C Language track

Lesson 04 · Level 1: Basics

Operators

Arithmetic, comparison, logical, bitwise and assignment operator precedence

Level 1: Basics5 min readFree lesson

Adding values, comparing them, logically combining them, and changing their format are all done by operators. The operators covered in this section become the basis for all subsequent sections.

Learning Objectives

  • Distinguish between Arithmetic/Relationship/Logic/Assignment/Increment/Decrement/Bit operators.
  • Be aware of operator priority.
  • Avoid the pitfalls of integer division and type casting.

Why do operators need a separate section?

Although it looks like a familiar arithmetic operation on the surface, there are many pitfalls in C.

  • The result of 7 / 2 is 3 (integer division), not 3.5.
  • a == b and a = b are completely different (comparison vs substitution).
  • Expressions like a++ + ++a may have different results across compilers (undefined behavior).

In this section, you will learn in advance why the results are the way they are.

Key concepts

1) Arithmetic operators

operator Meaning Yes
+ - * / Four basic operations 7 / 2 → 3 (integer)
% remainder (integer types only) 7 % 2 → 1

Integer / Integer = Integer, Real / Integer = Real, Integer / Real = Real.

2) Increment/decrement operator (prefix/postfix)

int a = 5;
int b = a++;     //Postfix: b is 5, a is 6
int c = ++a;     //Potential: After making a to 7, c also becomes 7.

Because the timing is different, it is dangerous to write ++/-- twice for the same variable on one line.

3) Relational operators

==, !=, <, >, <=, >=. The result is 0 or 1.

4) Logical operators and shorthand evaluation

operator Meaning
&& AND. If the left side is false, the right side is not evaluated
|| OR. If the left side is true, the right side is not evaluated
! NOT 
if (p != NULL && *p > 0) { /*If p is NULL, *p is not evaluated and is therefore safe.*/ }

5) Assignment operator family

=, +=, -=, *=, /=, %=. a += 3 is the same as a = a + 3.

6) Bitwise operators (reference)

&, |, ^, ~, <<, >>. It is often used in the Embedded/Bit flag.

7) Type conversion (casting)

int total = 95, count = 4;
double avg = (double)total / count;   // 23.75

If (double) accidentally promotes one side, the rest will also be automatically promoted by mistake.

See as example

Example 1 — ex01_arith.c: Four basic operations and remainder

int a = 17, b = 5;

Execution result

a = 17, b = 5
a + b = 22
a - b = 12
a * b = 85
a / b = 3 (integer division)
a % b = 2 (remainder)
a / (double)b = 3.400

Key point: The results (3 vs 3.4) vary depending on the same It-s-17/5 data type.

Example 2 — ex02_increment.c: Prefix and postfix difference

a = 5; b = a++;    //rearguard
a = 5; b = ++a;    //potential
a += 10; a *= 2;

Execution result

Postfix: a=6, b=5
Potential: a=6, b=6
a += 10 -> 16
a *= 2  -> 32

Key point: postfix “use then increase”, prefix “increase then use”. As a result, a is both 6.

Example 3 — ex03_logical.c: Relationships and Logic

int age = 25, has_license = 1;

Execution result

a == b : 0
a != b : 1
a <  b : 1
a >= b : 0
Adult and licensed: 1
Minor or No License: 0

Key point: The result of the logical operation is 0/1-and, all non-zero values are treated as true.

Example 4 — ex04_cast.c: The power of casting

int total = 95, count = 4;
double avg_wrong = total / count;          // 23
double avg_right = (double)total / count;  // 23.75

Execution result

Bad average (integer division): 23.000
Average Correct (Casting): 23.750
(int)3.99 = 3 (truncate)

Key point: Casting to int is truncation, not rounding.

A different perspective — a one-line summary of priorities

High → Low
( ) [ ] -> .                       (Parentheses/Approach)
! ++ -- (cast) sizeof &(address) (unary)
* / % (multiplication type)
+ - (addition type)
< <= > >= (relationship)
== != (equivalent)
&& ||                               (logic)
?: (ternary)
= += -= *= /= (assignment)
, (comma)

If it's difficult to remember, always use parentheses. a * b + c is clearer if written as (a * b) + c.

Common Mistakes

  1. Integer division trap: When calculating the average, write sum / count and the decimal point disappears.
  2. Confusion between == and =: if (a = 5) is always true + side effects.
  3. Unknown shortened evaluation: It is safe to put the guard condition first like if (count != 0 && total / count > 10).
  4. Compare sign There-is/Not-there: When comparing int -1 and unsigned 1, -1 is interpreted as a very large positive number.
  5. Multiple use of ++: a = a++ + ++a; has undefined behavior.

Cleanup

  • In arithmetic operations, when data types are mixed, the resulting data type is determined (usually promoted to a larger one).
  • integer/integer division truncates the decimal part. Casting is essential for average, etc.
  • The result of the Relationship/Logic operator is 0 or 1, and a non-0 value is true.
  • Safety guards such as NULL checks can be created with shortened evaluation.
  • If ambiguous, specify priority in parentheses.

