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Use a Fixed Collection

Use a Fixed Collection

A fixed-size array stores a known number of elements.

Its first position is index 0, so a three-subject report uses indexes 0, 1 and 2.

Before this lesson, review Organize Names and Declarations when needed.

What you’ll learn

  • Use arrays to use a Fixed Collection.
  • Explain how the complete example produces 70 → 3.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

Its first position is index 0, so a three-subject report uses indexes 0, 1 and 2.

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 <array>

int main()  {
  std::array<int, 3> marks {
    70, 80, 90
  }
  ;
  std::cout << marks.at(0) << '\n' << marks.size() << '\n';
  return 0;
}

Expected output

70
3
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>
#include <array>

int main()  {
  std::array<int, 3> marks {
    70, 80, 90
  }
  ;
  std::cout << marks.at(0) << '\n' << marks.size() << '\n';
  return 0;
}

Expected output

70
3

A fixed-size array stores a known number of elements. Its first position is index 0, so a three-subject report uses indexes 0, 1 and 2.

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

Important mistakes and boundaries

An index equal to size is already out of range. at checks the bound; unchecked subscripting requires you to guarantee a valid position.

Change the school example

C++17 · edit this complete example

#include <iostream>
#include <array>

int main()  {
  std::array<int, 3> marks {
    70, 80, 90
  }
  ;
  std::cout << marks.at(0) << '\n' << marks.size() << '\n';
  return 0;
}

Your task: Replace the first element with 60. 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 first element with 60. The lines become 60 and 3.

A fixed-size array stores a known number of elements. Its first position is index 0, so a three-subject report uses indexes 0, 1 and 2.

An index equal to size is already out of range. at checks the bound; unchecked subscripting requires you to guarantee a valid position.

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 <array>

int main()  {
  std::array<int, 3> marks {
    60, 80, 90
  }
  ;
  std::cout << marks.at(0) << '\n' << marks.size() << '\n';
  return 0;
}

Expected output

60
3
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>
#include <array>

int main()  {
  std::array<int, 3> marks {
    70, 80, 90
  }
  ;
  std::cout << marks.at(0) << '\n' << marks.size() << '\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

70 3

A fixed-size array stores a known number of elements. Its first position is index 0, so a three-subject report uses indexes 0, 1 and 2. An index equal to size is already out of range. at checks the bound; unchecked subscripting requires you to guarantee a valid position.

What to remember

A fixed-size array stores a known number of elements. An index equal to size is already out of range. at checks the bound; unchecked subscripting requires you to guarantee a valid position.

Useful connections: Organize Names and Declarations, Name and Update Values.

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: Grow a Class List →