String concatenation in MATLAB - Deep Dive

Overview - String concatenation

What is it?

String concatenation is the process of joining two or more pieces of text together to form one longer text. In MATLAB, strings are sequences of characters, and concatenation combines them into a single string. This is useful when you want to build messages, labels, or any text output dynamically. It helps computers handle and display text in a flexible way.

Why it matters

Without string concatenation, computers would struggle to create meaningful messages or combine data labels dynamically. Imagine trying to display a sentence without joining words or numbers together; it would be very limited and hard to read. String concatenation allows programs to communicate clearly and adapt text based on data or user input, making software more interactive and user-friendly.

Where it fits

Before learning string concatenation, you should understand what strings are and how MATLAB stores text data. After mastering concatenation, you can explore string formatting, text processing functions, and how to manipulate strings for data analysis or visualization.

Mental Model

Core Idea

String concatenation is like linking building blocks of text end-to-end to create a longer, meaningful message.

Think of it like...

Think of string concatenation like snapping together LEGO bricks. Each brick is a small piece of text, and when you snap them together, you build a bigger structure—a complete sentence or phrase.

┌─────────┐   ┌─────────┐   ┌───────────────┐
│ 'Hello' │ + │ ' '     │ + │ 'World!'      │
└─────────┘   └─────────┘   └───────────────┘
          ↓ Concatenate
      ┌─────────────────────┐
      │ 'Hello World!'      │
      └─────────────────────┘

Build-Up - 6 Steps

1

Foundation

Understanding MATLAB Strings

Concept: Learn what strings are in MATLAB and how they represent text.

In MATLAB, strings are sequences of characters enclosed in single quotes, like 'data'. They store text information. You can create strings by typing them directly or using functions like string().

Result

You can create and display text data in MATLAB.

Understanding what strings are is essential before combining them; they are the basic units of text data.

2

Foundation

Basic Concatenation with Square Brackets

Concept: Use square brackets [] to join strings side by side.

In MATLAB, you can concatenate strings by placing them inside square brackets. For example, ['Hello', 'World'] joins 'Hello' and 'World' into 'HelloWorld'. Spaces are not added automatically.

Result

Concatenated string without spaces: 'HelloWorld'

Square brackets are the simplest way to join strings, but you must add spaces manually if needed.

3

Intermediate

Adding Spaces Between Strings

🤔Before reading on: Do you think ['Hello', ' ', 'World'] adds a space between words automatically or do you need to include it explicitly? Commit to your answer.

Concept: Include space characters explicitly when concatenating to separate words.

To add spaces between words, include a space character ' ' as a separate string inside the brackets. For example, ['Hello', ' ', 'World'] results in 'Hello World'.

Result

Concatenated string with space: 'Hello World'

Knowing that spaces are just characters helps you control exactly how the final text looks.

4

Intermediate

Concatenating String Arrays

🤔Before reading on: Can you concatenate multiple strings stored in a string array using square brackets? Predict yes or no.

Concept: String arrays can be concatenated element-wise or combined into one string using functions.

MATLAB supports string arrays like ["Hello", "World"]. To join them into one string, use the strjoin() function, e.g., strjoin(["Hello", "World"], ' ') returns "Hello World".

Result

Joined string from array: 'Hello World'

Using strjoin simplifies combining multiple strings with separators, improving code clarity.

5

Advanced

Using plus (+) Operator for String Concatenation

🤔Before reading on: Does the plus (+) operator work for concatenating strings in MATLAB? Commit to yes or no.

Concept: MATLAB allows using + to concatenate string objects (not character arrays).

For string objects (created with double quotes), you can use + to concatenate. For example, "Hello" + " " + "World" results in "Hello World". This does not work with character arrays (single quotes).

Result

Concatenated string object: "Hello World"

Knowing the difference between character arrays and string objects is key to using + correctly.

6

Expert

Performance and Memory Considerations

🤔Before reading on: Do you think concatenating many strings repeatedly with + or [] is equally efficient? Commit to your answer.

Concept: Repeated concatenation can be inefficient; preallocation or using join functions improves performance.

When concatenating many strings in a loop, using + or [] repeatedly creates new copies each time, slowing down the program. Preallocating string arrays or using strjoin after collecting strings is faster and uses less memory.

Result

Efficient concatenation with strjoin or preallocation improves speed and memory use.

Understanding performance helps write faster, scalable MATLAB code for large text data.

