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The do-while loop

Learn loops that run at least once with do-while.

The do-while loop

When you want to run the body at least once you use the do-while loop. Its form is do { ... } while (condition);; the body runs first, then the condition is checked.

For example int n = 0; do { std::cout << n; n++; } while (n < 3); prints 0, 1 and 2. The difference from while is that the condition is checked at the end, so the body always runs at least once. This makes it ideal for asking the user at least one question.

do-while vs while

The key difference between do-while and while is where the condition is checked. In while it is checked at the top, in do-while at the bottom. Because of this, do-always runs its body at least once.

This matters for menu display: show the options once, get the answer, ask again if invalid. The pattern do { ... } while (choice != 0); shows the menu again until option 0 is entered.

Practice Print once (do-while)

The n variable is set to 5. Write a do { std::cout << n; n++; } while (n < 3); loop. Even though the condition is false, the body runs once; the output must be 5.

Starter code

main.cpp
#include <iostream>

int main() {
    int n = 5;
    // Print with do-while
    return 0;
}

Sample solution

main.cpp
#include <iostream>

int main() {
    int n = 5;
    do {
        std::cout << n;
        n++;
    } while (n < 3);
    std::cout << std::endl;
    return 0;
}

Expected output

5

Questions and answers

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

1 Write the do-while closing

Question 
do {
    std::cout << n;
    n++;
} ____ (n < 3);
Answer 
do {
    std::cout << n;
    n++;
} while (n < 3);

Explanation: do-while ends with while and a semicolon.

2 At least how many times does a do-while loop run?

main.cpp
do { ... } while (cond);
  • 1 time Correct answer
  • 0 times
  • 2 times
  • Forever

Explanation: The body runs first; even if the condition is false it loops at least once.

3 Match each do-while part to its role

Term Meaning
do Starts the body
{ ... } Loop body
while Asks the condition
(n < 3) Continue condition

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