Dynamic field names in MATLAB - Deep Dive

Overview - Dynamic field names

What is it?

Dynamic field names in MATLAB allow you to create or access fields in a structure using variable names instead of fixed names. This means you can decide the field name while the program runs, not just when you write the code. It helps when you want to work with data where field names change or come from user input. This makes your code more flexible and adaptable.

Why it matters

Without dynamic field names, you would have to write separate code for each possible field name, which is slow and error-prone. Dynamic field names let you handle many cases with less code, making your programs easier to maintain and extend. This is especially useful when working with datasets that have varying or unknown field names, like data from different sources or user-generated content.

Where it fits

Before learning dynamic field names, you should understand basic MATLAB structures and how to access fixed fields. After mastering dynamic field names, you can explore advanced data manipulation, automated data processing, and building flexible data models in MATLAB.

Mental Model

Core Idea

Dynamic field names let you use variables to name or access structure fields, making your data handling flexible and adaptable at runtime.

Think of it like...

It's like having a filing cabinet where you can label each drawer with any name you want on the spot, instead of only using pre-labeled drawers.

Structure
┌───────────────┐
│ structVar     │
│ ┌───────────┐ │
│ │ fieldName │ │  <-- variable holding the field name
│ └───────────┘ │
│ structVar.(fieldName) = value;  <-- dynamic access
└───────────────┘

Build-Up - 7 Steps

1

Foundation

Understanding MATLAB structures

Concept: Learn what a structure is and how to access fixed fields.

A structure in MATLAB is like a container with named boxes called fields. You can access a field by writing structName.fieldName. For example, person.name = 'Alice'; person.age = 30; accesses fixed fields 'name' and 'age'.

Result

You can store and retrieve data using fixed field names in a structure.

Knowing how fixed fields work is essential before using dynamic field names because dynamic fields build on this concept.

2

Foundation

Using variables to hold field names

Concept: Store field names as strings in variables.

Instead of writing the field name directly, you can save it in a variable: fname = 'age'; Then you can use fname to refer to the field name.

Result

You have a variable that holds the name of a field as text.

Separating field names into variables is the first step to dynamic field access.

3

Intermediate

Accessing fields dynamically

🤔Before reading on: Do you think structVar.(varName) accesses the field named by varName or the literal field named 'varName'? Commit to your answer.

Concept: Use parentheses and a variable to access a field dynamically.

If you have a structure s and a variable fname = 'score', you can get the value with s.(fname). This accesses the field named 'score' inside s. For example, s.score = 95; fname = 'score'; val = s.(fname); % val is 95

Result

You can read or write any field by changing the variable fname.

Understanding that parentheses allow variable field names unlocks flexible data access.

4

Intermediate

Creating fields dynamically

🤔Before reading on: Can you add a new field to a structure using a variable as the field name? Yes or no? Commit to your answer.

Concept: Assign to a dynamic field name to create or update fields.

You can add a new field by assigning to s.(fname) = value; For example, fname = 'height'; s.(fname) = 180; adds a field 'height' with value 180 to s.

Result

Structures can grow with new fields decided at runtime.

Knowing you can create fields dynamically lets you build flexible data structures.

5

Intermediate

Looping with dynamic field names

🤔Before reading on: Do you think you can use dynamic field names inside a loop to process multiple fields? Commit to your answer.

Concept: Use dynamic field names to iterate over fields programmatically.

Suppose you have a list of field names in a cell array: fields = {'score', 'height', 'weight'}; You can loop: for i = 1:length(fields), fname = fields{i}; disp(s.(fname)); end This prints each field's value.

Result

You can process many fields without hardcoding their names.

Looping with dynamic fields enables scalable and generic code for data processing.

6

Advanced

Handling missing dynamic fields safely

🤔Before reading on: What happens if you try to access a dynamic field that does not exist? Will MATLAB error or return empty? Commit to your answer.

Concept: Check if a dynamic field exists before accessing it to avoid errors.

Use isfield(s, fname) to check if the field exists. For example, if isfield(s, fname), val = s.(fname); else val = NaN; end This prevents runtime errors when fields are missing.

Result

Your code becomes robust to missing or unexpected fields.

Knowing to check field existence prevents common runtime errors in dynamic field usage.

7

Expert

Performance and limitations of dynamic fields

🤔Before reading on: Do you think dynamic field access is as fast as fixed field access in MATLAB? Commit to your answer.

Concept: Dynamic field names add overhead and have limitations in code generation and optimization.

Accessing fields dynamically requires MATLAB to look up the field name at runtime, which is slower than fixed access. Also, some MATLAB features like code generation (for C code) do not support dynamic fields well. Understanding these limits helps you decide when to use dynamic fields or alternatives like containers.Map.

Result

You can write efficient and compatible code by balancing flexibility and performance.

Knowing the tradeoffs of dynamic fields guides better design decisions in production code.

