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Read and Write a Report File

Read and Write a Report File

File streams connect stream operations to files.

Check both opening and writing, close the output before reading it, then parse the stored report through a separate input stream.

Before this lesson, review Handle a Recoverable Failure when needed.

What you’ll learn

  • Use file input output to read and Write a Report File.
  • 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

Check both opening and writing, close the output before reading it, then parse the stored report through a separate input stream.

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 <fstream>

int main()  {
  {
    std::ofstream out("marks.txt");
    if (!out)  {
      return 1;
    }
    out << 72 << '\n';
    if (!out)  {
      return 1;
    }
  }
  std::ifstream in("marks.txt");
  int mark {
  }
  ;
  if (in >> mark)  {
    std::cout << mark << '\n';
  }
  else  {
    return 1;
  }
  return 0;
}

Expected output

72
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>
#include <fstream>

int main()  {
  {
    std::ofstream out("marks.txt");
    if (!out)  {
      return 1;
    }
    out << 72 << '\n';
    if (!out)  {
      return 1;
    }
  }
  std::ifstream in("marks.txt");
  int mark {
  }
  ;
  if (in >> mark)  {
    std::cout << mark << '\n';
  }
  else  {
    return 1;
  }
  return 0;
}

Expected output

72

File streams connect stream operations to files. Check both opening and writing, close the output before reading it, then parse the stored report through a separate input stream.

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

This writes marks.txt in the working directory. Real files can fail due to permission, space or encoding; do not overwrite valuable files during practice.

Change the school example

C++17 · edit this complete example

#include <iostream>
#include <fstream>

int main()  {
  {
    std::ofstream out("marks.txt");
    if (!out)  {
      return 1;
    }
    out << 72 << '\n';
    if (!out)  {
      return 1;
    }
  }
  std::ifstream in("marks.txt");
  int mark {
  }
  ;
  if (in >> mark)  {
    std::cout << mark << '\n';
  }
  else  {
    return 1;
  }
  return 0;
}

Your task: Write 80 instead. 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: Write 80 instead. A successful read prints 80.

File streams connect stream operations to files. Check both opening and writing, close the output before reading it, then parse the stored report through a separate input stream.

This writes marks.txt in the working directory. Real files can fail due to permission, space or encoding; do not overwrite valuable files during practice.

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 <fstream>

int main()  {
  {
    std::ofstream out("marks.txt");
    if (!out)  {
      return 1;
    }
    out << 80 << '\n';
    if (!out)  {
      return 1;
    }
  }
  std::ifstream in("marks.txt");
  int mark {
  }
  ;
  if (in >> mark)  {
    std::cout << mark << '\n';
  }
  else  {
    return 1;
  }
  return 0;
}

Expected output

80
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>
#include <fstream>

int main()  {
  {
    std::ofstream out("marks.txt");
    if (!out)  {
      return 1;
    }
    out << 72 << '\n';
    if (!out)  {
      return 1;
    }
  }
  std::ifstream in("marks.txt");
  int mark {
  }
  ;
  if (in >> mark)  {
    std::cout << mark << '\n';
  }
  else  {
    return 1;
  }
  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

File streams connect stream operations to files. Check both opening and writing, close the output before reading it, then parse the stored report through a separate input stream. This writes marks.txt in the working directory. Real files can fail due to permission, space or encoding; do not overwrite valuable files during practice.

What to remember

File streams connect stream operations to files. This writes marks.txt in the working directory. Real files can fail due to permission, space or encoding; do not overwrite valuable files during practice.

Useful connections: Handle a Recoverable Failure, 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: Validate a Complete Record →