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Convert Values Deliberately

Convert Values Deliberately

A conversion changes how a value is represented.

static_cast makes an intentional numeric conversion visible, such as displaying the whole-number part of a decimal average.

Before this lesson, review Protect Fixed Values when needed.

What you’ll learn

  • Use type conversion to convert Values Deliberately.
  • Explain how the complete example produces 72.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

static_cast makes an intentional numeric conversion visible, such as displaying the whole-number part of a decimal average.

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()  {
  double average = 72.9;
  int whole = static_cast<int>(average);
  std::cout << whole << '\n';
  return 0;
}

Expected output

72
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>

int main()  {
  double average = 72.9;
  int whole = static_cast<int>(average);
  std::cout << whole << '\n';
  return 0;
}

Expected output

72

A conversion changes how a value is represented. static_cast makes an intentional numeric conversion visible, such as displaying the whole-number part of a decimal average.

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

Important mistakes and boundaries

Conversion to int truncates a representable fractional value toward zero; it does not round. Out-of-range conversions need validation, not wishful casting.

Change the school example

C++17 · edit this complete example

#include <iostream>

int main()  {
  double average = 72.9;
  int whole = static_cast<int>(average);
  std::cout << whole << '\n';
  return 0;
}

Your task: Change the average to 75.2. 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 average to 75.2. The displayed whole part is 75.

A conversion changes how a value is represented. static_cast makes an intentional numeric conversion visible, such as displaying the whole-number part of a decimal average.

Conversion to int truncates a representable fractional value toward zero; it does not round. Out-of-range conversions need validation, not wishful casting.

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()  {
  double average = 75.2;
  int whole = static_cast<int>(average);
  std::cout << whole << '\n';
  return 0;
}

Expected output

75
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>

int main()  {
  double average = 72.9;
  int whole = static_cast<int>(average);
  std::cout << whole << '\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

72

A conversion changes how a value is represented. static_cast makes an intentional numeric conversion visible, such as displaying the whole-number part of a decimal average. Conversion to int truncates a representable fractional value toward zero; it does not round. Out-of-range conversions need validation, not wishful casting.

What to remember

A conversion changes how a value is represented. Conversion to int truncates a representable fractional value toward zero; it does not round. Out-of-range conversions need validation, not wishful casting.

Useful connections: Protect Fixed 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: Let the Compiler Deduce a Type →