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Pass a Small Operation

Pass a Small Operation

A lambda creates an unnamed callable object.

Capturing a school threshold by value lets an algorithm count passing marks using a rule supplied at the call site.

Before this lesson, review Order and Find Records when needed.

What you’ll learn

  • Use lambda expressions to pass a Small Operation.
  • Explain how the complete example produces 2.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

Capturing a school threshold by value lets an algorithm count passing marks using a rule supplied at the call site.

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>
#include <algorithm>

int main()  {
  std::vector<int> marks {
    40, 72, 80
  }
  ;
  int threshold = 50;
  auto count = std::count_if(marks.begin(), marks.end(), [threshold](int mark)  {
    return mark >= threshold;
  }
  );
  std::cout << count << '\n';
  return 0;
}

Expected output

2
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>
#include <vector>
#include <algorithm>

int main()  {
  std::vector<int> marks {
    40, 72, 80
  }
  ;
  int threshold = 50;
  auto count = std::count_if(marks.begin(), marks.end(), [threshold](int mark)  {
    return mark >= threshold;
  }
  );
  std::cout << count << '\n';
  return 0;
}

Expected output

2

A lambda creates an unnamed callable object. Capturing a school threshold by value lets an algorithm count passing marks using a rule supplied at the call site.

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

Important mistakes and boundaries

Reference captures must not outlive their referenced objects. Capturing by value snapshots the value rather than tracking later changes.

Change the school example

C++17 · edit this complete example

#include <iostream>
#include <vector>
#include <algorithm>

int main()  {
  std::vector<int> marks {
    40, 72, 80
  }
  ;
  int threshold = 50;
  auto count = std::count_if(marks.begin(), marks.end(), [threshold](int mark)  {
    return mark >= threshold;
  }
  );
  std::cout << count << '\n';
  return 0;
}

Your task: Use threshold 75. 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 threshold 75. Only the mark 80 passes, so the count is 1.

A lambda creates an unnamed callable object. Capturing a school threshold by value lets an algorithm count passing marks using a rule supplied at the call site.

Reference captures must not outlive their referenced objects. Capturing by value snapshots the value rather than tracking later changes.

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>
#include <algorithm>

int main()  {
  std::vector<int> marks {
    40, 72, 80
  }
  ;
  int threshold = 75;
  auto count = std::count_if(marks.begin(), marks.end(), [threshold](int mark)  {
    return mark >= threshold;
  }
  );
  std::cout << count << '\n';
  return 0;
}

Expected output

1
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>
#include <vector>
#include <algorithm>

int main()  {
  std::vector<int> marks {
    40, 72, 80
  }
  ;
  int threshold = 50;
  auto count = std::count_if(marks.begin(), marks.end(), [threshold](int mark)  {
    return mark >= threshold;
  }
  );
  std::cout << count << '\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

A lambda creates an unnamed callable object. Capturing a school threshold by value lets an algorithm count passing marks using a rule supplied at the call site. Reference captures must not outlive their referenced objects. Capturing by value snapshots the value rather than tracking later changes.

What to remember

A lambda creates an unnamed callable object. Reference captures must not outlive their referenced objects. Capturing by value snapshots the value rather than tracking later changes.

Useful connections: Order and Find Records, 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: Summarize and Convert Values →