ProgramGuru

Make Evaluation Clear

Make Evaluation Clear

Precedence groups expressions: multiplication groups before addition.

Parentheses make a different intended grouping explicit and help a learner check a calculation independently.

Before this lesson, review Combine Conditions when needed.

What you’ll learn

  • Use operator precedence to make Evaluation Clear.
  • Explain how the complete example produces 20 → 30.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

Parentheses make a different intended grouping explicit and help a learner check a calculation independently.

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()  {
  std::cout << 10 + 5 * 2 << '\n' << (10 + 5) * 2 << '\n';
  return 0;
}

Expected output

20
30
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>

int main()  {
  std::cout << 10 + 5 * 2 << '\n' << (10 + 5) * 2 << '\n';
  return 0;
}

Expected output

20
30

Precedence groups expressions: multiplication groups before addition. Parentheses make a different intended grouping explicit and help a learner check a calculation independently.

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

Important mistakes and boundaries

Grouping rules are not a license to rely on unclear evaluation order or multiple side effects on the same variable.

Change the school example

C++17 · edit this complete example

#include <iostream>

int main()  {
  std::cout << 10 + 5 * 2 << '\n' << (10 + 5) * 2 << '\n';
  return 0;
}

Your task: Change 10 to 20. 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 10 to 20. The two results become 30 and 50.

Precedence groups expressions: multiplication groups before addition. Parentheses make a different intended grouping explicit and help a learner check a calculation independently.

Grouping rules are not a license to rely on unclear evaluation order or multiple side effects on the same variable.

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()  {
  std::cout << 20 + 5 * 2 << '\n' << (20 + 5) * 2 << '\n';
  return 0;
}

Expected output

30
50
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>

int main()  {
  std::cout << 10 + 5 * 2 << '\n' << (10 + 5) * 2 << '\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

20 30

Precedence groups expressions: multiplication groups before addition. Parentheses make a different intended grouping explicit and help a learner check a calculation independently. Grouping rules are not a license to rely on unclear evaluation order or multiple side effects on the same variable.

What to remember

Precedence groups expressions: multiplication groups before addition. Grouping rules are not a license to rely on unclear evaluation order or multiple side effects on the same variable.

Useful connections: Combine Conditions, 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: Store Student Names →