C Language - Cheat Sheet

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This C language cheat sheet gives a quick overview of C language concepts starting from the basics to the advanced level. This cheat sheet is very useful for students, developers, and those who are preparing for an interview. Go through this cheat sheet to learn all basic and advanced concepts of C programming language.

Basis Structure of C Program

The basic structure of a C program gives you an idea about the basic statements that you need to use to write a program in C language. The following is the basic structure of a C program −

// Preprocessor directive/header file inclusion section
#include <stdio.h>

// Global declaration section

// the main() function
int main() {
   // Variable declarations section 
   int x, y;
    
   // other code statements section

   // Return o
   return 0;
}
// Other user-defined function definition section

#include <stdio.h>

#include is a preprocessor directive that includes the header file in the C program. The stdio.h is a header file where all input and output-related functions are defined.

main() Function

The main() function is an entry point of a C program, the program's executions start from the main() function.

The below is the syntax of the main() function −

int main() {
   return 0;
}

Comments

There are two types of comments in C language. Single-line and multi-line comments. Comments are ignored by the compilers.

Single-line Comments

Use // to write a single-line comment.

// This is a single-line comment

Multi-line Comments

Use /* and */ before and after the text to write multi-line comments in C language.

/*
This is line 1
This is line 2
..
*/

Printing (printf() Function)

The printf() function is a library function to print the formatted text on the console output. Whenever you want to print anything, use the printf().

Example

printf("Hello world");

User Input (scanf() Function)

The scanf() function is used to take various types of inputs from the user.

Here is the syntax of the scanf() function −

scanf("format_specifier", &variable_name);

Format Specifiers

The following is the list of C format specifiers that are used in printf() and scanf() functions to print/input specific type of values.

Example

#include <stdio.h>

int main(){

   int age = 18;

   float percent = 67.75;

   printf("Age: %d \nPercent: %f", age, percent);

   return 0;
}

Output

Age: 18 
Percent: 67.750000

Data Types

The data types specify the type and size of the data to be stored in a variable. Data types are categorized in 3 sections −

• Basic Data Types
• Derived Data Types
• User-defined Data Types

Basic Data Types

The basic data types are the built-in data types in C language and they are also used to create derived data types.

Derived Data Types

The derived data types are derived from the basic data types. The derived data types are −

• Array
• Pointer

User-defined Data Types

The user-defined data types are created by the programmer to handle data of different type and based on the requirements. The user-defined data types are −

• Structures
• Unions
• Enumerations

Basic Input & Output

For basic input and output in C language, we use printf() and scanf() functions.

The printf() function is used to print the formatted text on the console.

printf("Hello world");

The scanf() function is used to take input from the user.

scanf("%d", &x); // Integer input
scanf("%f", &y); // float input
scanf("%c", &z); // Character Input
scanf("%s", name); // String input

Example of Basic Input and Output

#include <stdio.h>

int main() {
   int num;

   printf("Input any integer number: ");
   scanf("%d", &num);

   printf("The input is: %d\n", num);

   return 0;
} 

Output

Input any integer number: The input is: 0

Identifiers

C identifiers are user-defined names for variables, constants, functions, etc. The following are the rules for defining identifiers −

• Keywords can't be used as identifiers.
• Only alphabets, underscore symbol (_), and digits are allowed in the identifier.
• The identifier must start either with an alphabet or an underscore.
• The same identifier can't be used as the name of two entities.
• Identifiers should be meaningful and descriptive.

Examples of Valid Identifiers

age, _name, person1, roll_no

Keywords

C keywords are the reversed words in the C compiler, which are used for specific purposes and must not be used as an identifier.

The following are the keywords in C language −

Variables

C variables are the name given to a storage area that our programs can use to access and manipulate the data.

Syntax of Declaring a Variable

data_type variable_name;

Escape Sequences

Escape sequences are the special characters followed by the escape (backward slash \). Escape sequences have special meanings and are used for printing those characters that cannot be printed normally.

Here is the list of escape sequences in C language −

Operators

Operators are the special symbols that are used to perform specific mathematical or logical operations.

