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Repeat Until a Condition Changes

Repeat Until a Condition Changes

A while loop checks before its body; do-while checks afterward.

That difference determines whether a school action can happen zero times or must happen at least once.

Before this lesson, review Repeat a Known Range when needed.

What you’ll learn

  • Use while and do while to repeat Until a Condition Changes.
  • Explain how the complete example produces 2 → 3.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

That difference determines whether a school action can happen zero times or must happen at least once.

Read the declarations above main before tracing its statements. Each declared name supplies the value, type or operation used later. The std:: prefix selects a name from the standard library, and cout << sends a value to the output stream.

C++17 · main.cpp · complete program

#include <iostream>

int main()  {
  int count = 0;
  while (count < 2)  {
    ++count;
  }
  std::cout << count << '\n';
  do  {
    ++count;
  }
  while (false);
  std::cout << count << '\n';
  return 0;
}

Expected output

2
3
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>

int main()  {
  int count = 0;
  while (count < 2)  {
    ++count;
  }
  std::cout << count << '\n';
  do  {
    ++count;
  }
  while (false);
  std::cout << count << '\n';
  return 0;
}

Expected output

2
3

A while loop checks before its body; do-while checks afterward. That difference determines whether a school action can happen zero times or must happen at least once.

Trace the result: Read the complete program below from its starting values to the output operations. Each std::cout statement displays the value supplied at that point; \n begins the next line. The resulting lines, in order, are 2 → 3. In this example, distinguish what the program calculates or changes from what it actually displays.

Important mistakes and boundaries

A body must eventually reach a state that stops repetition. A false do-while condition does not undo its first body execution.

Change the school example

C++17 · edit this complete example

#include <iostream>

int main()  {
  int count = 0;
  while (count < 2)  {
    ++count;
  }
  std::cout << count << '\n';
  do  {
    ++count;
  }
  while (false);
  std::cout << count << '\n';
  return 0;
}

Your task: Change the while bound to 3. Before running your edited program, predict the new output and explain which stored value, condition or object changes. Keep the rest of the example intact.

Use the complete source repeated here; this task does not depend on remembering code from another section.

Worked answer and reasoning

Worked variation: Change the while bound to 3. The two printed counts become 3 and 4.

A while loop checks before its body; do-while checks afterward. That difference determines whether a school action can happen zero times or must happen at least once.

A body must eventually reach a state that stops repetition. A false do-while condition does not undo its first body execution.

For a baseline run, save this source as main.cpp and use clang++ -std=c++17 -Wall -Wextra -pedantic main.cpp -o school, or equivalent g++ flags. Recompile after editing so that you run the updated executable.

The source and output below belong to the edited program. Compare them with the original example to see the consequence of this single change.

C++17 · worked variation · complete program

#include <iostream>

int main()  {
  int count = 0;
  while (count < 3)  {
    ++count;
  }
  std::cout << count << '\n';
  do  {
    ++count;
  }
  while (false);
  std::cout << count << '\n';
  return 0;
}

Expected output

3
4
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>

int main()  {
  int count = 0;
  while (count < 2)  {
    ++count;
  }
  std::cout << count << '\n';
  do  {
    ++count;
  }
  while (false);
  std::cout << count << '\n';
  return 0;
}
Before making the practice edit, what does the original C++17 program print?
Choose one answer. You can try again.

Answer and explanation

2 3

A while loop checks before its body; do-while checks afterward. That difference determines whether a school action can happen zero times or must happen at least once. A body must eventually reach a state that stops repetition. A false do-while condition does not undo its first body execution.

What to remember

A while loop checks before its body; do-while checks afterward. A body must eventually reach a state that stops repetition. A false do-while condition does not undo its first body execution.

Useful connections: Repeat a Known Range, Give a Task a Name.

Language and design guidance: C++ Core Guidelines and C++ working draft. The draft describes an evolving language; these examples deliberately target C++17.

Reaching the recap and completing the quizzes are separate milestones.

    Next lesson: Stop or Skip an Iteration →