C Programming language Cheatsheet

The C programming language is a computer programming language that was developed to do system programming for the operating system UNIX and is an imperative programming language. C was developed in the early 1970s by Ken Thompson and Dennis Ritchie at Bell Labs.

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

Sample C program

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

• #include is a keyword which is used to include the library file <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

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

Literals

Escape sequences

Arrays

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)

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

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.

Strings

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

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 

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);

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