Below are the operators used in C language −

Example of Operators

result = num1 + num2;
if(result >=10){
   printf("Greater than 10.");
}

Conditional Statements

C language provides the following conditional statements −

• if Statement
• if-else Statement
• if-else-if Statement
• Nested if-else Statement
• Switch Statement
• Ternary Operator

if Statement

An if statement consists of a Boolean expression followed by one or more statements.

The syntax of if statement is −

if(boolean_expression) {
   /* statement(s) will execute if the boolean expression is true */
}

if-else statement

An if-else statement can be followed by an optional else statement, which executes when the Boolean expression is false.

The syntax of the if statement is −

if (Boolean expr){
   Expression;
   . . .
}
else{
   Expression;
   . . .
}

if-else-if Statement

The if-else-if statement is also known as the ladder if-else. It is used to check multiple conditions when a condition is not true.

The syntax of if-else-if statement −

if(condition1){
}
else if(condition2){
}
…
else{
}

Nested if Statements

By using the nested if statements, you can use one if or else-if statement inside another if or else-if statement(s).

The syntax of nested if statements −

if (expr1){
   if (expr2){
      block to be executed when 
      expr1 and expr2 are true
   }
   else{
      block to be executed when 
      expr1 is true but expr2 is false
   }        
}

Switch Statement

A switch statement allows a variable to be tested for equality against a list of values.

The syntax of the switch statement is −

switch (Expression){

   // if expr equals Value1
   case Value1: 
      Statement1;
      Statement2;
      break;
   
   // if expr equals Value2
   case Value2: 
      Statement1;
      Statement2;
      break;
      .
      . 
   // if expr is other than the specific values above
   default:
      Statement1;
      Statement2;
}

Ternary Operator

The ternary operator (?:) is also known as the conditional operator. It can be used as a replacement for an if-else statement.

The syntax of the ternary operator is −

(condition) ? true_block: false_block;

Loops

C loops are used to execute blocks of one or more statements respectively a specified number of times, or till a certain condition is reached. The following are the loop statements in C language −

• while Loop
• do...while Loop
• for Loop

while Loop

The while loop is an entry-control loop where the condition is checked before executing the loop body.

The syntax of the while loop is −

while(test_condition){
   // Statement(s);
}

do…while Loop

The do…while loop is an exit control loop where the body of the loop executes before checking the condition.

The syntax of do…while loop is −

do{
   // Statement(s);
}while(test_condition);

for Loop

The for loop is also an entry-controlled loop where the elements (initialization, test condition, and increment) are placed together to form a for loop inside the parenthesis with the for keyword.

The syntax of the for loop is −

for(initialization ; test condition ; increment){
   // Statement (s);
}

Jump Statements

Jump statements are used to transfer the flow of the program from one place to another. The following are the jump statements in C language −

• goto Statement
• break Statement
• continue Statement

goto Statement

The goto statement transfers the program's control to a specific label. You need to define a label followed by the colon (:). The goto statement can transfer the program's flow up or down.

The syntax of the goto statement is −

label_name:

//Statement(s)
if(test_condition){
   goto label_name;
}

break Statement

The break statement can be used with loops and switch statements. The break statement terminates the loop execution and transfers the program's control outside of the loop body.

The syntax of the break statement is −

while(condition1){
   . . .
   . . .
   if(condition2)
   break;
      . . .
      . . .
}

continue Statement

The continue statement is used to skip the execution of the rest of the statement within the loop in the current iteration and transfer it to the next loop iteration.

The syntax of the continue statement is −

while (expr){
   . . .
   . . .
   if (condition)
      continue;
   . . .
}

User-defined Functions

The user-defined function is defined by the user to perform specific tasks to achieve the code reusability and modularity.

Example of user-defined function

#include <stdio.h>

// Function declaration
int add(int, int);

// Function definition
int add(int a, int b) { return (a + b); }

int main() {
   int num1 = 10, num2 = 10;
   int res_add;

   // Calling the function
   res_add = add(num1, num2);

   // Printing the results
   printf("Addition is : %d\n", res_add);

   return 0;
}

Output

Addition is : 20

Arrays

An array is a collection of data items of similar data type which are stored at a contiguous memory location. The data item may be primary data types (int, float, char), or user-defined types such as struct or pointers can be stored in an array.

