Lesson 2

Variables and input

Short answer: a variable is a named box in memory holding one value of one type. Declare it (int age;), give it a value (age = 20;), and from then on the name is that value.

Four types cover almost all beginner code

The type decides how many bytes the box takes and how the bits are read. That is why 'A' and "A" differ — single quotes make a char, double quotes make a string literal (the long version).

Declare, assign, print

#include <iostream>
using namespace std;

int main() {
    int score = 95;
    double price = 19.99;
    char grade = 'A';
    bool passed = true;

    cout << "score:  " << score << endl;
    cout << "price:  " << price << endl;
    cout << "grade:  " << grade << endl;
    cout << "passed: " << passed << endl;   // bools print as 1 / 0
    return 0;
}

That last line is the first surprise: bool prints as 1 or 0, not true/false. It is how cout treats booleans by default.

Reading input with cin

cin >> name reads one value from the keyboard. The arrow points into the variable — a useful way to remember the direction:

#include <iostream>
using namespace std;

int main() {
    int a, b;
    cin >> a;
    cin >> b;
    cout << "sum = " << a + b << endl;
    return 0;
}

Change the numbers in the input box: the program is not a calculator app, it is four lines of arithmetic that happen to run in a browser.

Integer division, once more

The most common numeric surprise: int / int is an int, fraction discarded.

#include <iostream>
using namespace std;

int main() {
    int a = 7, b = 2;
    cout << a / b << endl;             // 3 — fraction discarded
    cout << (double)a / b << endl;     // 3.5 — one side is double
    return 0;
}

Why, including negatives and %: why 7 / 2 is 3.

Errors you will hit this week

• Used before declared. C++ reads top to bottom; using x before int x; is a compile error, not zero.
• Uninitialised. int sum; sum += 5; adds 5 to whatever was in memory. Always write int sum = 0;.
• Overflow. int stops near 2.1 billion — 20! does not fit. Our factorial problem makes you hit that wall on purpose.
• Double quotes for a char. char c = "A"; fails; write 'A'.