Module 1 : Introduction to C

Learning Objectives

1. Introduction: From Python to C

1.1 Key Differences Overview

Aspect Python C
Compilation Interpreted Compiled
Type System Dynamic typing Static typing
Memory Management Automatic Manual
Syntax Style Indentation-based Brace-based
Performance Slower execution Faster execution
Development Speed Faster to write More verbose

Some of you might ask, What does it mean by Static Typing vs Dynamic Typing ?

Dynamic Typing ( Python ):

# Python - Dynamic Typing
x = 5        # x is an integer
x = "Hello"  # Now x is a string (allowed!)
x = 3.14     # Now x is a float (also allowed!)

Static Typing ( C ):

// C - Static Typing
int x = 5;           // x is declared as integer
x = "Hello";         // ERROR! Cannot assign string to integer variable
float y = 3.14f;     // y must be declared as float to store decimal numbers

Advantages of Static Typing ( C ):

Advantages of Dynamic Typing ( Python ):

1.2 Basic Program Structure

Python:

# Simple Python program
print("Hello, World!")

C:

#include <stdio.h>

int main() {
    printf("Hello, World!\n");
    return 0;
}

Key Points:

2. Input/Output Operations

2.1 Output Operations

2.1.1 Basic Output - printf()

Function Signature:

int printf(const char *format, ...);

Python vs C Comparison:

Python C
print("Hello") printf("Hello\n");
print("Value:", x) printf("Value: %d\n", x);
print(f"x = {x}") printf("x = %d\n", x);

Keep in mind that print() in python automatically creates a new line by default

2.1.2 Format Specifiers

Data Type Format Specifier Example
int %d or %i printf("%d", 42);
float %f printf("%.2f", 3.14);
double %lf printf("%.2lf", 3.14159);
char %c printf("%c", 'A');
string %s printf("%s", "Hello");
hexadecimal %x or %X printf("%x", 255);
unsigned int %u printf("%u", 42u);

2.1.3 Advanced printf() Features

Width and Precision:

printf("%5d", 42);        // Right-aligned in 5 characters: "   42"
printf("%-5d", 42);       // Left-aligned in 5 characters: "42   "
printf("%05d", 42);       // Zero-padded: "00042"
printf("%.2f", 3.14159);  // 2 decimal places: "3.14"
printf("%8.2f", 3.14159); // 8 characters, 2 decimals: "    3.14"

2.1.4 Escape Characters

Escape characters are special character sequences that represent characters that are difficult or impossible to type directly. They start with a backslash (\).

Common Escape Characters:

Escape Sequence Character Description Example
\n Newline Moves cursor to next line printf("Line 1\nLine 2");
\t Tab Horizontal tab (8 spaces) printf("Name:\tJohn");
\" Double Quote Literal double quote printf("He said \"Hello\"");
\' Single Quote Literal single quote printf("It\'s working");
\\ Backslash Literal backslash printf("Path: C:\\Program Files");
\r Carriage Return Return to beginning of line printf("Loading\rDone");
\b Backspace Move cursor back one position printf("ABC\bD"); → "ABD"
\0 Null Character String terminator char str[] = "Hi\0lo";
\a Alert (Bell) System beep/alert sound printf("\aError!");
\f Form Feed Page break printf("Page 1\fPage 2");
\v Vertical Tab Vertical tab printf("Line 1\vLine 2");

Python vs C Escape Characters:

Purpose Python C
New line print("Line 1\nLine 2") printf("Line 1\nLine 2");
Tab spacing print("Name:\tAge") printf("Name:\tAge");
Quote in string print("She said \"Hi\"") printf("She said \"Hi\"");
Backslash print("C:\\folder") printf("C:\\\\folder");

Practical Examples:

// Creating formatted output with escape characters
printf("Student Information:\n");
printf("Name:\t\tJohn Doe\n");
printf("Age:\t\t20\n");
printf("GPA:\t\t3.75\n");

// Output:
// Student Information:
// Name:		John Doe
// Age:		20
// GPA:		3.75

// Using quotes within strings
printf("The teacher said, \"Programming is fun!\"\n");
// Output: The teacher said, "Programming is fun!"

// File paths (especially important for Windows)
printf("Save file to: C:\\Documents\\Programs\\myfile.txt\n");
// Output: Save file to: C:\Documents\Programs\myfile.txt

Important Notes:

2.2 Input Operations

2.2.1 Basic Input - scanf()

Function Signature:

int scanf(const char *format, ...);

Python vs C Comparison:

Python C
x = int(input()) scanf("%d", &x);
name = input() scanf("%s", name);
x = float(input()) scanf("%f", &x);

2.2.2 Important scanf() Considerations

Address Operator (&):

int age;
char name[50];
float height;

scanf("%d", &age);     // & required for int
scanf("%s", name);     // & NOT needed for string
scanf("%f", &height);  // & required for float

Input Buffer Issues and Whitespace Handling:

The Whitespace Problem: When you press Enter after typing input, scanf() reads the data but leaves the newline character (\n) in the input buffer. This can cause problems with subsequent input operations.