C Arrays can be of two types −

• One-dimensional (1D) Array − A one-dimensional array is a single list of data items of the same data type.
• Multi-dimensional Arrays − A multi-dimensional array such as a two-dimensional array is an array of arrays.

Syntax of Arrays

The following is the syntax of declarations of different types of arrays −

type array_name [size1];   // One-dimensional array
type array_name [size1][size2];   // Two-dimensional arrays
type array_name [size1][size2][size3];    // Three-dimensional arrays

Example of One-dimensional Array

#include <stdio.h>

int main(){
   int numbers[5] = {10, 20, 30, 40, 50};

   int i;  // loop counter

   // Printing array elements
   printf("The array elements are : ");
   for (i = 0; i < 5; i++) {
      printf("%d ", numbers[i]);
   }

   return 0;
}

Output

The array elements are : 10 20 30 40 50 

Example of Two-dimensional Arrays

#include <stdio.h>

int main () {

   /* an array with 5 rows and 2 columns*/
   int a[5][2] = { {0,0}, {1,2}, {2,4}, {3,6},{4,8}};
   int i, j;
 
   /* output each array element's value */
   for ( i = 0; i < 5; i++ ) {
      for ( j = 0; j < 2; j++ ) {
         printf("a[%d][%d] = %d\n", i,j, a[i][j] );
      }
   }
   
   return 0;
}

Output

a[0][0] = 0
a[0][1] = 0
a[1][0] = 1
a[1][1] = 2
a[2][0] = 2
a[2][1] = 4
a[3][0] = 3
a[3][1] = 6
a[4][0] = 4
a[4][1] = 8

Strings

C string is a sequence of characters i.e., an array of char data type, terminated by "null character" represented by '\0'. To read and print the string using scanf() and printf() functions, you will have to use the "%s" format specifier.

String Declaration

char string_name[size];

Reading String

scanf("%s", string_name);

Printing String

printf("%s", string_name);

Example of C strings

#include <stdio.h>

int main() {
   char name[20];

   printf("Enter a name: ");
   scanf("%s", name);

   printf("You entered: %s", name);

   return 0;
}

Strings Functions

C standard library string.h provides various functions to work with the strings. Here is the list of C string functions −

Structures

C structures are the collection of different data types. Structures are considered user-defined data types where you can group data items of different data types.

Structure Declaration Syntax

struct struct_name {
   type1 item1;
   type2 item2;
   .
   .
}structure_variable;

Example of Structure

#include <stdio.h>

struct book{
   char title[10];
   char author[20];
   double price;
   int pages;
};

int main(){
   struct book book1 = {"Learn C", "Dennis Ritchie", 675.50, 325};
   
   printf("Title:  %s \n", book1.title);
   printf("Author: %s \n", book1.author);
   printf("Price:  %lf\n", book1.price);
   printf("Pages:  %d \n", book1.pages);
   printf("Size of book struct: %d", sizeof(struct book));
   return 0;
}

Output

Title:  Learn C 
Author: Dennis Ritchie 
Price:  675.500000
Pages:  325 
Size of book struct: 48

Unions

C union is a user-defined data type that allows to store set of data items of different data types in the same memory location.

Syntax of Union Declaration

union [union tag]{
   member definition;
   member definition;
   ...
   member definition;
} [one or more union variables];

Example of Union

#include <stdio.h>

union Data{
   int i;
   float f;
};

int main(){
   union Data data;        
   
   data.i = 10;
   data.f = 220.5;
   
   printf("data.i: %d \n", data.i);
   printf("data.f: %f \n", data.f);
   return 0;
}

Output

data.i: 1130135552 
data.f: 220.500000 

Enumerations (enums)

C enumeration (enum) is an enumerated data type that consists of a group of integral constants.

