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C Programming Basics Cheat Sheet (DRAFT) by

Basics of C, to be used as a reference

This is a draft cheat sheet. It is a work in progress and is not finished yet.

How to Intialize an Variable

data_type (var_name);
data_type (var_name) = initia­l_v­alue;

Functions

Syntax:
 
return­_type (func_­name) (datatype name1, datatype name2, ...) { 
     . 
     . 
     return something; 
 }
 
int diff(int a, int b) {
 return a - b; 
}
A function that takes two integers and performs subtra­ction

How to use Struct

struct (struc­t_name) {
     (datatype) name1;
     (datatype) name2; 
     . 
 };
A struct is a type of pointer. Similar to making a class.
 
struct struct­_name (name) = { val1, val2, ..};
Initia­lizes a struct with the following values
 
p->x;
The arrow can be used to access members of a struct. This code accesses x from a pointer struct called p. Equivalent to (*p).x

<st­dio.h> and <st­rin­g.h> functions

strlen­(st­ring)
Counts the total char in a string
 
char msg[] = "­mes­sag­e"; 
printf("%s", strlen(msg));
 // This code prints 7
strcat­(st­ring1, string2)
Is used to join two strings together string1 adds the text of string2 to it
 
char msg1[] = "My name is ";
char msg2[] = "Vin";
strcat(msg1, msg2);
printf("%s", msg1);
//This code prints "My name is Vin"
strcmp()
Is used to tell if two strings are equal. Returns 0 if they are equal. Returns a non-zero value if they are not.
 
char msg1[] = "xyz";
char msg2[] = "xyz";
int result = strcmp­(msg1, msg2);
printf("%d", result);
//This code prints 0
strchr()
Is used to find first occurrence of a char in a string
 
char word = "wolves";
void res = strchr­(word, 's');
printf("%s", res);
 //This code returns a pointer to s
strstr()
Is used to find first occurrence of a strin in a string
 
char sent[] = "The world is cold";
void* ptr = strstr­(sent, "orl");
if (sent != NULL) { 
printf("%s", ptr);
}
//This code prints if substring is found
strcpy()
One string copies another
 
char msg[] = "Jim";
char* nmsg = malloc­(st­rle­n(msg) * sizeof(char));
strcpy(nmsg, msg);
printf("%s\n", nmsg);
//This code prints "­Jim­", nmsg was changed

DataTypes

char
refers to characters or strings
Ex. "­car­", 'c'
int
refers to an integer
Ex. 1, 2, 390
float
refers to a decimal, up to 6 digits
Ex. 1.909034
double
refers to a decimal, up to 15 digits
Ex. 1.9090­341111
void
Empty

Pointers

Descri­ption:
A variable that stores another variables address
 
*
The Derefe­rence Operat­or:­access the value stored at the address a pointer is pointing to
 
&
The Address Operator: It is used to get the memory address of a variable.
 
int num = 6; 
 int *ptr = &num;
Return the address of num to ptr, and then derefe­rence it. Essent­ially creating a pointer

Loops

for Loop
 
for (int x = 0; x < 5; x++) { /body/ };
while Loop
 
while (x<5) { /body/ };

How to use malloc()

Allocates space in memory of a specific block size. Returns a void pointer if succes­sful. Remember to free the pointer when done.
 
int nums = malloc­(si­zeo­f(int)  10);
Creates an array of size 10

How to use realloc()

Re-all­ocated space of a given malloc() block space, will preserve data that's already there as long the new space is not smaller
 
int* arr = malloc(5 * sizeof(int));
arr = reallo­c(arr, 10 * sizeof­(int));
The array that was created with malloc() is increased to be able to contain 5 more elements using realloc()
 

Arrays

Syntax:
(datatype) (var_name)[]
int arr[10];
An integer array with space for 10 integers
int nums[] = {1,2};
An integer array with elements declared
int arr[5] = {0};
An integer array of all zeroes
int arr3[5] = {1, 2};
An integer array, first two elements are set, others are 0
char word[] = "­Hel­lo"
A char array which is basically a string

Format Specifiers

%c
Used for character data
char
%d
Used for signed integer data
int
%u
Used for unsigned integer data
unsigned int
%f or %.(num)f
Used for float or double, can insert a number before "­f" for precision
float or double
%s
Used for string data
char (string)[] or char* (string)
%p
Used for printing the address of a pointer
void *(pointer)

Importing files

To import files use #include ... at the top of the file, these are .h files
Use < ... > if importing from standard c librarby
Use " ... " if importing your personal file
 
#include <st­dio.h>
Standard Input Output library
printf(); scanf();
 
#include <st­rin­g.h>
A library with sting manipu­lation functions
strlen(); strcpy(); strcat(); memcpy(); memset();
 
#include <st­dli­b.h>
Standard Library
malloc(); realloc(); free(); rand();