ProgramGuru

Choose Processing Order

Choose Processing Order

A stack serves the most recently added item first; a queue serves the earliest added item first.

Undo history and a school help desk need different orders.

Before this lesson, review Look Up Keys and Unique Values when needed.

What you’ll learn

  • Use stacks and queues to choose Processing Order.
  • Explain how the complete example produces 2 → 1.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

Undo history and a school help desk need different orders.

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>
#include <stack>
#include <queue>

int main()  {
  std::stack<int> undo;
  undo.push(1);
  undo.push(2);
  std::queue<int> help;
  help.push(1);
  help.push(2);
  std::cout << undo.top() << '\n' << help.front() << '\n';
  return 0;
}

Expected output

2
1
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>
#include <stack>
#include <queue>

int main()  {
  std::stack<int> undo;
  undo.push(1);
  undo.push(2);
  std::queue<int> help;
  help.push(1);
  help.push(2);
  std::cout << undo.top() << '\n' << help.front() << '\n';
  return 0;
}

Expected output

2
1

A stack serves the most recently added item first; a queue serves the earliest added item first. Undo history and a school help desk need different orders.

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 → 1. In this example, distinguish what the program calculates or changes from what it actually displays.

Important mistakes and boundaries

Check empty before top, front or pop. Removing an item and retrieving its value are separate operations.

Change the school example

C++17 · edit this complete example

#include <iostream>
#include <stack>
#include <queue>

int main()  {
  std::stack<int> undo;
  undo.push(1);
  undo.push(2);
  std::queue<int> help;
  help.push(1);
  help.push(2);
  std::cout << undo.top() << '\n' << help.front() << '\n';
  return 0;
}

Your task: Push roll 3 afterward. 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: Push roll 3 afterward. The stack’s top becomes 3 while the queue’s front remains 1.

A stack serves the most recently added item first; a queue serves the earliest added item first. Undo history and a school help desk need different orders.

Check empty before top, front or pop. Removing an item and retrieving its value are separate operations.

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>
#include <stack>
#include <queue>

int main()  {
  std::stack<int> undo;
  undo.push(1);
  undo.push(2);
  std::queue<int> help;
  help.push(1);
  help.push(2);
  undo.push(3); help.push(3); std::cout << undo.top() << '\n' << help.front() << '\n';
  return 0;
}

Expected output

3
1
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>
#include <stack>
#include <queue>

int main()  {
  std::stack<int> undo;
  undo.push(1);
  undo.push(2);
  std::queue<int> help;
  help.push(1);
  help.push(2);
  std::cout << undo.top() << '\n' << help.front() << '\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 1

A stack serves the most recently added item first; a queue serves the earliest added item first. Undo history and a school help desk need different orders. Check empty before top, front or pop. Removing an item and retrieving its value are separate operations.

What to remember

A stack serves the most recently added item first; a queue serves the earliest added item first. Check empty before top, front or pop. Removing an item and retrieving its value are separate operations.

Useful connections: Look Up Keys and Unique Values, 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: Point to an Existing Object →