operators_expressions.md

Operators and Expressions

An expression is anything that evaluates to a value: a literal like 42, a variable name, a function call, or some combination of these joined by operators.

int x = 10;
int y = 5;
int sum = x + y;       // expression: x + y → 15
bool ok  = (sum > 10); // expression: sum > 10 → true

This chapter covers the operators you will use day-to-day, and the rule that decides what happens when several of them appear in the same expression.

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Arithmetic

Operator	Meaning	Example	Result
+	addition	5 + 3	8
-	subtraction	5 - 3	2
*	multiplication	5 * 3	15
/	division	10 / 3	3 (with int) or 3.333… (with double)
%	remainder (modulo)	10 % 3	1

Integer division truncates. This is the single most common arithmetic surprise:

int    a = 10 / 3;        // 3, fractional part discarded
double b = 10 / 3;        // 3.0, not 3.333 — divided as int first, then converted
double c = 10.0 / 3;      // 3.333…, at least one operand is double

If you want a decimal result, at least one operand must be a double or float.

% (modulo) only works on integer types. 10 % 3 is 1; 10.0 % 3 is a compile error.

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Assignment and compound assignment

int total = 0;
total = total + 5;    // long form
total += 5;           // same thing, shorter

The compound forms +=, -=, *=, /=, %= all work the same way: read the current value, apply the operation, write back.

++ and -- increment or decrement by one:

int i = 0;
++i;   // i is now 1, preferred form
i++;   // also works, prefer ++i when used on its own

For built-in types ++i and i++ behave identically when used as a standalone statement. They differ when used inside a larger expression (++i returns the new value, i++ returns the old one), but using ++ inside an expression you also assign from is a quick way to confuse yourself. Don't.

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Comparison

Comparison operators produce a bool:

Operator	Meaning
==	equal
!=	not equal
<	less than
>	greater than
<=	less than or equal
>=	greater than or equal

bool adult = (age >= 18);

The most common bug here: writing = (assignment) when you mean == (comparison).

if (x = 5) { ... }   // assigns 5 to x, then tests if 5 is true, always runs
if (x == 5) { ... }  // tests whether x equals 5

Modern compilers warn about this if you enable warnings. Turn them on.

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Logical operators

Used to combine boolean expressions:

Operator	Meaning
&&	AND: true only if both sides are true
||	OR: true if either side is true
!	NOT: flips true and false

if (temperature > 80 && pressure < 5) {
    // both conditions must hold
}

if (!ready) {
    // 'ready' is false
}

&& and || are short-circuiting: they evaluate the right-hand side only if needed. This is useful — and occasionally essential:

if (count != 0 && total / count > threshold) {
    // safe: total / count runs only when count is not zero
}

If count is zero, the right side never runs, so the division is skipped. Swap the two conditions and the program divides by zero.

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The ternary operator

A compact if/else that produces a value:

int max = (a > b) ? a : b;

Reads as: "if a > b, the value is a; otherwise it is b." Convenient for short choices. For anything more complex, use a real if/else statement; readability beats brevity.

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Precedence: why parentheses save you

When several operators appear in the same expression, precedence decides which binds tighter.

int x = 2 + 3 * 4;   // 14, not 20, * binds tighter than +

The full precedence table is long. You do not need to memorise it. You need to remember two rules and you will not get in trouble:

1. *, /, % bind tighter than + and - (standard maths).
2. When in doubt, use parentheses. They cost nothing and make intent explicit.

Operators of equal precedence evaluate left to right, which can catch you out:

int result = totalSeconds / 60 * 60;

You might read that as "divide by 60, then multiply by 60 — back where you started." It is not: the / runs first, and integer division discards the remainder. With totalSeconds = 125, 125 / 60 is 2, and 2 * 60 is 120 — not 125. Now a different trap, where the grouping is unclear:

double rate = a + b / c + d;

Did you mean (a + b) / (c + d) or a + (b/c) + d? They give different answers. Write the parentheses you mean.

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Mixing types

If you combine values of different types, C++ converts them following well-defined rules. Two cases worth knowing:

int    i = 5;
double d = 2.0;
double result = i + d;   // i is promoted to double; result is 7.0

That is the safe direction: int to double loses nothing.

double pi = 3.14;
int    n  = pi;          // truncates to 3, fractional part lost

Going the other direction (double to int) silently loses information. Brace initialisation refuses it; ordinary assignment does not. If you want to truncate, make it explicit with a cast:

int n = static_cast<int>(pi);

static_cast is the polite way to ask for a conversion the compiler would otherwise warn about. It also flags to a reader that the truncation is intentional.

Avoid the C-style cast. You will also see the older form, (int)pi. Prefer static_cast: a C-style cast silently performs whatever conversion is needed — including unsafe ones that a named cast would reject — and a bare (int) is nearly impossible to search your code for. static_cast<int>(pi) states exactly what you mean and lets the compiler keep you honest.

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Summary

• Arithmetic on integers truncates; mix in a double to get decimal results.
• == compares, = assigns. They are not the same.
• && and || short-circuit, which is useful for guarding against null/invalid values.
• Precedence exists, but parentheses are free. Use them.
• Conversions from larger to smaller types lose data silently; make them explicit with static_cast, not the old C-style (int)x.

Floating-point arithmetic has surprises of its own: 0.1 + 0.2 does not exactly equal 0.3, and comparing floats with == is almost never what you want. See the Floating-Point Pitfalls reference for the full list of gotchas.