C Notes

Hub Applications of C Programming Language Features of C Programming Language History of C Programming Language What is C Language?Why Learn C Programming in 2024?First C Program - Hello World in C with Explanation GCC Compiler - Complete Guide to GNU C Compiler Install C Compiler on Linux - GCC Setup Guide for All Distros Install C Compiler on macOS - Xcode Command Line Tools and Homebrew GCC Install C Compiler on Windows - MinGW, MSYS2, and WSL Guide Setup VS Code for C Programming - Complete Configuration Guide C Syntax Rules — Complete Guide Comments in C Programming Constants in C Programming Data Types in C Identifiers in C Programming Keywords in C Programming Structure of a C Program Variables in C Programming Arithmetic Operators in C Programming Assignment Operators in C Programming Bitwise Operators in C Programming Increment and Decrement Operators in C Logical Operators in C Programming Relational Operators in C Programming Ternary Operator in C Programming Break Statement in C Programming Continue Statement in C Programming Do-While Loop in C Programming For Loop in C Programming Goto Statement in C Programming if-else Statement in C Programming if Statement in C Programming Nested if Statements in C Programming Switch Statement in C Programming While Loop in C Programming Function Arguments in C - Pass by Value vs Pass by Reference Function Call in C Programming Function Declaration (Prototype) in C Function Definition in C Programming Introduction to Functions in C Programming Recursion in C Programming Return Statement in C Programming Storage Classes in C Programming Array Operations in C - Searching, Sorting, Insert and Delete Array Programs in C - Reverse, Rotate, Duplicates, Merge Introduction to Arrays in C Programming Multidimensional Arrays in C Programming One Dimensional Arrays in C Programming Passing Arrays to Functions in C Two Dimensional Arrays in C Programming String Comparison in C - strcmp, strncmp, Case-Insensitive String Functions in C - strlen, strcpy, strcat, strcmp C String Handling Library - Complete string.h Reference String Manipulation in C - Reverse, Uppercase, Palindrome String Programs in C - Anagram, Word Count, Vowels, Frequency Strings in C Programming - Complete Introduction Double Pointers in C - Pointer to Pointer Pointer Arithmetic in C Programming Pointer Declaration and Initialization in C Pointer Programs in C - Swap, String Reversal, Dynamic Allocation Pointers and Arrays in C - The Relationship Explained Pointers and Functions in C - Pass by Reference Introduction to Pointers in C Programming Void Pointers in C - Generic Pointers Guide calloc() in C – Contiguous Memory Allocation Dynamic Arrays in C – Runtime-Sized Arrays free() in C – Releasing Dynamic Memory malloc() in C – Dynamic Memory Allocation Memory Management Best Practices in C realloc() in C – Resizing Dynamic Memory Binary Files in C – fread and fwrite File Handling in C – Introduction File Handling Projects in C – Student Records & Phone Book File Modes in C – Complete Guide fopen() and fclose() in C – Opening and Closing Files Reading Files in C – fgetc, fgets, fscanf, fread Writing Files in C – fputc, fputs, fprintf, fwrite Conditional Compilation in C – #ifdef, #ifndef, #if #define Macros in C – Constants and Macro Functions Header Files in C – Creating Custom Headers #include Directive in C – File Inclusion Macro Functions in C – Parameterized Macros C Preprocessor – Overview and Directives Graphs in C – Adjacency Matrix, Adjacency List, BFS, and DFS Hash Tables in C – Implementation, Collision Handling, and Chaining Linked List in C – Singly, Doubly, and Circular with Implementation Queue in C – Linear, Circular, and Priority Queue Implementation Stack in C – Array and Linked List Implementation with Applications Trees in C – Binary Search Tree Implementation with Traversals C Programming Best Practices – Coding Standards, Safety, and Performance Bit Fields in C – Definition, Use Cases, and Protocol Headers C Standard Library Overview – All Headers and Common Functions Command Line Arguments in C – argc and argv Explained Debugging Techniques in C – GDB, Valgrind, and Sanitizers Function Pointers in C – Complete Guide with Examples Memory Layout of a C Program – Text, Data, BSS, Heap, Stack Bank Management System in C – Complete Project with File Storage File Encryption Tool in C – XOR Cipher Implementation Library Management System in C – Complete Project with Source Code Mini Compiler in C – Lexer and Parser for Simple Expressions Snake Game in C – Console-Based Game with Source Code Student Management System in C – Complete Project with Source Code C Programming Cheatsheet – Complete Quick Reference Guide Top 50 C Programming Interview Questions with Answers (2025)Memory Management Interview Questions in C with Answers Output Prediction Questions in C – What Will This Program Print?Pointer Interview Questions in C – Tricky Problems with Answers Common C Programming Errors and Solutions – Debugging Guide Important C Programs – 50+ Frequently Asked Programs with Solutions Quick Revision Notes for C Programming – Complete C in 10 Minutes Recommended C Programming Books – From Beginner to Expert Format Specifiers in C - Complete Reference getchar() and putchar() in C Programming gets() and puts() in C Programming C Input Output Programs and Examples printf() Function in C Programming scanf() Function in C Programming Arrays of Structures in C – Managing Collections Enums in C – Enumerated Data Types Nested Structures in C – Struct Within Struct Pointers to Structures in C – Arrow Operator Structures in C – User-Defined Data Types typedef in C – Creating Type Aliases Unions in C – Shared Memory Data Types

