finally, we display the value of person.age, which outputs 30. The key point is that using parentheses with a variable allows MATLAB to use the variable's value as the field name, not the literal variable name. Changing the variable fieldName later does not affect the existing struct 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