C++ Build the Logic

What is Logic?

In the previous chapter, you learned the four steps of problem solving. For many beginners, step 2 is the hardest: you understand the problem, but you do not know what code to write.

The plan is called the logic of the program: the steps and decisions that turn the input into the output.

The good news: many problems are built from the same few building blocks. When you can spot them, the code is much easier to write.

The Building Blocks

A building block is a small piece of code that does one common job. You can use it again and again, in many different programs.

Each block below uses a loop to look at the values one at a time. With each value, it does one small job: count it, add it, compare it, or look for it. The words in the question tell you which block you need:

This example uses all four blocks on the points of five players in a game:

#include <iostream>
using namespace std;

int main() {
  int points[5] = {72, 95, 61, 88, 79};
  int count = 0;
  int sum = 0;
  int max = points[0];
  bool found = false;

  for (int p : points) {
    if (p > 80) {    // Count
      count++;
    }
    sum += p;        // Sum
    if (p > max) {   // Max
      max = p;
    }
    if (p == 100) {  // Search
      found = true;
    }
  }

  cout << "More than 80 points: " << count << "\n";
  cout << "Total points: " << sum << "\n";
  cout << "Best score: " << max << "\n";
  cout << "Anyone with 100? " << (found ? "Yes" : "No") << "\n";
  return 0;
}

More than 80 points: 2
Total points: 395
Best score: 95
Anyone with 100? No

Which Block Do You Need?

Spotting the right block gets easier with practice. Try it now: for each question about the game, choose the block you need:

Tips for Building Logic

• Look for the question words: "how many" (Count), "total" (Sum), "highest" or "lowest" (Max or Min), and "is there" (Search).
• Choose the start value first: count = 0, sum = 0, max = points[0] or found = false.
• Many problems use more than one block. An average, for example, is a sum divided by the number of values.

Many of our C++ Practice Problems are built from these blocks, like Sum of Numbers, Average Score and Min and Max.