Relational Operators in C Programming

Master relational (comparison) operators in C: equal to, not equal, greater than, less than, and their combinations. Learn how they return 0 or 1, common mistakes, and usage in conditions.

Relational operators — also called comparison operators — are what give your programs the ability to make decisions. Every if statement, every while loop, every conditional check relies on these operators to compare values and return a true or false result. In C, "true" is any non-zero value (typically 1), and "false" is 0.

List of Relational Operators in C

C provides six relational operators that compare two operands:

Operator	Meaning	Example	Result
`==`	Equal to	`5 == 5`	1 (true)
`!=`	Not equal to	`5 != 3`	1 (true)
`>`	Greater than	`7 > 3`	1 (true)
`<`	Less than	`2 < 8`	1 (true)
`>=`	Greater than or equal to	`5 >= 5`	1 (true)
`<=`	Less than or equal to	`3 <= 2`	0 (false)

How Relational Operators Work

Every relational expression evaluates to either 1 (true) or 0 (false). This is important to understand — in C, there's no dedicated boolean type in C89 (C99 introduced _Bool), so relational expressions produce integer results.

c example

#include <stdio.h>

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

    return 0;
}

Output

a == b : 0
a != b : 1
a > b  : 0
a < b  : 1
a >= b : 0
a <= b : 1

Equal To Operator (==)

The double equals == checks whether two values are identical. This is one of the most commonly used — and most commonly confused — operators in C.

c example

#include <stdio.h>

int main() {
    int x = 5;
    float y = 5.0;
    char ch = 'A';

    printf("x == 5 : %d\n", x == 5);
    printf("x == 10 : %d\n", x == 10);
    printf("y == 5.0 : %d\n", y == 5.0);
    printf("ch == 'A' : %d\n", ch == 'A');
    printf("ch == 65 : %d\n", ch == 65);  // 'A' is ASCII 65

    return 0;
}

Output

x == 5 : 1
x == 10 : 0
y == 5.0 : 1
ch == 'A' : 1
ch == 65 : 1

The Classic Mistake: `=` vs `==`

One of the most dangerous bugs in C is accidentally using assignment (=) instead of comparison (==):

c example

#include <stdio.h>

int main() {
    int x = 5;

    // WRONG: This assigns 10 to x, then checks if 10 is non-zero (always true)
    if (x = 10) {
        printf("This always executes! x is now %d\n", x);
    }

    // CORRECT: This compares x with 10
    x = 5;
    if (x == 10) {
        printf("This won't execute\n");
    } else {
        printf("x is not 10, x = %d\n", x);
    }

    return 0;
}

Output

This always executes! x is now 10
x is not 10, x = 5

Pro tip: Some programmers write if (10 == x) instead of if (x == 10). This way, if you accidentally write =, the compiler will catch it because you can't assign to a constant. This is called a "Yoda condition."

