C Programming language Cheatsheet

Basics

• scanf("%d", &x); -- read value into the variable x from input stream
• printf("%d",x); -- printf value to the output stream
• gets(str); -- reads a line from input stream into a variable
• scanf("%[^\n]",s); --reads a line from input using scanf only

Sample C program

#include <stdio.h>    
int main() {    
    printf("Hello World!!");    
    return 0;   
}  

• #include is a keyword which is used to include header files like : <stdlib.h> and <stdio.h>.
• <stdio.h> library file is used to read the data from terminal and to display the data on terminal. It has several in-built functions like printf(), scanf() etc.
• main() function is the entry point of any C program.
• printf and scanf are inbuilt library functions which are used for input and output in C language. They are defined in stdio.h header file.
• return 0 is used to terminate the main() function and returns the value 0
• // -- single line comment
• /* comments */ -- Multi line comment

Data types

1. Basic Data types

2. Derived Data types

Arrays:

data-type array-name[size];

Pointers:

datatype *pointername;

Structures:

struct structure_name {

   member definition;
   member definition;
   ...
   member definition;
} [one or more structure variables]; 

struct structure_name variable name;

3. Enumeration Data types non-negative

enum name{constant1, constant2, constant3, ....... };

4. Void Data types

Void specifies that there is no return value. Generally used when function returns no value, pointer with type as void represents the address of an object but not it's type.

Variables

Syntax:

data-type variable-name = value;

Example:

int x = 10; // declaring int variable and assigning value 10 to it
char grade = 'A'; // declaring char variable and assigning value A to it 

Based on scope, Variables can be classified into two types: 
Local Variables 
Global Variables

Literals

Literals are the constant values assigned to the constant variables.

Escape sequences

Arrays

Array in C can be defined as a method of clubbing multiple entities of similar type into a larger group.

One dimentional Array:

data-type array-name[size];

Example

int a[5] = {1,2,3,4,5};

Two dimensional array:

data-type array-name[size][size];

Example

int a[2][3] = {
                {1,2,3},
                {4,5,6}
              };

Operators

Keywords(reserved words)

Keywords are words that have special meaning to the C compiler. These words help us to use the functionality of C language.
There are 32 keywords in C language.

auto         double      int        struct
break        else        long       switch
case         enum        register   typedef
char         extern      return     union
const        float       short      unsigned
continue     for         signed     void
default      goto        sizeof     volatile
do           if          static     while
alignas      nullptr     typeof     alignof

Identifiers

Identifiers are user defined names for variables, functions and arrays.

Rules:

• They must be less than or equal to 31 characters.
• No special characters.
• Must start with a letter or under score.
• Can contain letters, digits, or underscore only.
• We can not use C keywords as a variable name.

Strings

Strings are an array of characters ended with null character. Enclosed in double quotes.

Declaration

    char str[]="onecompiler";

Constants

Constants are the fixed values. They can be declared in two ways as shown below:

const datatype <constant-name> = <constant-value>;

#define <constant-name> <constant-value>

Special characters

• {} : specifies start and end of code blocks
• [] : used for arrays
• () : used for functions
• , : used to seperate variables, constants etc
• * : used for pointers
• # : used as a macro processor.

Conditional Statements

1. If

if(conditional-expression)
{
    //code
}

2. If-else

if(conditional-expression)
{
    //code
} else {
    //code
}

3. If-else-if ladder

if(conditional-expression-1)
{
    //code
} else if(conditional-expression-2) {
    //code
} else if(conditional-expression-3) {
    //code
}
....
else {
    //code
}

4. Switch

switch(conditional-expression){    
case value1:    
 //code    
 break;  //optional  
case value2:    
 //code    
 break;  //optional  
...    
    
default:     
 //code to be executed when all the above cases are not matched;    
} 

Loops

1. For

for(Initialization; Condition; Increment/decrement){  
//code  
} 

2. While

while (condition){  
//code 
}  

3. Do-While

do {  
//code 
} while (condition); 

Functions

Function is a sub-routine which contains set of statements.

// declaring a function
return_type function_name(parameters);

// defining a function
return_type function_name(parameters){  
//code
}

// calling a function
function_name (parameters)

Pointers

Pointer is a variable which holds the memory information(address) of another variable of same data type.

datatype *pointername;

Example

int x = 10, *ptr;

/*ptr = x; // Error because ptr is adress and x is value
*ptr = &x;  // Error because x is adress and ptr is value */

ptr = &x; // valid because &x and ptr are addresses
*ptr = x; // valid because both x and *ptr values 

int a[10];
ptr = a;  // since a is the address of the first element in array (a[0])
ptr++;    // pointer points to next array element (a[1])

Types of Pointers

Structures

Structure is a user-defined data type where it allows you to combine data of different data types.

struct structure_name {

   member definition;
   member definition;
   ...
   member definition;
} [one or more structure variables]; 

struct structure_name variable name; //declaring structure variables

Unions

Union is a user-defined datatype similar to structs which allows to store different data types in the same memory location. In Unnions, one member can contain a value at any given time.

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

union union_name variable name; // Declaring Union Variables

File handling

File operations like create, update, read, and deleting files which are stored on the local file system can be performed in C.

FILE *fptr; //declaring a pointer of type File
fptr = fopen("filename",mode); //opening a file
fscanf(fptr, "format specifier", data); //read a file
fprintf(fptr, "format specifier", data); //write a file
fclose(fptr);

Dynamic Memory Allocation

A set of functions for dynamic memory allocation from the heap. These methods are used to use the dynamic memory which makes our C programs more efficient

Header Files

Mathematical Functions

All functions defined under <math.h> header file.

Randomize Functions

All functions defined under <stdlib.h> header file. These randomized functions are pseudo-random (time-based random functions).

• random() : returns a random long
• srandom(seed) : seeds the random generator with a new random seed

How To Compile in GCC

• gcc prog.c : compiles prog.c into a.out, run result with ./a.out.
• gcc -o prog prog.c : compiles prog.c into prog, run result with ./prog.
• gcc -g -o prog prog.c : as above, but allows for debugging.

use of fflush()

*used to clear the buffer
*used to accept the next string