Lesson 3

Making decisions

Short answer: if (condition) runs the next block only when the condition is true; else runs the alternative. A condition is just an expression that evaluates to true or false.

Comparisons produce bools

You can print a comparison to see its 1/0 value — a quick way to check what C++ actually thinks:

#include <iostream>
using namespace std;

int main() {
    int x = 7;
    cout << (x > 5) << endl;    // 1 (true)
    cout << (x == 5) << endl;   // 0 (false)
    cout << (x != 5) << endl;   // 1
    return 0;
}

Three-way choice

else if chains tests in order and stops at the first true one:

#include <iostream>
using namespace std;

int main() {
    int score;
    cin >> score;

    if (score >= 90)      cout << "A" << endl;
    else if (score >= 80) cout << "B" << endl;
    else if (score >= 70) cout << "C" << endl;
    else                  cout << "F" << endl;
    return 0;
}

Try 95, 85, 75, 60 in the input box. Order matters: once score >= 90 is true, the later tests never run.

Combining conditions

&& means and (both must be true), || means or (at least one), ! means not. The leap-year rule is the classic three-way combination:

#include <iostream>
using namespace std;

int main() {
    int year;
    cin >> year;

    bool leap = (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
    if (leap) cout << "leap year" << endl;
    else      cout << "not a leap year" << endl;
    return 0;
}

Try 1900 (not leap), 2000 (leap), 2024 (leap). The full story of why the century rule exists is in this article's cousin — or just solve the problem.

The traps

• = vs ==. if (x = 5) assigns 5 and is always true — a silent bug. Read if (5 == x) if you want the compiler to catch it.
• Missing braces. if (a) if (b) x(); else y(); — the else binds to the nearest if. Always use braces.
• Chained comparisons. 0 < x < 10 compiles and is wrong (it means (0 < x) < 10). Write 0 < x && x < 10.