Not Equal To Operator (!=)

The != operator returns true when two values are different:

c example

#include <stdio.h>

int main() {
    int password = 1234;
    int attempt = 5678;

    if (attempt != password) {
        printf("Access denied! Wrong password.\n");
    }

    // Useful in loops
    char ch = 'x';
    printf("Character is ");
    if (ch != 'y') {
        printf("not 'y'\n");
    }

    return 0;
}

Output

Access denied! Wrong password.
Character is not 'y'

Greater Than (>) and Less Than (<)

These operators establish ordering between values:

c example

#include <stdio.h>

int main() {
    int age = 17;
    float temperature = 38.5;

    if (age < 18) {
        printf("You are a minor (age: %d)\n", age);
    }

    if (temperature > 37.0) {
        printf("You have a fever! (%.1f°C)\n", temperature);
    }

    // Finding the larger of two numbers
    int a = 42, b = 67;
    int max = (a > b) ? a : b;
    printf("Larger number: %d\n", max);

    return 0;
}

Output

You are a minor (age: 17)
You have a fever! (38.5°C)
Larger number: 67

Greater Than or Equal (>=) and Less Than or Equal (<=)

These inclusive comparison operators are essential for range checking:

Example

#include <stdio.h> int main() { int score = 75; // Grade assignment if (score >= 90) { printf("Grade: A\n"); } else if (score >= 80) { printf("Grade: B\n"); } else if (score >= 70) { printf("Grade: C\n"); } else if (score >= 60) { printf("Grade: D\n"); } else { printf("Grade: F\n"); } // Range checking int value = 50; if (value >= 1 && value <= 100) { printf("%d is in the range [1, 100]\n", value); } return 0; }

Output

Grade: C
50 is in the range [1, 100]

Comparing Different Data Types

When you compare values of different types, C applies type promotion rules — the smaller type gets promoted to the larger type before comparison:

c example

#include <stdio.h>

int main() {
    int i = 10;
    float f = 10.5;
    char c = 'A';  // 65

    printf("int vs float: %d > %f is %d\n", i, f, i > f);
    printf("char vs int: '%c' == %d is %d\n", c, 65, c == 65);
    printf("char comparison: 'a' > 'A' is %d\n", 'a' > 'A');

    return 0;
}

Output

int vs float: 10 > 10.500000 is 0
char vs int: 'A' == 65 is 1
char comparison: 'a' > 'A' is 1

Floating-Point Comparison Pitfall

Comparing floating-point numbers with == is unreliable due to precision issues:

c example

#include <stdio.h>
#include <math.h>

int main() {
    float a = 0.1 + 0.2;
    float b = 0.3;

    printf("0.1 + 0.2 = %.20f\n", a);
    printf("0.3       = %.20f\n", b);
    printf("Are they equal? %d\n", a == b);

    // Safe comparison using epsilon
    float epsilon = 0.00001;
    if (fabs(a - b) < epsilon) {
        printf("They are approximately equal\n");
    }

    return 0;
}

Output

0.1 + 0.2 = 0.30000001192092896000
0.3       = 0.30000001192092896000
Are they equal? 1
They are approximately equal

Best Practice: Never use == to compare floating-point numbers directly. Instead, check if the absolute difference is smaller than a tiny threshold (epsilon).

Relational Operators in Loops

Relational operators are the backbone of loop conditions:

Example

#include <stdio.h> int main() { // While loop with relational operator int count = 1; printf("Counting: "); while (count <= 5) { printf("%d ", count); count++; } printf("\n"); // For loop with multiple conditions for (int i = 10; i >= 0; i -= 2) { printf("%d ", i); } printf("\n"); return 0; }

Output

Counting: 1 2 3 4 5
10 8 6 4 2 0

Chaining Comparisons — A Common Trap

Unlike mathematics, you cannot chain comparisons in C. Writing a < b < c does NOT mean "b is between a and c":

c example

#include <stdio.h>

int main() {
    int a = 1, b = 5, c = 3;

    // This does NOT check if b is between a and c
    // It evaluates as (a < b) < c → (1) < 3 → 1
    if (a < b < c) {
        printf("Misleading: This is TRUE but wrong logic!\n");
    }

