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Read and Display Input

Read and Display Input

A stream extracts typed values from text.

Check extraction before using a student mark, so missing or nonnumeric input cannot silently become a valid report.

Before this lesson, review Find and Fix Errors when needed.

What you’ll learn

  • Use input output to read and Display Input.
  • 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 extraction before using a student mark, so missing or nonnumeric input cannot silently become a valid report.

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 <sstream>

int main()  {
  std::istringstream input("72");
  int marks {
  }
  ;
  if (input >> marks)  {
    std::cout << marks << '\n';
  }
  else  {
    std::cout << "Invalid input" << '\n';
  }
  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 <sstream>

int main()  {
  std::istringstream input("72");
  int marks {
  }
  ;
  if (input >> marks)  {
    std::cout << marks << '\n';
  }
  else  {
    std::cout << "Invalid input" << '\n';
  }
  return 0;
}

Expected output

72

A stream extracts typed values from text. Check extraction before using a student mark, so missing or nonnumeric input cannot silently become a valid report.

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 example uses a fixed text stream for reproducibility. std::cin reads live input; extraction success and mark-range validity are separate checks.

Change the school example

C++17 · edit this complete example

#include <iostream>
#include <sstream>

int main()  {
  std::istringstream input("72");
  int marks {
  }
  ;
  if (input >> marks)  {
    std::cout << marks << '\n';
  }
  else  {
    std::cout << "Invalid input" << '\n';
  }
  return 0;
}

Your task: Replace the input text 72 with absent. 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: Replace the input text 72 with absent. Extraction fails and Invalid input prints.

A stream extracts typed values from text. Check extraction before using a student mark, so missing or nonnumeric input cannot silently become a valid report.

This example uses a fixed text stream for reproducibility. std::cin reads live input; extraction success and mark-range validity are separate checks.

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 <sstream>

int main()  {
  std::istringstream input("absent");
  int marks {
  }
  ;
  if (input >> marks)  {
    std::cout << marks << '\n';
  }
  else  {
    std::cout << "Invalid input" << '\n';
  }
  return 0;
}

Expected output

Invalid input
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>
#include <sstream>

int main()  {
  std::istringstream input("72");
  int marks {
  }
  ;
  if (input >> marks)  {
    std::cout << marks << '\n';
  }
  else  {
    std::cout << "Invalid input" << '\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

72

A stream extracts typed values from text. Check extraction before using a student mark, so missing or nonnumeric input cannot silently become a valid report. This example uses a fixed text stream for reproducibility. std::cin reads live input; extraction success and mark-range validity are separate checks.

What to remember

A stream extracts typed values from text. This example uses a fixed text stream for reproducibility. std::cin reads live input; extraction success and mark-range validity are separate checks.

Useful connections: Find and Fix Errors, 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: Name and Update Values →