.NET Academy

Operators and expressions

Do math and logic in C# — and avoid the classic integer-division trap.

11 min read

You will learn

  • Use arithmetic operators, including integer division and remainder
  • Combine conditions with &&, || and !
  • Use compound assignment and increment operators
  • Know how operator precedence works

Expressions

An expression is anything that produces a value: 42, price * quantity, age >= 18. Operators combine values into new values. You already know most of them from math class — with a couple of programming twists.

Arithmetic

OperatorMeaningExampleResult
+Addition5 + 27
-Subtraction5 - 23
*Multiplication5 * 210
/Division5 / 22 (!)
%Remainder5 % 21

The integer division trap

When both sides of / are integers, C# does integer division and throws away the fraction:

Console.WriteLine(7 / 2);      // 3
Console.WriteLine(7 / 2.0);    // 3.5  — one double is enough
Console.WriteLine(7.0 / 2);    // 3.5
 
int total = 7, count = 2;
double average = total / count;          // 3 — the division happened as int!
double correct = (double)total / count;  // 3.5

Watch out

double average = total / count; is one of the most common beginner bugs. The type of the variable on the left doesn't matter — the division is computed with the types on the right.

Remainder % is surprisingly useful

int minutes = 135;
Console.WriteLine($"{minutes / 60}h {minutes % 60}m");  // 2h 15m
 
bool isEven = number % 2 == 0;        // even numbers have no remainder
int lastDigit = 1234 % 10;            // 4

Assignment shortcuts

int score = 10;
score += 5;   // score = score + 5   → 15
score -= 3;   //                     → 12
score *= 2;   //                     → 24
score /= 4;   //                     → 6
score++;      // add one             → 7
score--;      // subtract one        → 6

Comparison operators

Comparisons produce a bool:

OperatorMeaning
==equal to
!=not equal to
<, >less than, greater than
<=, >=less/greater than or equal
bool canVote = age >= 18;
bool isZero = balance == 0;

Note

= assigns, == compares. Writing if (x = 5) is a compile error in C# — the compiler protects you from this classic bug.

Logical operators

OperatorMeaningTrue when…
&&ANDboth sides are true
||ORat least one side is true
!NOTthe value is false
bool canRent = age >= 25 && hasLicense;
bool isWeekend = day == "Saturday" || day == "Sunday";
bool isClosed = !isOpen;

&& and || short-circuit: if the left side already decides the answer, the right side isn't evaluated. That makes this safe:

if (name != null && name.Length > 0) { ... }   // Length is never read when name is null

Precedence

Like in math, *, / and % bind tighter than + and -, and comparisons come before &&, which comes before ||:

int a = 2 + 3 * 4;       // 14
int b = (2 + 3) * 4;     // 20

When in doubt, add parentheses — they make intent obvious to the next reader, too.

The Math class

For anything beyond the basics, use Math:

Math.Max(3, 7);        // 7
Math.Abs(-5);          // 5
Math.Pow(2, 10);       // 1024
Math.Sqrt(81);         // 9
Math.Round(2.567, 2);  // 2.57

Try it

Program.cs
Loading editor…

C# runtime not loaded

Key takeaways

  • / between two integers drops the fraction — make one side a double when you need decimals.
  • % gives the remainder: great for even/odd checks and splitting units.
  • &&, || and ! combine conditions, and &&/|| short-circuit.
  • Use parentheses whenever precedence isn't obvious.

Exercise

Implement three small methods. ToHoursAndMinutes(125) should return "2h 5m". CelsiusToFahrenheit converts using F = C × 9 / 5 + 32. IsLeapYear returns true for years divisible by 4, except years divisible by 100 — unless they're also divisible by 400.

Program.cs
Loading editor…

C# runtime not loaded

Hint 1

Integer division drops the remainder: 125 / 60 is 2. The remainder operator gives what's left: 125 % 60 is 5.

Hint 2

For Fahrenheit, celsius is a double so the math stays in floating point: celsius * 9 / 5 + 32.

Hint 3

Translate the rule literally: (divisible by 4 AND NOT divisible by 100) OR divisible by 400.

Show solution
string ToHoursAndMinutes(int totalMinutes) => $"{totalMinutes / 60}h {totalMinutes % 60}m";

double CelsiusToFahrenheit(double celsius) => celsius * 9 / 5 + 32;

bool IsLeapYear(int year) => (year % 4 == 0 && year % 100 != 0) || year % 400 == 0;

Console.WriteLine(ToHoursAndMinutes(125));
Console.WriteLine(CelsiusToFahrenheit(100));
Console.WriteLine(IsLeapYear(2024));

Quiz

4 questions · 3 correct to pass

  1. 1.What is the value of `7 / 2` in C#?

  2. 2.What is `17 % 5`?

  3. 3.After `int x = 5; x += 3; x++;`, what is x?

  4. 4.What does `2 + 3 * 4` evaluate to?

To complete this lesson

  • ○ Solve the exercise
  • ○ Pass the quiz (3/4 correct)