    // Correct way to check range
    if (a < b && b < c) {
        printf("b is between a and c\n");
    } else {
        printf("b is NOT between a and c\n");
    }

    return 0;
}

Output

Misleading: This is TRUE but wrong logic!
b is NOT between a and c

Precedence of Relational Operators

Priority	Operators	Associativity
Higher	`<`, `<=`, `>`, `>=`	Left to Right
Lower	`==`, `!=`	Left to Right

Relational operators have lower precedence than arithmetic operators, so a + b > c * d is evaluated as (a + b) > (c * d).

Interview Questions

Q1: What is the return type of relational expressions in C?

Relational expressions in C return an integer value — 1 for true and 0 for false. There's no built-in boolean type in C89. C99 introduced _Bool and stdbool.h with true/false macros, but internally they're still integers.

Q2: What's wrong with if (x = 5) and how does it differ from if (x == 5)?

if (x = 5) is an assignment, not a comparison. It assigns 5 to x, and since 5 is non-zero, the condition is always true. if (x == 5) correctly compares x with 5. This is one of the most common bugs in C programs.

Q3: Why should you avoid using == to compare floating-point numbers?

Floating-point numbers have limited precision and may not store exact values. Due to rounding errors, 0.1 + 0.2 might not exactly equal 0.3. Instead, use epsilon comparison: fabs(a - b) < epsilon.

Q4: What is the output of printf("%d", 3 > 2 > 1)?

The output is 0. The expression evaluates left to right: 3 > 2 gives 1, then 1 > 1 gives 0. This demonstrates why chaining comparisons doesn't work as expected in C.

Q5: What is the precedence relationship between arithmetic and relational operators?

Arithmetic operators (+, -, *, /, %) have higher precedence than relational operators. So in 5 + 3 > 6, the addition is performed first: 8 > 6 evaluates to 1.

Summary

Relational operators compare two values and return 1 (true) or 0 (false)
The == operator checks equality — never confuse it with = (assignment)
Floating-point comparisons with == are unreliable due to precision issues
You cannot chain comparisons like a < b < c — use logical AND instead
Relational operators have lower precedence than arithmetic operators
These operators are fundamental to decision-making (if/else) and loops (while/for)

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Relational Operators in C Programming.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this C Programming topic.

Search Terms

c-programming, c programming, programming, operators, relational, relational operators in c programming

Continue learning this concept

OperatorsArithmetic Operators in C ProgrammingLearn all arithmetic operators in C including addition, subtraction, multiplication, division, and modulus. Understand integer division traps, operator precedence, and type casting with practical examples.OperatorsAssignment Operators in C ProgrammingLearn all assignment operators in C including simple assignment, compound assignment operators (+=, -=, *=, /=, %=, &=, |=, ^=, <<=, >>=). Understand their syntax, behavior, and practical usage.OperatorsBitwise Operators in C ProgrammingComplete guide to bitwise operators in C: AND, OR, XOR, NOT, left shift, and right shift. Learn practical applications like flag manipulation, masking, and power-of-2 operations with examples.OperatorsIncrement and Decrement Operators in CMaster increment (++) and decrement (--) operators in C. Understand the difference between prefix and postfix, common pitfalls, undefined behavior, and practical usage in loops and expressions.OperatorsLogical Operators in C ProgrammingLearn logical operators in C: AND (&&), OR (||), and NOT (!). Understand short-circuit evaluation, truth tables, operator precedence, and practical examples for decision-making.OperatorsTernary Operator in C ProgrammingLearn the ternary (conditional) operator (?:) in C. Understand syntax, nested ternary expressions, when to use ternary vs if-else, and practical examples for concise conditional logic.

PreviousLogical Operators in C Programming NextTernary Operator in C Programming

C Notes

Relational Operators in C Programming

Master relational (comparison) operators in C: equal to, not equal, greater than, less than, and their combinations. Learn how they return 0 or 1, common mistakes, and usage in conditions.