Under the Hood

MATLAB stores character arrays as sequences of ASCII or Unicode codes in memory. When concatenating with [], MATLAB creates a new array combining the codes from each input. For string objects, MATLAB manages memory differently, allowing the + operator to join strings by creating a new string object internally. Each concatenation involves copying data to a new memory space, which can be costly if done repeatedly.

Why designed this way?

MATLAB's design separates character arrays (older style) and string objects (newer style) to balance backward compatibility and modern text handling. Using [] for character arrays is simple and consistent with MATLAB's array operations. The + operator for string objects was introduced later to provide more intuitive syntax similar to other languages.

Character arrays concatenation:
[ 'H' 'e' 'l' 'l' 'o' ] + [ ' ' ] + [ 'W' 'o' 'r' 'l' 'd' ]
       ↓
[ 'H' 'e' 'l' 'l' 'o' ' ' 'W' 'o' 'r' 'l' 'd' ]

String objects concatenation:
"Hello" + " " + "World"
       ↓
"Hello World"

Myth Busters - 3 Common Misconceptions

Quick: Does the plus (+) operator work for concatenating character arrays in MATLAB? Commit to yes or no.

Common Belief: Many believe + can concatenate any strings in MATLAB.

Reality: The + operator only works for string objects (double quotes), not character arrays (single quotes).

Why it matters: Using + with character arrays causes errors, confusing beginners and breaking code.

Quick: Does concatenating strings with [] automatically add spaces between words? Commit to yes or no.

Common Belief: Some think [] adds spaces automatically when joining strings.

Reality: [] joins strings exactly as given; spaces must be added explicitly.

Why it matters: Assuming automatic spaces leads to merged words and unreadable text output.

Quick: Is repeatedly concatenating strings in a loop with [] or + efficient? Commit to yes or no.

Common Belief: Many assume repeated concatenation is fast and memory-friendly.

Reality: Repeated concatenation creates new copies each time, causing slowdowns and high memory use.

Why it matters: Ignoring this leads to slow programs and crashes with large text data.

Expert Zone

1

String objects and character arrays behave differently; mixing them can cause subtle bugs.

2

Using strjoin with a delimiter is often more efficient and clearer than manual concatenation in loops.

3

MATLAB's internal memory copying during concatenation can be a bottleneck in large-scale text processing.

When NOT to use

Avoid using [] or + for concatenating very large numbers of strings repeatedly in loops. Instead, collect strings in arrays and use strjoin or preallocate memory. For complex text formatting, use sprintf or compose functions.

Production Patterns

In real-world MATLAB code, string concatenation is often combined with formatting functions to build dynamic labels, file paths, or messages. Efficient concatenation patterns use string arrays and strjoin to handle large datasets or generate reports.

Connections

Array concatenation

String concatenation uses the same square bracket syntax as numeric array concatenation in MATLAB.

Understanding array concatenation helps grasp how strings, as arrays of characters, are joined in MATLAB.

Text formatting

String concatenation often pairs with formatting functions like sprintf to create complex text outputs.

Knowing concatenation basics makes it easier to build formatted strings for reports or user interfaces.

Memory management in programming

Concatenation performance relates to how memory is allocated and copied during string operations.

Understanding memory helps optimize string handling, preventing slowdowns in large data processing.

Common Pitfalls

#1 Trying to concatenate character arrays with + operator.

Wrong approach:

result = 'Hello' + 'World';

Correct approach:

result = ['Hello', 'World'];

Root cause:

Confusing string objects with character arrays and assuming + works for both.

#2 Concatenating strings without adding spaces, resulting in merged words.

Wrong approach:

result = ['Hello', 'World'];

Correct approach:

result = ['Hello', ' ', 'World'];

Root cause:

Assuming concatenation adds spaces automatically.

#3 Concatenating many strings repeatedly inside a loop using + or [].

Wrong approach:

for i = 1:n
result = [result, newString];
end

Correct approach:

strings = stringsArray; % collect strings
result = strjoin(strings, ' ');

Root cause:

Not understanding that repeated concatenation copies data each time, causing inefficiency.

Key Takeaways

String concatenation joins pieces of text to form longer messages, essential for dynamic text handling.

In MATLAB, square brackets [] concatenate character arrays, but spaces must be added explicitly.

The plus (+) operator concatenates string objects (double quotes), not character arrays (single quotes).

Repeated concatenation in loops is inefficient; use string arrays and strjoin for better performance.

Understanding the difference between character arrays and string objects prevents common errors.