Variables

By the end of this lesson you'll be able to store text, numbers, and true/false values in C++, choose the right type, convert safely between them, and read input from the user — the foundation of every C++ program you'll ever write.

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A variable is a labelled container in a kitchen. A jar labelled "Sugar" ( string ) holds text; a measuring cup labelled "Cups" ( int ) holds whole numbers; a scale labelled "Weight" ( double ) holds decimals. Each container only holds one kind of thing — you can't pour flour into a liquid measuring cup. C++ is statically typed , which means every variable's type is fixed when you declare it and the compiler checks it before your program ever runs. That strictness is a feature: it catches whole classes of mistakes early.

Notice string uses "double quotes" but char uses 'single quotes' — mixing these up is the #1 beginner error, so it's worth burning in now. ( string lives in the header.)

1. Declaring & Initializing Variables

Every variable needs a type and a name , and you usually give it a value on the same line: type name = value; . Use descriptive camelCase names like firstName or totalPrice — code is read far more often than it's written, so a clear name pays for itself. Read this worked example, run it, then check the output against the comments.

There's more than one way to give a variable its first value. The modern brace initialization — int x{' 5 '} — is preferred because it refuses to silently lose data (it blocks "narrowing", like squeezing 3.9 into an int ). And auto lets the compiler pick the type for you from the value on the right.

Your turn. The program below is almost complete — fill in the three blanks marked ___ using the hints in the comments, then run it.

2. Constants, Signedness & Sizes

Some values should never change — a tax rate, a maximum number of lives, pi. Mark them const and the compiler will reject any attempt to reassign them. constexpr goes one step further: the value must be known at compile time, which lets the compiler use it as a fixed size or array length. Numbers are also signed by default (they can be negative); marking one unsigned drops the negatives and reaches a bigger positive range with the same memory. sizeof tells you exactly how many bytes a type uses.

3. Type Conversion & Casting

Sometimes a value is the wrong type and you need to convert it. Implicit conversion happens automatically when it's safe (an int fits inside a double ). When the conversion can lose data — like double to int — C++ may still do it but the decimals just vanish. To convert deliberately you use static_cast<type>(value) , which states your intent clearly and is far safer than old C-style (int) casts. The classic trap: dividing two int values gives an int result.

Now you try. The two values below are int , so a plain / would chop the decimals off. Cast one side to double so the answer keeps its fraction:

4. Scope & Reading Input with cin

Scope is simply where a variable is visible . A variable lives inside the nearest pair of {' '} braces and disappears at the closing brace — try to use it outside and the compiler won't know what you mean. You read keyboard input with : the operator pulls what the user typed into your variable, automatically matching the variable's type (a number into an int , a word into a string ).

auto asks the compiler to deduce the type from the value: auto city = string("London"); is exactly a string . It's still strongly typed — once deduced, the type is fixed. Use auto to cut noise when the type is obvious or very long, but prefer the explicit type when it makes beginner code clearer.

const and constexpr both mean "do not reassign". const is read-only at run time; constexpr is fixed at compile time. Constants are written in UPPER_SNAKE_CASE by convention so they stand out.

No blanks this time — just a brief and a blank canvas (with an outline to keep you on track). Build it, run it, and check your output against the example in the comments. This is exactly the kind of small program real apps are made of.

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