String searching (contains, strfind) in MATLAB - Mini Project: Build & Apply

String Searching with contains and strfind in MATLAB

📖 Scenario: You work in a library and need to find books that mention certain keywords in their titles.

🎯 Goal: Build a MATLAB program that searches for a keyword in a list of book titles using contains and strfind.

📋 What You'll Learn

Create a cell array of book titles

Create a keyword variable to search for

Use contains to find which titles include the keyword

Use strfind to find the position of the keyword in each title

Display the results clearly

💡 Why This Matters

🌍 Real World

Searching for keywords in book titles helps librarians quickly find relevant books for readers.

💼 Career

Knowing how to search strings is useful in data analysis, text processing, and software development.

1

Create a list of book titles

Create a cell array called bookTitles with these exact strings: 'Matlab Basics', 'Advanced Programming', 'Data Science', 'Machine Learning', and 'Deep Learning'.

MATLAB

% Create the cell array bookTitles with the given titles
% Your code here

Hint

Use curly braces {} to create a cell array of strings in MATLAB.

2

Set the keyword to search

Create a variable called keyword and set it to the string 'Learning'.

MATLAB

bookTitles = {'Matlab Basics', 'Advanced Programming', 'Data Science', 'Machine Learning', 'Deep Learning'};
% Create the keyword variable with the value 'Learning'
% Your code here

Hint

Assign the string 'Learning' to the variable keyword.

3

Find titles containing the keyword using contains and strfind

Use contains with bookTitles and keyword to create a logical array called hasKeyword. Then use a for loop with index i from 1 to length of bookTitles to find the position of keyword in each title using strfind. Store the result in a cell array called positions.

MATLAB

bookTitles = {'Matlab Basics', 'Advanced Programming', 'Data Science', 'Machine Learning', 'Deep Learning'};
keyword = 'Learning';
% Use contains to find titles with the keyword
% Use a for loop with i to find positions using strfind
% Your code here

Hint

contains returns a logical array. Use strfind inside a loop to get positions.

4

Display the search results

Use a for loop with index i from 1 to length of bookTitles to print each title. If hasKeyword(i) is true, print 'Found at position(s):' followed by the positions from positions{i}. Otherwise, print 'Keyword not found'.

MATLAB

bookTitles = {'Matlab Basics', 'Advanced Programming', 'Data Science', 'Machine Learning', 'Deep Learning'};
keyword = 'Learning';
hasKeyword = contains(bookTitles, keyword);
positions = cell(1, length(bookTitles));
for i = 1:length(bookTitles)
    positions{i} = strfind(bookTitles{i}, keyword);
end
% Print each title and search result
% Your code here

Hint

Use fprintf to print each title and the search results. Use mat2str to convert positions to string.

Practice

1. Which MATLAB function returns a logical true or false indicating if a substring exists within a string?

easy

A. contains

B. strfind

C. findstr

D. substring

Solution

1. Step 1: Understand the purpose of contains

   contains returns true or false if a substring is found inside a string.

2. Step 2: Compare with strfind

   strfind returns positions, not logical true/false.

3. Final Answer:

   contains -> Option A

4. Quick Check:

   Logical check for substring = contains [OK]

Hint: Use contains for true/false substring check [OK]

Common Mistakes:

• Confusing strfind with contains
• Thinking strfind returns true/false
• Using findstr which is outdated

2. Which of the following is the correct syntax to find the starting indices of substring 'cat' in string str = 'concatenate' using strfind?

easy

A. contains(str, 'cat')

B. strfind('cat', str)

C. strfind(str, 'cat')

D. strfind(str, cat)

Solution

1. Step 1: Recall strfind syntax

   The first argument is the main string, second is the substring to find.

2. Step 2: Check each option

   strfind(str, 'cat') correctly uses strfind(str, 'cat'). strfind('cat', str) reverses arguments. contains(str, 'cat') uses contains, not strfind. strfind(str, cat) misses quotes around 'cat'.

3. Final Answer:

   strfind(str, 'cat') -> Option C

4. Quick Check:

   strfind(mainString, substring) = strfind(str, 'cat') [OK]

Hint: First argument main string, second substring in strfind [OK]

Common Mistakes:

• Swapping arguments in strfind
• Forgetting quotes around substring
• Using contains instead of strfind for indices

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

str = 'abracadabra';
pos = strfind(str, 'ra');
disp(pos);

medium

A. [3 10]

B. [2 9]

C. [4 11]

D. []

Solution

1. Step 1: Identify substring positions in 'abracadabra'

   Look for 'ra' in the string: 'abracadabra'. The substring 'ra' starts at index 3 and 10.

2. Step 2: Confirm output of strfind

   strfind returns starting indices of matches, so output is [3 10].

3. Final Answer:

   [3 10] -> Option A

4. Quick Check:

   Positions of 'ra' = [3 10] [OK]

Hint: Count string indices carefully for strfind output [OK]

Common Mistakes:

• Counting indices starting at 0 instead of 1
• Confusing substring length with position
• Expecting logical output instead of indices

4. Identify the error in this MATLAB code snippet:

str = 'hello world';
pos = strfind('world', str);
disp(pos);

medium

A. disp cannot display arrays

B. Arguments of strfind are reversed

C. strfind cannot search for 'world'

D. Missing semicolon after strfind

Solution

1. Step 1: Check strfind argument order

   strfind expects the main string first, then the substring. Here, arguments are reversed.

2. Step 2: Understand impact of reversed arguments

   Searching 'world' inside 'hello world' is correct, but reversed arguments cause wrong or empty output.

3. Final Answer:

   Arguments of strfind are reversed -> Option B

4. Quick Check:

   strfind(mainString, substring) correct order [OK]

Hint: Main string first, substring second in strfind [OK]

Common Mistakes:

• Swapping arguments in strfind
• Assuming disp can't show arrays
• Thinking semicolon is mandatory for correctness

5. You want to find all positions of the substring 'ana' in str = 'banana bandana' and check if it exists. Which code correctly does this?

hard

A. positions = strfind('ana', str); exists = contains('ana', str);

B. positions = contains(str, 'ana'); exists = strfind(str, 'ana');

C. positions = find(str, 'ana'); exists = contains(str, 'ana');

D. positions = strfind(str, 'ana'); exists = contains(str, 'ana');

Solution

1. Step 1: Use strfind to get positions

   strfind(str, 'ana') returns all starting indices of 'ana' in the string.

2. Step 2: Use contains to check existence

   contains(str, 'ana') returns true if 'ana' is found anywhere in str.

3. Step 3: Verify options

   positions = strfind(str, 'ana'); exists = contains(str, 'ana'); correctly uses both functions. positions = contains(str, 'ana'); exists = strfind(str, 'ana'); swaps roles. positions = strfind('ana', str); exists = contains('ana', str); reverses arguments. positions = find(str, 'ana'); exists = contains(str, 'ana'); uses invalid function find for strings.

4. Final Answer:

   positions = strfind(str, 'ana'); exists = contains(str, 'ana'); -> Option D

5. Quick Check:

   Positions with strfind, existence with contains [OK]

Hint: Use strfind for positions, contains for true/false [OK]

Common Mistakes:

• Swapping arguments in strfind or contains
• Using find instead of strfind
• Confusing output types of contains and strfind