Syntax of enum Declaration

enum myenum {val1, val2, val3, val4};

Example of Enumeration (enum)

#include <stdio.h>

enum status_codes { OKAY = 1, CANCEL = 0, ALERT = 2 };

int main() {
   // Printing values
   printf("OKAY = %d\n", OKAY);
   printf("CANCEL = %d\n", CANCEL);
   printf("ALERT = %d\n", ALERT);

   return 0;
}

Output

OKAY = 1
CANCEL = 0
ALERT = 2

Pointers

C pointers are derived data types that are used to store the address of another variable and can also be used to access and manipulate the variable's data stored at that location.

Syntax of Pointer Declaration

data_type *pointer_name;

Syntax of Pointer Initialization

If you are declared a pointer, below is the syntax to initialize a pointer with the address of another variable −

pointer_name = &variable_name;

Pointer Example

#include <stdio.h>

int main() {
   int x = 10;

   // Pointer declaration and initialization
   int * ptr = & x;

   // Printing the current value
   printf("Value of x = %d\n", * ptr);

   // Changing the value
   * ptr = 20;

   // Printing the updated value
   printf("Value of x = %d\n", * ptr);

   return 0;
}

Output

Value of x = 10
Value of x = 20

Type of Pointers

There are various types of pointers in C language. They are −

• Null pointer
• Wild pointer
• Void pointer
• Dangling pointer
• Complex pointers
• Far pointer
• File pointers
• Double pointers
• Near pointer

Dynamic Memory Allocations

Memories for variables are declared at compile-time. C language provides some functions for dynamic memory allocations that allow to allocation of memory for the variables at run time.

The functions for dynamic memory allocation are −

• malloc()
• calloc()
• realloc()
• free()

malloc() Function

The malloc() function allocates the requested memory (number of blocks of the specified size) and returns a pointer to it.

The syntax of malloc() function is −

malloc (size_t size);
calloc() Function

calloc() Function

The calloc() function allocates the requested memory (number of blocks of the specified size) and returns the void pointer. The calloc() function sets allocated memory to zero.

The syntax of calloc() function is −

void *calloc(size_t nitems, size_t size)

realloc() Function

The realloc() function attempts to resize the memory block pointed to by a pointer that was previously allocated with a call to malloc() or calloc() function.

The syntax of realloc() function is −

void *calloc(size_t nitems, size_t size)

free() Function

The free() function deallocates the memory previously allocated by a call to calloc(), malloc(), or realloc().

The syntax of realloc() function is −

void *calloc(size_t nitems, size_t size)

File Handling

File handling refers to perform various operations on files such as creating, writing, reading, deleting, moving, renaming files, etc. C language provides various functions for file handling.

File Operations

The following are the operations that can perform a file using C language file handling functions −

• Creating a new file
• Opening an existing file
• Writing data to a file
• Appending data to a file
• Reading data from a file
• Renaming a file
• Deleting a file
• Closing a file

File Handling Functions

Following is the list of file handling functions in C −

Example of File Handling

Here is an example of file handling in C language −

#include <stdio.h>
#include <stdlib.h>

int main() {
   FILE *file;
   char file_name[] = "my_file.txt";
   char write_data[100] = "Tutorials Point";
   char read_data[100];

   // Opening file in write mode
   file = fopen("file_name.txt", "w");
   if (file == NULL) {
      printf("Error\n");
      return 1;
   }
   // Writing to the file
   fprintf(file, "%s", write_data);
   // Closing the file
   fclose(file);

   // Again, opening the file in read mode
   file = fopen("file_name.txt", "r");
   if (file == NULL) {
      printf("Error.\n");
      return 1;
   }

   // Reading data from the file
   if (fgets(read_data, 100, file) != NULL) {
      // Printing it on the screen
      printf("File's data:\n%s\n", read_data);
   }
   fclose(file);

   return 0;
}

Output

File's data:
Tutorials Point

Preprocessor Directives

The preprocessor directives are part of pre-compilation and start with the hash (#) character. These directives instruct the compiler to expand include and expand the code before starting the process of compilation.

Here is the list of preprocessor directives −

Example of Preprocessor Directive

This is an example of #define which is one of the preprocessor directives in C language −

#define MAX_ARRAY_LENGTH 20

Standard Libraries

Here is the list of commonly used libraries (C header files) −