// Problematic code:
int num;
char ch;

printf("Enter a number: ");
scanf("%d", &num);              // User types "5" and presses Enter
                                // Buffer now contains: \n (leftover newline)
printf("Enter a character: ");
scanf("%c", &ch);               // This reads the leftover \n, not user input!
printf("Character: %c\n", ch);  
// Outputs: 
// Character: (it shows nothing because it prints a newline)

What happens step by step:

  1. User types "5" and presses Enter → Input buffer: 5\n
  2. scanf("%d", &num) reads "5" → Buffer remaining: \n
  3. scanf("%c", &ch) immediately reads the leftover \n
  4. Program doesn't wait for new character input

Solutions:

Solution 1: Space before %c

int num;
char ch;

printf("Enter a number: ");
scanf("%d", &num);
printf("Enter a character: ");
scanf(" %c", &ch);  // Space before %c consumes all whitespace (spaces, tabs, newlines)

Solution 2: Explicit buffer clearing

int num;
char ch;

printf("Enter a number: ");
scanf("%d", &num);

// Clear the input buffer
while (getchar() != '\n');  // Read and discard until newline

printf("Enter a character: ");
scanf("%c", &ch);

Solution 3: Using getchar() to consume newline

int num;
char ch;

printf("Enter a number: ");
scanf("%d", &num);
getchar();  // Consume the leftover newline

printf("Enter a character: ");
scanf("%c", &ch);

Whitespace Characters in C:

Important scanf() Whitespace Rules:

Advanced scanf() Format Specifiers:

1. Character Set Specifiers [...]:

char name[50];

// Read only alphabetic characters
scanf("%[a-zA-Z]", name);

// Read everything except newline
scanf("%[^\n]", name);  // Reads entire line including spaces

// Read only digits
scanf("%[0-9]", name);

// Read only vowels
scanf("%[aeiouAEIOU]", name);

2. Excluding Character Sets [^...]:

char input[100];

// Read everything EXCEPT newline (gets full line with spaces)
scanf("%[^\n]", input);

// Read everything EXCEPT spaces and tabs
scanf("%[^ \t]", input);

// Read everything EXCEPT digits
scanf("%[^0-9]", input);

// Read until comma is encountered
scanf("%[^,]", input);

3. Width Specifiers:

char buffer[10];

// Read maximum 9 characters (leaving room for \0)
scanf("%9s", buffer);

// Read exactly 5 characters
scanf("%5c", buffer);

4. Practical Examples:

// Example 1: Reading full name (including spaces)
char full_name[100];
printf("Enter your full name: ");
scanf(" %[^\n]", full_name);  // Space before % consumes previous newline

// Example 2: Reading until specific delimiter
char email[50];
printf("Enter email: ");
scanf("%[^@]", email);  // Read until @ symbol

// Example 3: Input validation
char grade[10];
printf("Enter grade (A, B, C, D, F): ");
scanf("%[ABCDFabcdf]", grade);  // Only accept valid grades

5. Combining Multiple Inputs:

int day, month, year;
char separator;

// Reading date in format: dd/mm/yyyy or dd-mm-yyyy
printf("Enter date (dd/mm/yyyy or dd-mm-yyyy): ");
scanf("%d%c%d%c%d", &day, &separator, &month, &separator, &year);

// Alternative: reading with specific separators
printf("Enter date (dd/mm/yyyy): ");
scanf("%d/%d/%d", &day, &month, &year);

2.2.3 Alternative Input Methods

getchar() and putchar():

char ch;
ch = getchar();  // Read single character
putchar(ch);     // Output single character

fgets() for Safe String Input:

char name[50];
printf("Enter your name: ");
fgets(name, sizeof(name), stdin);

3. Variables and Data Types

3.1 Variable Declaration

Python vs C:

Python C
x = 5 int x = 5;
name = "John" char name[] = "John";
pi = 3.14 float pi = 3.14f;

3.2 Basic Data Types

3.2.1 Integer Types

Type Size (bytes) Range Usage
char 1 -128 to 127 Small integers, characters
short 2 -32,768 to 32,767 Small integers
int 4 -2,147,483,648 to 2,147,483,647 Standard integers
long 4/8 System dependent Large integers
long long 8 Very large range Very large integers

