String searching (contains, strfind) in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the contains function do in MATLAB?

The contains function checks if a string or array of strings contains a specified substring and returns a logical true or false.

beginner

How does strfind differ from contains in MATLAB?

strfind returns the starting indices of the substring within the string, while contains returns a logical value indicating presence or absence.

beginner

What is the output of strfind('hello world', 'o')?

It returns the indices where 'o' appears: [5 8].

intermediate

Can contains handle arrays of strings in MATLAB?

Yes, contains can check each element of a string array or cell array of character vectors and return a logical array.

beginner

What happens if strfind does not find the substring?

It returns an empty array [] indicating no matches.

What does contains('apple', 'p') return?

A true

B false

C [2 3]

D []

contains returns true if the substring is found anywhere in the string.

What is the output of strfind('banana', 'na')?

A [3 5]

B [2 4]

C [4 6]

D []

strfind returns the starting indices of each occurrence of 'na' in 'banana', which are positions 3 and 5.

If strfind does not find the substring, what does it return?

A 0

B false

C empty array []

D NaN

strfind returns an empty array [] when no match is found.

Which function returns a logical array when searching multiple strings?

A strfind

B contains

C find

D strcmp

contains returns a logical array indicating presence of the substring in each element.

What is the difference between contains and strfind?

A <code>contains</code> returns indices, <code>strfind</code> returns logical

B Both return logical arrays

C Both return indices

D <code>contains</code> returns logical, <code>strfind</code> returns indices

contains returns true/false, while strfind returns the starting positions of the substring.

Explain how to use contains to check if a substring exists in multiple strings.

Think about searching in a list of words.

Describe what strfind returns and how it can be used to find substring positions.

Imagine looking for where a word starts inside a sentence.

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