ProgramGuru

Visit Each Element

Visit Each Element

A range-based for loop visits a sequence’s elements.

Use a value variable for a copy and a reference variable when the school intends to update stored marks.

Before this lesson, review Grow a Class List when needed.

What you’ll learn

  • Use range based for to visit Each Element.
  • Explain how the complete example produces 71 → 81.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

Use a value variable for a copy and a reference variable when the school intends to update stored marks.

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 <vector>

int main()  {
  std::vector<int> marks {
    70, 80
  }
  ;
  for (int& mark : marks)  {
    ++mark;
  }
  for (int mark : marks)  {
    std::cout << mark << '\n';
  }
  return 0;
}

Expected output

71
81
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>
#include <vector>

int main()  {
  std::vector<int> marks {
    70, 80
  }
  ;
  for (int& mark : marks)  {
    ++mark;
  }
  for (int mark : marks)  {
    std::cout << mark << '\n';
  }
  return 0;
}

Expected output

71
81

A range-based for loop visits a sequence’s elements. Use a value variable for a copy and a reference variable when the school intends to update stored marks.

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

Important mistakes and boundaries

Changing a copied int loop variable does not replace the stored element. Structural container changes during traversal can invalidate the iteration.

Change the school example

C++17 · edit this complete example

#include <iostream>
#include <vector>

int main()  {
  std::vector<int> marks {
    70, 80
  }
  ;
  for (int& mark : marks)  {
    ++mark;
  }
  for (int mark : marks)  {
    std::cout << mark << '\n';
  }
  return 0;
}

Your task: Use mark += 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: Use mark += 2. The stored marks become 72 and 82.

A range-based for loop visits a sequence’s elements. Use a value variable for a copy and a reference variable when the school intends to update stored marks.

Changing a copied int loop variable does not replace the stored element. Structural container changes during traversal can invalidate the iteration.

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 <vector>

int main()  {
  std::vector<int> marks {
    70, 80
  }
  ;
  for (int& mark : marks)  {
    mark += 2;
  }
  for (int mark : marks)  {
    std::cout << mark << '\n';
  }
  return 0;
}

Expected output

72
82
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>
#include <vector>

int main()  {
  std::vector<int> marks {
    70, 80
  }
  ;
  for (int& mark : marks)  {
    ++mark;
  }
  for (int mark : marks)  {
    std::cout << mark << '\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

71 81

A range-based for loop visits a sequence’s elements. Use a value variable for a copy and a reference variable when the school intends to update stored marks. Changing a copied int loop variable does not replace the stored element. Structural container changes during traversal can invalidate the iteration.

What to remember

A range-based for loop visits a sequence’s elements. Changing a copied int loop variable does not replace the stored element. Structural container changes during traversal can invalidate the iteration.

Useful connections: Grow a Class List, 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: Inspect and Edit Names →