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Substring

Learn to take part of a string with .substr().

The substr method

To take only part of a string you use the substr method. It is called as s.substr(start, len); start is the starting index and len is how many characters to take. Indices are zero-based.

For example std::string s = "Computer"; s.substr(0, 4) returns "Comp". If you omit the second parameter, s.substr(start) returns everything from the start index to the end of the string.

Single-parameter substr

If you pass only the start index to substr, you get the rest of the string from that index on. std::string s = "Programming"; s.substr(7) returns "ming".

This short form is handy for separating file extensions or suffixes. Negative indices are not supported in C++; the index must be between 0 and s.length().

Practice Print the first four characters

The text variable is set to "Computer". Take the first four characters with text.substr(0, 4) into the part variable and print it with std::cout. The output must be Comp.

Starter code

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

int main() {
    std::string text = "Computer";
    // Take the first four characters into part and print
    return 0;
}

Sample solution

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

int main() {
    std::string text = "Computer";
    std::string part = text.substr(0, 4);
    std::cout << part << std::endl;
    return 0;
}

Expected output

Comp

Questions and answers

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

1 What does std::string s = "Computer"; s.substr(4, 3) return?

main.cpp
std::string s = "Computer";
s.substr(4, 3)
  • "Com"
  • "ute" Correct answer
  • "Comp"
  • "uter"

Explanation: From index 4 take 3 characters: "ute".

2 Write the substr start parameter

Question 
std::string s = "Welcome";
std::string part = s.substr(____, 3);
Answer 
std::string s = "Welcome";
std::string part = s.substr(0, 3);

Explanation: substr(0, 3) returns the first three characters.

3 Match each substr call to its result

Term Meaning
s.substr(0, 3) on "Welcome" "Wel"
s.substr(3) on "Welcome" "come"
s.substr(0, 1) on "Welcome" "W"
s.substr(4, 2) on "Welcome" "om"

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