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Visit Rows and Columns

Visit Rows and Columns

A nested loop runs an inner traversal for each outer value.

A classroom seating grid can visit every row-column pair rather than only matching equal indexes.

Before this lesson, review Stop or Skip an Iteration when needed.

What you’ll learn

  • Use nested loops to visit Rows and Columns.
  • Explain how the complete example produces 1,1 → 1,2 → 2,1 → 2,2.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

A classroom seating grid can visit every row-column pair rather than only matching equal indexes.

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>

int main()  {
  for (int row = 1; row <= 2; ++row)  {
    for (int column = 1; column <= 2; ++column)  {
      std::cout << row << ',' << column << '\n';
    }
  }
  return 0;
}

Expected output

1,1
1,2
2,1
2,2
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>

int main()  {
  for (int row = 1; row <= 2; ++row)  {
    for (int column = 1; column <= 2; ++column)  {
      std::cout << row << ',' << column << '\n';
    }
  }
  return 0;
}

Expected output

1,1
1,2
2,1
2,2

A nested loop runs an inner traversal for each outer value. A classroom seating grid can visit every row-column pair rather than only matching equal indexes.

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 1,1 → 1,2 → 2,1 → 2,2. In this example, distinguish what the program calculates or changes from what it actually displays.

Important mistakes and boundaries

Two loops with r rows and c columns usually perform r times c inner visits. Work can grow quickly as both dimensions increase.

Change the school example

C++17 · edit this complete example

#include <iostream>

int main()  {
  for (int row = 1; row <= 2; ++row)  {
    for (int column = 1; column <= 2; ++column)  {
      std::cout << row << ',' << column << '\n';
    }
  }
  return 0;
}

Your task: Restrict the outer loop to row 1. 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: Restrict the outer loop to row 1. The remaining pairs are 1,1 and 1,2.

A nested loop runs an inner traversal for each outer value. A classroom seating grid can visit every row-column pair rather than only matching equal indexes.

Two loops with r rows and c columns usually perform r times c inner visits. Work can grow quickly as both dimensions increase.

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>

int main()  {
  for (int row = 1; row <= 1; ++row)  {
    for (int column = 1; column <= 2; ++column)  {
      std::cout << row << ',' << column << '\n';
    }
  }
  return 0;
}

Expected output

1,1
1,2
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>

int main()  {
  for (int row = 1; row <= 2; ++row)  {
    for (int column = 1; column <= 2; ++column)  {
      std::cout << row << ',' << column << '\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

1,1 1,2 2,1 2,2

A nested loop runs an inner traversal for each outer value. A classroom seating grid can visit every row-column pair rather than only matching equal indexes. Two loops with r rows and c columns usually perform r times c inner visits. Work can grow quickly as both dimensions increase.

What to remember

A nested loop runs an inner traversal for each outer value. Two loops with r rows and c columns usually perform r times c inner visits. Work can grow quickly as both dimensions increase.

Useful connections: Stop or Skip an Iteration, 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: Give a Task a Name →