Dynamic field names in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are dynamic field names in MATLAB structures?

Dynamic field names allow you to create or access structure fields using variable names instead of fixed names. This means you can decide the field name while the program runs.

beginner

How do you assign a value to a dynamic field name in a MATLAB structure?

Use parentheses and a string variable for the field name, like this: structVar.(fieldName) = value; where fieldName is a variable holding the field name as a string.

intermediate

What happens if you try to access a dynamic field name that does not exist in a MATLAB structure?

MATLAB will give an error saying the field does not exist. You can check if the field exists first using isfield(structVar, fieldName) to avoid errors.

intermediate

Example: How to create a structure with dynamic field names from a list of names?

You can loop through the list and assign values like this:

names = {'age', 'height'};
for i = 1:length(names)
  s.(names{i}) = i*10;
end

This creates fields 'age' and 'height' with values 10 and 20.

beginner

Why are dynamic field names useful in MATLAB?

They let you write flexible code that can handle different data fields without hardcoding names. This is helpful when field names come from user input or external data.

How do you access a dynamic field named stored in variable fname of structure s?

A s[fname]

B s.fname

C s{fname}

D s.(fname)

Use parentheses and dot notation with the variable holding the field name: s.(fname).

What MATLAB function checks if a dynamic field exists in a structure?

A fieldexists

B isfield

C hasfield

D checkfield

The function isfield(structure, fieldName) returns true if the field exists.

Which of these is the correct way to assign 100 to a dynamic field fname in structure s?

A s.(fname) = 100;

B s.fname = 100;

C s{fname} = 100;

D s[fname] = 100;

Use s.(fname) = 100; to assign value to a dynamic field.

What error occurs if you try to access a non-existing dynamic field without checking?

A Syntax error

B Index exceeds matrix dimensions

C Field does not exist error

D No error, returns empty

MATLAB throws a 'Field does not exist' error if you access a missing field.

Why use dynamic field names instead of fixed field names?

A To make code flexible for different field names

B To speed up code execution

C To hardcode field names

D To avoid using structures

Dynamic field names let you write flexible code that adapts to different field names.

Explain how to create and access dynamic field names in a MATLAB structure.

Think about how you use variables to name fields.

Describe a situation where dynamic field names are helpful in MATLAB programming.

Consider working with data that changes structure.

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