String replacement in MATLAB - Step-by-Step Execution

Concept Flow - String replacement

Start with original string

↓

Identify substring to replace

↓

Find all occurrences

↓

Replace each occurrence with new substring

↓

Return modified string

↓

End

The program starts with an original string, finds all parts matching the target substring, replaces them with a new substring, and returns the updated string.

Execution Sample

MATLAB

str = "Hello world!";
newStr = replace(str, "world", "MATLAB");
disp(newStr);

This code replaces the word "world" with "MATLAB" in the string and displays the result.

Execution Table

Step	Action	Input String	Substring to Replace	Replacement Substring	Resulting String
1	Start	Hello world!	world	MATLAB	Hello world!
2	Find substring 'world'	Hello world!	world	MATLAB	Found at position 7
3	Replace 'world' with 'MATLAB'	Hello world!	world	MATLAB	Hello MATLAB!
4	Display result	Hello MATLAB!	world	MATLAB	Hello MATLAB!

💡 All occurrences of 'world' replaced; execution ends.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
str	"Hello world!"	"Hello world!"	"Hello world!"	"Hello world!"
newStr	undefined	undefined	"Hello MATLAB!"	"Hello MATLAB!"

Key Moments - 2 Insights

Why does the original string 'str' not change after replacement?

In MATLAB, the replace function returns a new string with replacements; it does not modify the original string variable 'str' (see execution_table step 3 and variable_tracker).

What happens if the substring to replace is not found?

If the substring is not found, the replace function returns the original string unchanged, so no replacement occurs (not shown in this example but implied by step 2).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of newStr after step 3?

A "Hello world!"

B "MATLAB world!"

C "Hello MATLAB!"

D "Hello!"

💡 Hint

Check the 'Resulting String' column in execution_table row for step 3.

At which step does the program find the substring to replace?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column in execution_table to find when substring is located.

If the substring to replace was 'hello' (lowercase), what would happen?

A No replacement would happen because 'hello' does not match 'Hello' exactly.

B An error would occur.

C The replacement would still happen because replace is case-insensitive.

D The entire string would be replaced.

💡 Hint

MATLAB's replace function is case-sensitive; check variable_tracker and execution_table for case matching.

Concept Snapshot

String replacement in MATLAB:
Use replace(originalString, oldSubstring, newSubstring)
Returns a new string with all occurrences of oldSubstring replaced
Original string remains unchanged
Case-sensitive matching
Useful for simple text substitutions

Full Transcript

This example shows how MATLAB replaces parts of a string. We start with the string 'Hello world!'. The program looks for the substring 'world'. It finds it starting at position 7. Then it replaces 'world' with 'MATLAB', creating a new string 'Hello MATLAB!'. The original string stays the same. Finally, the new string is displayed. This process helps change text inside strings without altering the original data.

Practice

1. What does the MATLAB function strrep('hello world', 'world', 'MATLAB') do?

easy

A. Replaces 'world' with 'MATLAB' resulting in 'hello MATLAB'

B. Removes the word 'world' from the string

C. Returns an error because 'world' is not a variable

D. Converts the entire string to uppercase

Solution

1. Step 1: Understand strrep function

   The function strrep replaces all occurrences of a substring with a new substring in a given string.

2. Step 2: Apply strrep to the example

   It replaces 'world' with 'MATLAB' in 'hello world', resulting in 'hello MATLAB'.

3. Final Answer:

   Replaces 'world' with 'MATLAB' resulting in 'hello MATLAB' -> Option A

4. Quick Check:

   strrep('hello world', 'world', 'MATLAB') = 'hello MATLAB' [OK]

Hint: Remember: strrep replaces all matching substrings [OK]

Common Mistakes:

• Thinking strrep removes substrings instead of replacing
• Confusing strrep with uppercase functions
• Assuming strrep only works on variables

2. Which of the following is the correct syntax to replace all occurrences of 'cat' with 'dog' in the string text = 'cat and cat'?

easy

A. replace(text, cat, dog)

B. strrep(text, 'cat', 'dog')

C. text.replace('cat', 'dog')

D. str_replace(text, 'cat', 'dog')

Solution

1. Step 1: Recall MATLAB string replacement syntax

   In MATLAB, the function to replace substrings is strrep(originalStr, oldSubstr, newSubstr).

2. Step 2: Match the correct syntax

   strrep(text, 'cat', 'dog') uses strrep correctly with the right order of arguments.

3. Final Answer:

   strrep(text, 'cat', 'dog') -> Option B

4. Quick Check:

   Correct MATLAB syntax for string replacement is strrep() [OK]

Hint: Use strrep(original, old, new) for replacements [OK]

Common Mistakes:

• Using Python or other language syntax by mistake
• Misspelling function names
• Using dot notation which MATLAB strings don't support

3. What is the output of the following MATLAB code?

str = 'apple banana apple';
newStr = strrep(str, 'apple', 'orange');
disp(newStr);

medium

A. orange banana apple

B. apple banana orange

C. orange banana orange

D. apple banana apple

Solution

1. Step 1: Identify the original string and replacement

   The original string is 'apple banana apple'. The function replaces all 'apple' with 'orange'.

2. Step 2: Apply strrep to replace all occurrences

   Both 'apple' words are replaced, resulting in 'orange banana orange'.

3. Final Answer:

   orange banana orange -> Option C

4. Quick Check:

   strrep replaces all matches, not just first [OK]

Hint: strrep replaces every match, not just one [OK]

Common Mistakes:

• Replacing only the first occurrence
• Confusing strrep with regex functions
• Expecting original string unchanged

4. The following MATLAB code is intended to replace 'dog' with 'cat' in the string text = 'dog and dog'. What is the error?

text = 'dog and dog';
newText = strrep(text, dog, cat);
disp(newText);

medium

A. Missing quotes around 'dog' and 'cat' causing an error

B. strrep cannot replace substrings in character arrays

C. disp cannot display strings

D. No error, code runs correctly

Solution

1. Step 1: Check argument types in strrep

   In MATLAB, substrings must be given as strings or character arrays enclosed in quotes.

2. Step 2: Identify missing quotes causing error

   The code uses dog and cat without quotes, which MATLAB treats as variables, causing an error.

3. Final Answer:

   Missing quotes around 'dog' and 'cat' causing an error -> Option A

4. Quick Check:

   String literals need quotes in strrep arguments [OK]

Hint: Always quote substrings in strrep arguments [OK]

Common Mistakes:

• Forgetting quotes around substrings
• Assuming variables are strings automatically
• Thinking disp cannot show strings

5. Given a string array words = ["red", "blue", "red", "green"], which MATLAB code replaces all occurrences of "red" with "yellow" correctly?

hard

A. newWords = strrep(words, red, 'yellow');

B. newWords = strrep(words, "red", yellow);

C. newWords = strrep(words, red, yellow);

D. newWords = replace(words, 'red', 'yellow');

Solution

1. Step 1: Understand string array replacement functions

   For string arrays, MATLAB's replace function works correctly, while strrep works on character arrays.

2. Step 2: Identify the correct function and syntax

   newWords = replace(words, 'red', 'yellow'); uses replace with proper quotes, which replaces all "red" with "yellow" in the string array.

3. Final Answer:

   newWords = replace(words, 'red', 'yellow'); -> Option D

4. Quick Check:

   Use replace() for string arrays, strrep() for char arrays [OK]

Hint: Use replace() for string arrays, strrep() for char arrays [OK]

Common Mistakes:

• Using strrep on string arrays (causes error)
• Forgetting quotes around substrings
• Confusing variable names with strings