List of Relational Operators in C

C provides six relational operators that compare two operands:

Operator	Meaning	Example	Result
`==`	Equal to	`5 == 5`	1 (true)
`!=`	Not equal to	`5 != 3`	1 (true)
`>`	Greater than	`7 > 3`	1 (true)
`<`	Less than	`2 < 8`	1 (true)
`>=`	Greater than or equal to	`5 >= 5`	1 (true)
`<=`	Less than or equal to	`3 <= 2`	0 (false)

How Relational Operators Work

c example

#include <stdio.h>

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

    return 0;
}

Output

a == b : 0
a != b : 1
a > b  : 0
a < b  : 1
a >= b : 0
a <= b : 1

Equal To Operator (==)

The double equals == checks whether two values are identical. This is one of the most commonly used — and most commonly confused — operators in C.

c example

#include <stdio.h>

int main() {
    int x = 5;
    float y = 5.0;
    char ch = 'A';

    printf("x == 5 : %d\n", x == 5);
    printf("x == 10 : %d\n", x == 10);
    printf("y == 5.0 : %d\n", y == 5.0);
    printf("ch == 'A' : %d\n", ch == 'A');
    printf("ch == 65 : %d\n", ch == 65);  // 'A' is ASCII 65

    return 0;
}

Output

x == 5 : 1
x == 10 : 0
y == 5.0 : 1
ch == 'A' : 1
ch == 65 : 1

The Classic Mistake: `=` vs `==`

One of the most dangerous bugs in C is accidentally using assignment (=) instead of comparison (==):

c example

#include <stdio.h>

int main() {
    int x = 5;

    // WRONG: This assigns 10 to x, then checks if 10 is non-zero (always true)
    if (x = 10) {
        printf("This always executes! x is now %d\n", x);
    }

    // CORRECT: This compares x with 10
    x = 5;
    if (x == 10) {
        printf("This won't execute\n");
    } else {
        printf("x is not 10, x = %d\n", x);
    }

    return 0;
}

Output

This always executes! x is now 10
x is not 10, x = 5

Not Equal To Operator (!=)

The != operator returns true when two values are different:

c example

#include <stdio.h>

int main() {
    int password = 1234;
    int attempt = 5678;

    if (attempt != password) {
        printf("Access denied! Wrong password.\n");
    }

    // Useful in loops
    char ch = 'x';
    printf("Character is ");
    if (ch != 'y') {
        printf("not 'y'\n");
    }

    return 0;
}

Output

Access denied! Wrong password.
Character is not 'y'

Greater Than (>) and Less Than (<)

These operators establish ordering between values:

c example

#include <stdio.h>

int main() {
    int age = 17;
    float temperature = 38.5;

    if (age < 18) {
        printf("You are a minor (age: %d)\n", age);
    }

    if (temperature > 37.0) {
        printf("You have a fever! (%.1f°C)\n", temperature);
    }

    // Finding the larger of two numbers
    int a = 42, b = 67;
    int max = (a > b) ? a : b;
    printf("Larger number: %d\n", max);

    return 0;
}

Output

You are a minor (age: 17)
You have a fever! (38.5°C)
Larger number: 67

Greater Than or Equal (>=) and Less Than or Equal (<=)

These inclusive comparison operators are essential for range checking:

Example

Output

Grade: C
50 is in the range [1, 100]

Comparing Different Data Types

When you compare values of different types, C applies type promotion rules — the smaller type gets promoted to the larger type before comparison:

c example

#include <stdio.h>

int main() {
    int i = 10;
    float f = 10.5;
    char c = 'A';  // 65

    printf("int vs float: %d > %f is %d\n", i, f, i > f);
    printf("char vs int: '%c' == %d is %d\n", c, 65, c == 65);
    printf("char comparison: 'a' > 'A' is %d\n", 'a' > 'A');

    return 0;
}

Output

int vs float: 10 > 10.500000 is 0
char vs int: 'A' == 65 is 1
char comparison: 'a' > 'A' is 1

Floating-Point Comparison Pitfall

Comparing floating-point numbers with == is unreliable due to precision issues:

c example

#include <stdio.h>
#include <math.h>

int main() {
    float a = 0.1 + 0.2;
    float b = 0.3;

    printf("0.1 + 0.2 = %.20f\n", a);
    printf("0.3       = %.20f\n", b);
    printf("Are they equal? %d\n", a == b);

    // Safe comparison using epsilon
    float epsilon = 0.00001;
    if (fabs(a - b) < epsilon) {
        printf("They are approximately equal\n");
    }

    return 0;
}

Output

0.1 + 0.2 = 0.30000001192092896000
0.3       = 0.30000001192092896000
Are they equal? 1
They are approximately equal

Best Practice: Never use == to compare floating-point numbers directly. Instead, check if the absolute difference is smaller than a tiny threshold (epsilon).