Unsigned Variants:

unsigned char uc;    // 0 to 255
unsigned int ui;     // 0 to 4,294,967,295
unsigned short us;   // 0 to 65,535

3.2.2 Floating-Point Types

Type Size Precision Range
float 4 bytes ~7 digits ±3.4 × 10^±38
double 8 bytes ~15 digits ±1.7 × 10^±308
long double 12/16 bytes Extended precision System dependent

3.2.3 Character and String Types

Single Characters:

char letter = 'A';        // Single character
char digit = '5';         // Character representation of digit
char newline = '\n';      // Escape sequence

Strings (Character Arrays):

char name[20] = "John";           // Fixed-size array
char message[] = "Hello World";   // Size determined by initializer
char buffer[100];                 // Uninitialized array

Python vs C String Comparison:

Python C
name = "John" char name[] = "John";
len(name) strlen(name)
name[0] name[0]
name + " Doe" strcat(name, " Doe");

3.3 Variable Declaration Rules

  1. Must declare before use (unlike Python)
  2. Case-sensitive (age ≠ Age)
  3. Cannot start with digits (2x is invalid)
  4. Cannot use keywords (int, if, while, etc.)
  5. Should use meaningful names (student_count not sc)

3.4 Constants

// Method 1: #define preprocessor directive
#define PI 3.14159
#define MAX_SIZE 100

// Method 2: const keyword
const int ARRAY_SIZE = 50;
const float GRAVITY = 9.81f;

Python vs C Constants:

Python C
PI = 3.14159 #define PI 3.14159
PI = 3.14159 const float PI = 3.14159f;

4. Arithmetic Operators

4.1 Basic Arithmetic Operators

Operator Operation Python Example C Example
+ Addition a + b a + b
- Subtraction a - b a - b
* Multiplication a * b a * b
/ Division a / b a / b
% Modulus a % b a % b

4.2 Important Division Differences

Integer Division:

# Python 3
print(7 / 3)   # Output: 2.333...
print(7 // 3)  # Output: 2 (floor division)

// C
printf("%f\n", 7.0 / 3.0);  // Output: 2.333333
printf("%d\n", 7 / 3);      // Output: 2 (integer division)
printf("%f\n", (float)7 / 3); // Output: 2.333333 (type casting)

4.3 Assignment Operators

Operator Equivalent Python C
= Basic assignment x = 5 x = 5;
+= Add and assign x += 3 x += 3;
-= Subtract and assign x -= 3 x -= 3;
*= Multiply and assign x *= 3 x *= 3;
/= Divide and assign x /= 3 x /= 3;
%= Modulus and assign x %= 3 x %= 3;

4.4 Increment and Decrement Operators

Pre-increment vs Post-increment:

int x = 5;
int y, z;

y = ++x;  // Pre-increment: x becomes 6, then y = 6
z = x++;  // Post-increment: z = 6, then x becomes 7

// Equivalent operations:
x = x + 1;  // Same as x++ or ++x when used alone
x += 1;     // Same as above

Python vs C:

Python C
x += 1 x++ or ++x or x += 1
x -= 1 x-- or --x or x -= 1

4.5 Operator Precedence

Priority Operators Associativity
1 (Highest) ++, -- (postfix) Left to right
2 ++, -- (prefix), +, - (unary) Right to left
3 *, /, % Left to right
4 +, - (binary) Left to right
5 (Lowest) =, +=, -=, etc. Right to left

5. Flow Control

5.1 Conditional Statements

5.1.1 if Statement

Python vs C Syntax:

Python:

if condition:
    statement1
    statement2
elif another_condition:
    statement3
else:
    statement4

C:

if (condition) {
    statement1;
    statement2;
} else if (another_condition) {
    statement3;
} else {
    statement4;
}

Key Differences:

5.1.2 Relational Operators

Operator Meaning Python C
== Equal to a == b a == b
!= Not equal to a != b a != b
< Less than a < b a < b
> Greater than a > b a > b
<= Less than or equal a <= b a <= b
>= Greater than or equal a >= b a >= b

5.1.3 Logical Operators

Operator Meaning Python C
&& Logical AND and or & &&
|| Logical OR or or | ||
! Logical NOT not or ~ !

