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String search

Learn to search in a string with .find().

The find method

To search for one string inside another you use the find method. The call s.find("needle") returns the index of the first occurrence of the needle. If it is not found, the method returns std::string::npos.

For example std::string s = "paris lyon marseille"; s.find("lyon") returns 6 because "lyon" starts at index 6. Indices count from zero and spaces count as characters too.

Checking with npos

When find cannot locate the searched string it returns std::string::npos, a special constant meaning "not found". To check whether a string contains another you use this pattern:

if (s.find("x") == std::string::npos) { /* not found */ }. This pattern is useful for banned word checks in user input or for looking up an extension in a file name.

Practice Find a city

The sentence variable is set to "paris lyon marseille" and word is set to "lyon". Assign the result of sentence.find(word) to pos and print it with std::cout. The output must be 6.

Starter code

main.cpp
#include <iostream>
#include <string>

int main() {
    std::string sentence = "paris lyon marseille";
    std::string word = "lyon";
    // Find word inside sentence, assign to pos and print
    return 0;
}

Sample solution

main.cpp
#include <iostream>
#include <string>

int main() {
    std::string sentence = "paris lyon marseille";
    std::string word = "lyon";
    int pos = sentence.find(word);
    std::cout << pos << std::endl;
    return 0;
}

Expected output

6

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Write the find call

Question 
std::string s = "Welcome";
int pos = s____("co");
Answer 
std::string s = "Welcome";
int pos = s.find("co");

Explanation: .find("co") returns the index of "co".

2 Match each find call to its result

Term Meaning
s.find("Wel") on "Welcome" 0
s.find("co") on "Welcome" 3
s.find("xy") on "Welcome" std::string::npos
s.find("om") on "Welcome" 4

3 What does find return when the needle is not in the string?

main.cpp
std::string s = "Welcome";
s.find("xy")
  • -1
  • 0
  • std::string::npos Correct answer
  • ""

Explanation: When not found, std::string::npos is returned.

All lessons in Beginner

1 C++ basics

  1. What is C++?
  2. Your first program
  3. std::cout
  4. std::cin
  5. Comments
  6. Summary - C++ basics

2 Variables

  1. Declaring variables
  2. auto
  3. const
  4. constexpr
  5. Type conversion
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. bool and char
  4. std::string
  5. sizeof
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Bitwise operators
  6. Summary - Operators

6 Strings

  1. std::string basics
  2. String concatenation
  3. String length
  4. Substring
  5. String search
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. The switch statement
  4. The ternary operator
  5. Nested conditionals
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. Range-based for
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding products
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. C-style arrays
  2. Array access
  3. std::array
  4. Multidimensional arrays
  5. Arrays and loops
  6. Summary - Arrays

11 std::vector

  1. Declaring vectors
  2. push_back
  3. size and at
  4. Vector access
  5. Vector loop
  6. Summary - std::vector

12 Functions

  1. Defining functions
  2. Return values
  3. Parameters
  4. Function overloading
  5. Default parameters
  6. Summary - Functions

13 References

  1. What is a reference?
  2. Declaring references
  3. Reference parameters
  4. const references
  5. Dangling references
  6. Summary - References

14 Pointer basics

  1. What is a pointer?
  2. The & address operator
  3. Pointer dereference
  4. nullptr
  5. Pointer arithmetic
  6. Summary - Pointer basics

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. String streams
  2. stoi and stod
  3. to_string
  4. String comparison
  5. String modification
  6. Summary - Advanced strings

17 Structs

  1. Defining structs
  2. Using structs
  3. Struct and function
  4. Array of structs
  5. Nested structs
  6. Summary - Structs

18 Exception basics

  1. try/catch
  2. throw
  3. std::exception
  4. Custom exceptions
  5. catch all
  6. Summary - Exceptions

19 Namespaces

  1. Defining namespaces
  2. using
  3. std namespace
  4. Nested namespaces
  5. Namespace alias
  6. Summary - Namespaces

20 Beginner final

  1. Project design
  2. Data model
  3. Business logic
  4. Error handling
  5. Putting it together