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Look Up Keys and Unique Values

Look Up Keys and Unique Values

A map associates keys with values, while a set stores unique keys.

The school can retrieve a mark by roll number and count distinct class labels.

Before this lesson, review Name a Fixed Set of States when needed.

What you’ll learn

  • Use maps and sets to look Up Keys and Unique Values.
  • Explain how the complete example produces 80 → 2.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

The school can retrieve a mark by roll number and count distinct class labels.

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 <string>
#include <map>
#include <set>

int main()  {
  std::map<int, int> marks {
    {
      1, 72
    }
    ,  {
      2, 80
    }
  }
  ;
  std::set<std::string> classes {
    "10A", "10A", "9B"
  }
  ;
  std::cout << marks.at(2) << '\n' << classes.size() << '\n';
  return 0;
}

Expected output

80
2
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>
#include <string>
#include <map>
#include <set>

int main()  {
  std::map<int, int> marks {
    {
      1, 72
    }
    ,  {
      2, 80
    }
  }
  ;
  std::set<std::string> classes {
    "10A", "10A", "9B"
  }
  ;
  std::cout << marks.at(2) << '\n' << classes.size() << '\n';
  return 0;
}

Expected output

80
2

A map associates keys with values, while a set stores unique keys. The school can retrieve a mark by roll number and count distinct class labels.

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

Important mistakes and boundaries

Map operator[] can insert a missing key. Use find or at when reading should not change the collection; unordered containers do not promise sorted iteration.

Change the school example

C++17 · edit this complete example

#include <iostream>
#include <string>
#include <map>
#include <set>

int main()  {
  std::map<int, int> marks {
    {
      1, 72
    }
    ,  {
      2, 80
    }
  }
  ;
  std::set<std::string> classes {
    "10A", "10A", "9B"
  }
  ;
  std::cout << marks.at(2) << '\n' << classes.size() << '\n';
  return 0;
}

Your task: Change roll 2’s mark to 85. 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: Change roll 2’s mark to 85. The lines become 85 and 2.

A map associates keys with values, while a set stores unique keys. The school can retrieve a mark by roll number and count distinct class labels.

Map operator[] can insert a missing key. Use find or at when reading should not change the collection; unordered containers do not promise sorted iteration.

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 <string>
#include <map>
#include <set>

int main()  {
  std::map<int, int> marks {
    {
      1, 72
    }
    ,  {
      2, 85
    }
  }
  ;
  std::set<std::string> classes {
    "10A", "10A", "9B"
  }
  ;
  std::cout << marks.at(2) << '\n' << classes.size() << '\n';
  return 0;
}

Expected output

85
2
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>
#include <string>
#include <map>
#include <set>

int main()  {
  std::map<int, int> marks {
    {
      1, 72
    }
    ,  {
      2, 80
    }
  }
  ;
  std::set<std::string> classes {
    "10A", "10A", "9B"
  }
  ;
  std::cout << marks.at(2) << '\n' << classes.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

80 2

A map associates keys with values, while a set stores unique keys. The school can retrieve a mark by roll number and count distinct class labels. Map operator[] can insert a missing key. Use find or at when reading should not change the collection; unordered containers do not promise sorted iteration.

What to remember

A map associates keys with values, while a set stores unique keys. Map operator[] can insert a missing key. Use find or at when reading should not change the collection; unordered containers do not promise sorted iteration.

Useful connections: Name a Fixed Set of States, 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: Choose Processing Order →