Examples in C :

// Python: if age >= 18 and score > 80:
if (age >= 18 && score > 80) {
    printf("Eligible for scholarship\n");
}

// Python: if not (x < 0 or x > 100):
if (!(x < 0 || x > 100)) {
    printf("Valid percentage\n");
}

5.1.4 Executing Code in if Conditions

While primarily used for conditions, C allows expressions that evaluate to a non-zero value (true) or zero (false) within the parentheses. This means you can sometimes perform assignments or function calls directly within the condition, though it's often discouraged for readability.

int x = 10;
if (x = 5) { // Assigns 5 to x, then evaluates to 5 (true)
    printf("x is now 5 and this code runs.\n");
}

5.1.5 Single Statement if:

If an if or else block contains only a single statement, the curly braces {} are optional. However, it's good practice to always use them to avoid ambiguity and potential errors when adding more statements later.

if (score > 90)
    printf("Excellent!\n");
else
    printf("Keep trying.\n");

5.1.6 switch Statement

C provides switch as an alternative to multiple if-else statements:

switch (variable) {
    case value1:
        // statements
        break;
    case value2:
        // statements
        break;
    default:
        // statements
        break;
}

Example:

int grade;
printf("Enter grade (1-5): ");
scanf("%d", &grade);

switch (grade) {
    case 5:
        printf("Excellent!\n");
        break;
    case 4:
        printf("Very Good!\n");
        break;
    case 3:
        printf("Good!\n");
        break;
    case 2:
        printf("Fair!\n");
        break;
    case 1:
        printf("Poor!\n");
        break;
    default:
        printf("Invalid grade!\n");
        break;
}

Understanding break and Fall-through: In C's switch statement, the break keyword is essential. If break is omitted from a case block, execution will "fall through" to the next case block (and subsequent ones) until a break is encountered or the end of the switch statement is reached. This "fall-through" behavior can be intentionally used for specific logic, but it's a common source of bugs if not intended.

Example of Fall-through:

int day = 2; // Monday
switch (day) {
    case 1:
        printf("Weekend!\n");
        break;
    case 2:
    case 3:
    case 4:
    case 5:
        printf("Weekday.\n"); // Execution falls through from case 2, 3, 4 to 5
        break;
    case 6:
        printf("Weekend!\n");
        break;
    default:
        printf("Invalid day.\n");
        break;
}
// Output for day = 2: Weekday.

This example shows how case 2, case 3, case 4, and case 5 all execute the same printf("Weekday.\n"); because there are no break statements between them.

5.2 Iteration Statements (Loops)

5.2.1 for Loop

Python vs C Syntax:

Python:

for i in range(5):
    print(i)

for i in range(1, 10, 2):
    print(i)

C:

// Basic for loop
for (int i = 0; i < 5; i++) {
    printf("%d\n", i);
}

// Step by 2
for (int i = 1; i < 10; i += 2) {
    printf("%d\n", i);
}

For Loop Structure:

for (initialization; condition; increment/decrement) {
    // loop body
}

5.2.2 while Loop

Python vs C:

Python:

i = 0
while i < 5:
    print(i)
    i += 1

C:

int i = 0;
while (i < 5) {
    printf("%d\n", i);
    i++;
}

5.2.3 do-while Loop

C provides do-while loop (not available in Python):

int choice;
do {
    printf("Enter choice (1-3): ");
    scanf("%d", &choice);
    
    if (choice < 1 || choice > 3) {
        printf("Invalid choice! Try again.\n");
    }
} while (choice < 1 || choice > 3);

Key Difference: do-while executes the loop body at least once, even if the condition is initially false.

5.2.4 Loop Control Statements

Statement Python C Purpose
break break break; Exit loop immediately
continue continue continue; Skip to next iteration

Example:

for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
        continue;  // Skip even numbers
    }
    if (i > 7) {
        break;     // Stop when i > 7
    }
    printf("%d ", i);  // Output: 1 3 5 7
}

6. More Migration Guide: From Python to C

6.1 Common Syntax Differences

Feature Python C
Comments # This is a comment // This is a comment
Block Comments """Multi-line""" /* Multi-line */
Code Blocks Indentation { } braces
Statement End Line break ; semicolon
Boolean Values True, False 1, 0 (or true, false with <stdbool.h>)

6.2 Variable Declaration Migration

Python to C Translation Examples:

# Python
age = 25
height = 5.9
name = "Alice"
is_student = True

// C
int age = 25;
float height = 5.9f;
char name[] = "Alice";
int is_student = 1;  // or use bool with #include <stdbool.h>

6.3 Function Definition Migration (We will learn more about this on the next module)

Python:

def calculate_area(length, width):
    return length * width

result = calculate_area(5, 3)
print(result)

C:

#include <stdio.h>

// Function declaration (prototype)
int calculate_area(int length, int width);

int main() {
    int result = calculate_area(5, 3);
    printf("%d\n", result);
    return 0;
}

// Function definition
int calculate_area(int length, int width) {
    return length * width;
}