Under the Hood

When MATLAB encounters s.(fname), it evaluates fname to get the field name string. Then it looks up this string in the structure's internal field list to find the corresponding data. This lookup happens at runtime, unlike fixed fields which are resolved at compile time. The structure stores fields as name-value pairs internally, so dynamic access requires searching the field names each time.

Why designed this way?

MATLAB structures were designed to be flexible containers with named fields. Allowing dynamic field names lets users write generic code that adapts to changing data schemas. The tradeoff is runtime overhead, but this design supports many data science tasks where field names are not fixed. Alternatives like tables or containers.Map exist but structures remain simple and widely used.

┌───────────────┐
│ Structure s   │
│ ┌───────────┐ │
│ │ Fields:   │ │
│ │ 'age'     │ │
│ │ 'score'   │ │
│ └───────────┘ │
│               │
│ Access s.(fname):
│   ┌─────────────┐
│   │ Evaluate fname -> 'score'
│   │ Search fields for 'score'
│   │ Return value stored in 'score'
│   └─────────────┘
└───────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Does s.(varName) access a field literally named 'varName' or the field named by the content of varName? Commit to your answer.

Common Belief: Many think s.(varName) accesses a field literally named 'varName'.

Reality: s.(varName) accesses the field whose name is the string stored in varName, not the literal text 'varName'.

Why it matters: Misunderstanding this causes bugs where code tries to access wrong fields or fails unexpectedly.

Quick: Can you use dynamic field names with non-string variables like numbers directly? Commit to your answer.

Common Belief: Some believe you can use numbers or other types directly as dynamic field names.

Reality: Dynamic field names must be strings or character vectors; numbers must be converted to strings first.

Why it matters: Failing to convert types leads to errors or unexpected behavior.

Quick: Is dynamic field access always as fast as fixed field access? Commit to your answer.

Common Belief: Dynamic field access is just as fast as fixed field access.

Reality: Dynamic field access is slower because MATLAB must look up the field name at runtime.

Why it matters: Ignoring performance differences can cause slowdowns in large or time-critical programs.

Quick: Can dynamic field names be used in MATLAB code generation for C code? Commit to your answer.

Common Belief: Dynamic field names work fine in all MATLAB features including code generation.

Reality: Dynamic field names are not supported in MATLAB code generation, limiting their use in embedded systems.

Why it matters: Using dynamic fields in code generation projects causes errors or unsupported code.

Expert Zone

1

Dynamic field names can cause subtle bugs if the variable holding the field name is empty or contains invalid characters.

2

Using dynamic field names inside nested structures requires careful syntax to avoid confusion and errors.

3

Combining dynamic field names with functions like fieldnames() and rmfield() allows powerful runtime structure manipulation.

When NOT to use

Avoid dynamic field names when performance is critical or when generating code for embedded systems. Instead, use fixed fields, tables, or containers.Map for key-value pairs with better performance and compatibility.

Production Patterns

In real-world MATLAB projects, dynamic field names are used for flexible data import/export, handling JSON or XML data, and building generic data processing pipelines that adapt to varying input schemas.

Connections

Hash maps (Data Structures)

Dynamic field names in MATLAB structures are similar to keys in hash maps or dictionaries in other languages.

Understanding dynamic fields helps grasp how key-value stores work in programming, enabling flexible data access.

Reflection (Programming)

Dynamic field names relate to reflection, where programs inspect and modify their own structure at runtime.

Knowing dynamic fields deepens understanding of runtime code flexibility and introspection.

Database column selection

Selecting columns dynamically in databases is conceptually like dynamic field access in structures.

This connection shows how dynamic selection of data elements is a common pattern across computing.

Common Pitfalls

#1 Trying to access a dynamic field without checking if it exists causes runtime errors.

Wrong approach:

val = s.(fname); % fname might not exist

Correct approach:

if isfield(s, fname), val = s.(fname); else val = NaN; end

Root cause:

Assuming all dynamic fields exist without verification leads to errors.

#2 Using non-string variables directly as dynamic field names causes errors.

Wrong approach:

num = 5; s.(num) = 10;

Correct approach:

num = 5; s.(string(num)) = 10;

Root cause:

Dynamic field names must be strings; forgetting to convert types causes failure.

#3 Confusing literal field names with dynamic field names leads to wrong data access.

Wrong approach:

fname = 'age'; val = s.varName; % tries to access 'varName' field

Correct approach:

fname = 'age'; val = s.(fname); % accesses 'age' field

Root cause:

Misunderstanding syntax for dynamic field access causes bugs.

Key Takeaways

Dynamic field names let you use variables to access or create structure fields, making your code flexible.

You must use parentheses around the variable holding the field name to access fields dynamically.

Always check if a dynamic field exists before accessing it to avoid runtime errors.

Dynamic field access is slower than fixed access and not supported in all MATLAB features like code generation.

Understanding dynamic field names connects to broader programming concepts like key-value stores and reflection.