ProgramGuru

Select a Labeled Case

Select a Labeled Case

Switch selects a case matching an integral or enumeration value.

A school timetable can map a day number to one subject without repeated equality tests.

Before this lesson, review Order Grade Rules when needed.

What you’ll learn

  • Use switch to select a Labeled Case.
  • Explain how the complete example produces Science.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

A school timetable can map a day number to one subject without repeated equality tests.

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 day = 2;
  switch (day)  {
    case 1: std::cout << "Maths" << '\n';
    break;
    case 2: std::cout << "Science" << '\n';
    break;
    default: std::cout << "No class" << '\n';
  }
  return 0;
}

Expected output

Science
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>

int main()  {
  int day = 2;
  switch (day)  {
    case 1: std::cout << "Maths" << '\n';
    break;
    case 2: std::cout << "Science" << '\n';
    break;
    default: std::cout << "No class" << '\n';
  }
  return 0;
}

Expected output

Science

Switch selects a case matching an integral or enumeration value. A school timetable can map a day number to one subject without repeated equality tests.

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

Important mistakes and boundaries

A case can fall through without break. Standard C++ switch does not directly switch on std::string.

Change the school example

C++17 · edit this complete example

#include <iostream>

int main()  {
  int day = 2;
  switch (day)  {
    case 1: std::cout << "Maths" << '\n';
    break;
    case 2: std::cout << "Science" << '\n';
    break;
    default: std::cout << "No class" << '\n';
  }
  return 0;
}

Your task: Change day to 1. 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 day to 1. Maths prints, and break prevents later cases from running.

Switch selects a case matching an integral or enumeration value. A school timetable can map a day number to one subject without repeated equality tests.

A case can fall through without break. Standard C++ switch does not directly switch on std::string.

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 day = 1;
  switch (day)  {
    case 1: std::cout << "Maths" << '\n';
    break;
    case 2: std::cout << "Science" << '\n';
    break;
    default: std::cout << "No class" << '\n';
  }
  return 0;
}

Expected output

Maths
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>

int main()  {
  int day = 2;
  switch (day)  {
    case 1: std::cout << "Maths" << '\n';
    break;
    case 2: std::cout << "Science" << '\n';
    break;
    default: std::cout << "No class" << '\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

Science

Switch selects a case matching an integral or enumeration value. A school timetable can map a day number to one subject without repeated equality tests. A case can fall through without break. Standard C++ switch does not directly switch on std::string.

What to remember

Switch selects a case matching an integral or enumeration value. A case can fall through without break. Standard C++ switch does not directly switch on std::string.

Useful connections: Order Grade Rules, 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: Choose a Short Value →