6.4 Common Pitfalls for Python Programmers

  1. Forgetting Semicolons:

    int x = 5  // ERROR: Missing semicolon
int x = 5; // CORRECT

  2. Using = instead of == in conditions:

    if (x = 5) { ... }   // ERROR: Assignment, not comparison
if (x == 5) { ... }  // CORRECT: Comparison

  3. Forgetting & in scanf():

    scanf("%d", x);   // ERROR: Missing &
scanf("%d", &x);  // CORRECT

  4. Array Index Out of Bounds: (We will learn more about this on the next module)

    int arr[5];
arr[5] = 10;  // ERROR: Index 5 is out of bounds (valid: 0-4)
arr[4] = 10;  // CORRECT: Last valid index

7. Best Basic Practices and Style Guidelines

7.1 Naming Conventions

7.2 Code Organization

#include <stdio.h>        // System headers
#include <stdlib.h>

#define MAX_SIZE 100      // Constants

// Function prototypes (We will learn more about this in the next module)
int add(int a, int b);
void print_result(int result);

int main() {
    // Main program logic
    return 0;
}

// Function definitions (We will learn more about this in the next module)
int add(int a, int b) {
    return a + b;
}

7.3 Error Handling

Input Validation:

int num;
printf("Enter a positive number: ");
scanf("%d", &num)

if (num < 0) {
    printf("Invalid input!\n");
}

8. Practical Examples

8.1 Complete Program Examples

Example 1: Simple Calculator

#include <stdio.h>

int main() {
    float num1, num2, result;
    char operator;
    
    printf("Enter first number: ");
    scanf("%f", &num1);
    
    printf("Enter operator (+, -, *, /): ");
    scanf(" %c", &operator);
    
    printf("Enter second number: ");
    scanf("%f", &num2);
    
    switch (operator) {
        case '+':
            result = num1 + num2;
            break;
        case '-':
            result = num1 - num2;
            break;
        case '*':
            result = num1 * num2;
            break;
        case '/':
            if (num2 != 0) {
                result = num1 / num2;
            } else {
                printf("Error: Division by zero!\n");
                return 1;
            }
            break;
        default:
            printf("Error: Invalid operator!\n");
            return 1;
    }
    
    printf("%.2f %c %.2f = %.2f\n", num1, operator, num2, result);
    return 0;
}

Example 2: Grade Classification

#include <stdio.h>

int main() {
    int score;
    
    printf("Enter your score (0-100): ");
    scanf("%d", &score);
    
    if (score < 0 || score > 100) {
        printf("Invalid score!\n");
    } else if (score >= 90) {
        printf("Grade: A (Excellent)\n");
    } else if (score >= 80) {
        printf("Grade: B (Very Good)\n");
    } else if (score >= 70) {
        printf("Grade: C (Good)\n");
    } else if (score >= 60) {
        printf("Grade: D (Fair)\n");
    } else {
        printf("Grade: F (Fail)\n");
    }
    
    return 0;
}

8.2 Loop Examples

Example 1: Sum of Numbers

#include <stdio.h>

int main() {
    int n, sum = 0;
    
    printf("Enter a positive integer: ");
    scanf("%d", &n);
    
    for (int i = 1; i <= n; i++) {
        sum += i;
    }
    
    printf("Sum of numbers from 1 to %d is: %d\n", n, sum);
    return 0;
}

Example 2: Multiplication Table

#include <stdio.h>

int main() {
    int num;
    
    printf("Enter a number: ");
    scanf("%d", &num);
    
    printf("Multiplication table for %d:\n", num);
    for (int i = 1; i <= 10; i++) {
        printf("%d x %d = %d\n", num, i, num * i);
    }
    
    return 0;
}

9. Common Debugging Tips

9.1 Compilation Errors

  1. Missing semicolons: Add ; at the end of statements
  2. Undeclared variables: Declare variables before using them
  3. Type mismatches: Ensure compatible types in assignments (this mostly happens in function parameters or calling, we will learn more about this on the next module)
  4. Missing headers: Include necessary header files

9.2 Runtime Errors

  1. Segmentation faults: Check array bounds and pointer usage.
    • this error is the hardest part when it comes to debugging because its highly related to a wrong memory allocation or pointer usage such as accessing an unaccessible variable or array index. We will learn more about this in the next module
  2. Infinite loops: Verify loop conditions and increment/decrement
  3. Wrong output: Check format specifiers in printf/scanf

9.3 Debugging Techniques

// Add debug prints to trace program execution
printf("Debug: x = %d, y = %d\n", x, y);

// Check intermediate results
int temp = a + b;
printf("Intermediate result: %d\n", temp);
int result = temp * c;