String replacement in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this string replacement code?

Consider the following MATLAB code that replaces a substring in a string. What will be the output?

MATLAB

str = 'I love apples';
newStr = strrep(str, 'apples', 'oranges');
disp(newStr);

A I love oranges

B I love apples

C I love applesoranges

D I love

💡 Hint

The function strrep replaces all occurrences of the old substring with the new substring.

The strrep function replaces 'apples' with 'oranges' in the original string, so the output is 'I love oranges'.

❓ Predict Output

intermediate

What does this code output after replacing multiple substrings?

This MATLAB code replaces multiple substrings sequentially. What is the final output?

MATLAB

str = 'cat bat rat';
str = strrep(str, 'bat', 'dog');
str = strrep(str, 'rat', 'fox');
disp(str);

A cat dog fox

B cat dog rat

C cat bat rat

D dog cat fox

💡 Hint

Each strrep call replaces all occurrences of the target substring.

First 'bat' is replaced by 'dog', then 'rat' is replaced by 'fox', resulting in 'cat dog fox'.

❓ Predict Output

advanced

What is the output of this code using regexprep for string replacement?

This MATLAB code uses regexprep to replace all digits with '#'. What is the output?

MATLAB

str = 'Room 123, Floor 4';
newStr = regexprep(str, '\\d', '#');
disp(newStr);

A Room #23, Floor 4

B Room 123, Floor 4

C Room ###, Floor #

D Room ###, Floor 4

💡 Hint

The pattern '\d' matches each digit individually.

regexprep replaces each digit with '#', so '123' becomes '###' and '4' becomes '#'.

❓ Predict Output

advanced

What error does this code produce when using strrep incorrectly?

What error will this MATLAB code produce?

MATLAB

str = 'hello world';
newStr = strrep(str, 'world');
disp(newStr);

A hello

B Error: Not enough input arguments.

C hello world

D Error: Undefined function or variable 'strrep'.

💡 Hint

Check the number of arguments strrep requires.

strrep requires three arguments: the original string, the substring to replace, and the replacement substring. Missing the third argument causes an error.

🧠 Conceptual

expert

How many replacements occur in this code using strrep with overlapping substrings?

Given the code below, how many replacements does strrep perform?

MATLAB

str = 'aaaa';
newStr = strrep(str, 'aa', 'b');
disp(newStr);

A 1

B 4

C 3

D 2

💡 Hint

strrep does not replace overlapping occurrences twice.

strrep replaces non-overlapping occurrences from left to right. 'aaaa' has two non-overlapping 'aa' substrings: positions 1-2 and 3-4, so 2 replacements occur.

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