Relational Operators in Loops

Relational operators are the backbone of loop conditions:

Example

Output

Counting: 1 2 3 4 5
10 8 6 4 2 0

Chaining Comparisons — A Common Trap

Unlike mathematics, you cannot chain comparisons in C. Writing a < b < c does NOT mean "b is between a and c":

c example

#include <stdio.h>

int main() {
    int a = 1, b = 5, c = 3;

    // This does NOT check if b is between a and c
    // It evaluates as (a < b) < c → (1) < 3 → 1
    if (a < b < c) {
        printf("Misleading: This is TRUE but wrong logic!\n");
    }

    // Correct way to check range
    if (a < b && b < c) {
        printf("b is between a and c\n");
    } else {
        printf("b is NOT between a and c\n");
    }

    return 0;
}

Output

Misleading: This is TRUE but wrong logic!
b is NOT between a and c

Precedence of Relational Operators

Priority	Operators	Associativity
Higher	`<`, `<=`, `>`, `>=`	Left to Right
Lower	`==`, `!=`	Left to Right

Relational operators have lower precedence than arithmetic operators, so a + b > c * d is evaluated as (a + b) > (c * d).

Interview Questions

Q1: What is the return type of relational expressions in C?

Q2: What's wrong with if (x = 5) and how does it differ from if (x == 5)?

Q3: Why should you avoid using == to compare floating-point numbers?

Q4: What is the output of printf("%d", 3 > 2 > 1)?

The output is 0. The expression evaluates left to right: 3 > 2 gives 1, then 1 > 1 gives 0. This demonstrates why chaining comparisons doesn't work as expected in C.

Q5: What is the precedence relationship between arithmetic and relational operators?

Arithmetic operators (+, -, *, /, %) have higher precedence than relational operators. So in 5 + 3 > 6, the addition is performed first: 8 > 6 evaluates to 1.

Summary

Relational operators compare two values and return 1 (true) or 0 (false)
The == operator checks equality — never confuse it with = (assignment)
Floating-point comparisons with == are unreliable due to precision issues
You cannot chain comparisons like a < b < c — use logical AND instead
Relational operators have lower precedence than arithmetic operators
These operators are fundamental to decision-making (if/else) and loops (while/for)

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Relational Operators in C Programming.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this C Programming topic.

Search Terms

c-programming, c programming, programming, operators, relational, relational operators in c programming

Continue learning this concept

PreviousLogical Operators in C Programming NextTernary Operator in C Programming

List of Relational Operators in C

How Relational Operators Work

Equal To Operator (==)

The Classic Mistake: = vs ==

Not Equal To Operator (!=)

Greater Than (>) and Less Than (<)

Greater Than or Equal (>=) and Less Than or Equal (<=)

Comparing Different Data Types

Floating-Point Comparison Pitfall

Relational Operators in Loops

Chaining Comparisons — A Common Trap

Precedence of Relational Operators

Interview Questions

Summary

Continue learning this concept

List of Relational Operators in C

How Relational Operators Work

Equal To Operator (==)

The Classic Mistake: = vs ==

Not Equal To Operator (!=)

Greater Than (>) and Less Than (<)

Greater Than or Equal (>=) and Less Than or Equal (<=)

Comparing Different Data Types

Floating-Point Comparison Pitfall

Relational Operators in Loops

Chaining Comparisons — A Common Trap

Precedence of Relational Operators

Interview Questions

Summary

Continue learning this concept

The Classic Mistake: `=` vs `==`

The Classic Mistake: `=` vs `==`