ProgramGuru

Summarize and Convert Values

Summarize and Convert Values

Accumulate combines a range into one result.

Transform applies an operation to each input position and writes corresponding outputs, making a marks adjustment explicit.

Before this lesson, review Pass a Small Operation when needed.

What you’ll learn

  • Use accumulate and transform to summarize and Convert Values.
  • Explain how the complete example produces 150 → 72.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

Transform applies an operation to each input position and writes corresponding outputs, making a marks adjustment explicit.

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>
#include <numeric>

int main()  {
  std::vector<int> marks {
    70, 80
  }
  ;
  int total = std::accumulate(marks.begin(), marks.end(), 0);
  std::transform(marks.begin(), marks.end(), marks.begin(), [](int mark)  {
    return mark + 2;
  }
  );
  std::cout << total << '\n' << marks.at(0) << '\n';
  return 0;
}

Expected output

150
72
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

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

int main()  {
  std::vector<int> marks {
    70, 80
  }
  ;
  int total = std::accumulate(marks.begin(), marks.end(), 0);
  std::transform(marks.begin(), marks.end(), marks.begin(), [](int mark)  {
    return mark + 2;
  }
  );
  std::cout << total << '\n' << marks.at(0) << '\n';
  return 0;
}

Expected output

150
72

Accumulate combines a range into one result. Transform applies an operation to each input position and writes corresponding outputs, making a marks adjustment explicit.

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

Important mistakes and boundaries

The initial accumulation value helps determine its result type. Output ranges must have sufficient space; writing past their end is invalid.

Change the school example

C++17 · edit this complete example

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

int main()  {
  std::vector<int> marks {
    70, 80
  }
  ;
  int total = std::accumulate(marks.begin(), marks.end(), 0);
  std::transform(marks.begin(), marks.end(), marks.begin(), [](int mark)  {
    return mark + 2;
  }
  );
  std::cout << total << '\n' << marks.at(0) << '\n';
  return 0;
}

Your task: Add 3 instead of 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: Add 3 instead of 2. The original total is still 150 and the adjusted first mark is 73.

Accumulate combines a range into one result. Transform applies an operation to each input position and writes corresponding outputs, making a marks adjustment explicit.

The initial accumulation value helps determine its result type. Output ranges must have sufficient space; writing past their end is invalid.

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>
#include <numeric>

int main()  {
  std::vector<int> marks {
    70, 80
  }
  ;
  int total = std::accumulate(marks.begin(), marks.end(), 0);
  std::transform(marks.begin(), marks.end(), marks.begin(), [](int mark)  {
    return mark + 3;
  }
  );
  std::cout << total << '\n' << marks.at(0) << '\n';
  return 0;
}

Expected output

150
73
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

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

int main()  {
  std::vector<int> marks {
    70, 80
  }
  ;
  int total = std::accumulate(marks.begin(), marks.end(), 0);
  std::transform(marks.begin(), marks.end(), marks.begin(), [](int mark)  {
    return mark + 2;
  }
  );
  std::cout << total << '\n' << marks.at(0) << '\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

150 72

Accumulate combines a range into one result. Transform applies an operation to each input position and writes corresponding outputs, making a marks adjustment explicit. The initial accumulation value helps determine its result type. Output ranges must have sufficient space; writing past their end is invalid.

What to remember

Accumulate combines a range into one result. The initial accumulation value helps determine its result type. Output ranges must have sufficient space; writing past their end is invalid.

Useful connections: Pass a Small Operation, 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: Represent Missing or Alternative Data →