Try it yourself

cd src
gcc -std=c11 -Wall -o ex01 ex01_arith.c     && ./ex01
gcc -std=c11 -Wall -o ex02 ex02_increment.c && ./ex02
gcc -std=c11 -Wall -o ex03 ex03_logical.c   && ./ex03
gcc -std=c11 -Wall -o ex04 ex04_cast.c      && ./ex04

Application:

  • Change the a and b values of ex01 to negative numbers and observe the resulting sign of %.
  • Predict and check what the results of (int)(-3.99) will be in ex04.

Example files

The runnable files used in this lesson. Copy them into your own project and experiment.

src/ex01_arith.cC
#include <stdio.h>

int main(void) {
    int a = 17, b = 5;

    printf("a = %d, b = %d\n", a, b);
    printf("a + b = %d\n", a + b);
    printf("a - b = %d\n", a - b);
    printf("a * b = %d\n", a * b);
    printf("a / b = %d (integer division)\n", a / b);
    printf("a %% b = %d (remainder)\n", a % b);
    printf("a / (double)b = %.3f\n", a / (double)b);

    return 0;
}
src/ex02_increment.cC
#include <stdio.h>

int main(void) {
    int a = 5;
    int b;

    b = a++;       //Postfix: increment after using a
    printf("Postfix: a=%d, b=%d\n", a, b);   // a=6, b=5

    a = 5;
    b = ++a;       //Potential: a increases first
    printf("Potential: a=%d, b=%d\n", a, b);   // a=6, b=6

    a += 10;       // a = a + 10
    printf("a += 10 -> %d\n", a);

    a *= 2;
    printf("a *= 2  -> %d\n", a);

    return 0;
}
src/ex03_logical.cC
#include <stdio.h>

int main(void) {
    int a = 10, b = 20;

    printf("a == b : %d\n", a == b);
    printf("a != b : %d\n", a != b);
    printf("a <  b : %d\n", a <  b);
    printf("a >= b : %d\n", a >= b);

    int age = 25;
    int has_license = 1;

    printf("Adult and licensed: %d\n", (age >= 18) && has_license);
    printf("Minor or Unlicensed: %d\n", (age < 18) || !has_license);

    return 0;
}
src/ex04_cast.cC
#include <stdio.h>

int main(void) {
    int total = 95, count = 4;

    double avg_wrong = total / count;
    double avg_right = (double)total / count;

    printf("Invalid average (integer division): %.3f\n", avg_wrong);  // 23.000
    printf("Correct average (casting): %.3f\n", avg_right); // 23.750

    double pi = 3.99;
    int trunc = (int)pi;
    printf("(int)3.99 = %d (decimal truncation)\n", trunc);

    return 0;
}

Assignment

Problem 1

Take two integers a and b as input and output the following.

  • Sum/difference/product/quotient/remainder

  • Real number result of dividing a by b (2 decimal places)

  • File name: hw01.c

Problem 2

Input an integer and output Even/odd and Positive/Negative/0 together. Try using only operator result (0/1), not conditional statements.

input: -4
Even=1, positive=0, negative=1
  • File name: hw02.c

Problem 3

Input the temperature in Fahrenheit (F) and convert it to Celsius and print it. Formula: C = (F - 32) * 5 / 9. Be careful with integer division.

  • File name: hw03.c
Show the reference solution

Try the assignment yourself first — comparing your attempt with a working solution is where most of the learning happens.

hw01.cC
#include <stdio.h>

int main(void) {
    int a, b;
    printf("Enter two integers:");
    scanf("%d %d", &a, &b);

    printf("Sum: %d\n", a + b);
    printf("Tea: %d\n", a - b);
    printf("Product: %d\n", a * b);
    if (b != 0) {
        printf("Share: %d\n", a / b);
        printf("Rest: %d\n", a % b);
        printf("Real division: %.2f\n", (double)a / b);
    } else {
        printf("Cannot be divided by 0.\n");
    }
    return 0;
}
hw02.cC
#include <stdio.h>

int main(void) {
    int n;
    printf("Integer input:");
    scanf("%d", &n);

    int even     = (n % 2 == 0);
    int positive = (n > 0);
    int negative = (n < 0);

    printf("Even=%d, positive=%d, negative=%d\n", even, positive, negative);
    return 0;
}
hw03.cC
#include <stdio.h>

int main(void) {
    double f;
    printf("Enter the Fahrenheit temperature:");
    scanf("%lf", &f);

    double c = (f - 32) * 5.0 / 9.0;
    printf("Celsius: %.2f\n", c);
    return 0;
}

Learn this lesson with a mentor

Work through the C Language track on your own, or join a guided cohort at our Pitampura campus and online.

Enroll nowTrack overview
Call NowEnroll Now