C

C is a procedural programming language for general-purpose programming.

Getting Started

hello.c

#include <stdio.h>

int main(void) {
  printf("Hello World!\n");

  return 0;
}

Compile hello.c file with gcc

$ gcc hello.c -o hello

Run the compiled binary hello

$ ./hello

Output => Hello World!

Variables

int myNum = 15;

int myNum2; // do not assign, then assign
myNum2 = 15;

int myNum3 = 15; // myNum3 is 15
myNum3 = 10;     // myNum3 is now 10

float myFloat = 5.99; // floating point number
char myLetter = 'D';  // character

int x = 5;
int y = 6;
int sum = x + y; // add variables to sum

// declare multiple variables
int x = 5, y = 6, z = 50;

Constants

const int minutesPerHour = 60;
const float PI = 3.14;

Best Practices

const int BIRTHYEAR = 1980;

Comment

// this is a comment
printf("Hello World!"); // Can comment anywhere in file

/*Multi-line comment, print Hello World!
to the screen, it's awesome */

Print text

printf("I am learning C.");
int testInteger = 5;
printf("Number = %d", testInteger);

float f = 5.99; // floating point number
printf("Value = %f", f);

short a = 0b1010110; // binary number
int b = 02713; // octal number
long c = 0X1DAB83; // hexadecimal number

// output in octal form
printf("a=%ho, b=%o, c=%lo\n", a, b, c);
// output => a=126, b=2713, c=7325603

// Output in decimal form
printf("a=%hd, b=%d, c=%ld\n", a, b, c);
// output => a=86, b=1483, c=1944451

// output in hexadecimal form (letter lowercase)
printf("a=%hx, b=%x, c=%lx\n", a, b, c);
// output => a=56, b=5cb, c=1dab83

// Output in hexadecimal (capital letters)
printf("a=%hX, b=%X, c=%lX\n", a, b, c);
// output => a=56, b=5CB, c=1DAB83

Control the number of spaces

int a1 = 20, a2 = 345, a3 = 700;
int b1 = 56720, b2 = 9999, b3 = 20098;
int c1 = 233, c2 = 205, c3 = 1;
int d1 = 34, d2 = 0, d3 = 23;

printf("%-9d %-9d %-9d\n", a1, a2, a3);
printf("%-9d %-9d %-9d\n", b1, b2, b3);
printf("%-9d %-9d %-9d\n", c1, c2, c3);
printf("%-9d %-9d %-9d\n", d1, d2, d3);

output result

20        345       700      
56720     9999      20098    
233       205       1        
34        0         23  

In %-9d, d means to output in 10 base, 9 means to occupy at least 9 characters width, and the width is not enough to fill with spaces, - means left alignment

Strings

char greetings[] = "Hello World!";
printf("%s", greetings);

access string

char greetings[] = "Hello World!";
printf("%c", greetings[0]);

modify string

char greetings[] = "Hello World!";
greetings[0] = 'J';

printf("%s", greetings);
// prints "Jello World!"

Another way to create a string

char greetings[] = {'H','e','l','l','\0'};

printf("%s", greetings);
// print "Hell!"

Creating String using character pointer (String Literals)

char *greetings = "Hello";
printf("%s", greetings);
// print "Hello!"

NOTE: String literals might be stored in read-only section of memory. Modifying a string literal invokes undefined behavior. You can’t modify it.!

C does not have a String type, use char type and create an array of characters

Condition

int time = 20;
if (time < 18) {
  printf("Goodbye!");
} else {
  printf("Good evening!");
}
// Output -> "Good evening!"
int time = 22;
if (time < 10) {
  printf("Good morning!");
} else if (time < 20) {
  printf("Goodbye!");
} else {
  printf("Good evening!");
}
// Output -> "Good evening!"

Ternary operator

int age = 20;
(age > 19) ? printf("Adult") : printf("Teenager");

Switch

int day = 4;

switch (day) {
  case 3: printf("Wednesday"); break;
  case 4: printf("Thursday"); break;
  default:
    printf("Weekend!");
}
// output -> "Thursday" (day 4)

While Loop

int i = 0;

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

NOTE: Don’t forget to increment the variable used in the condition, otherwise the loop will never end and become an “infinite loop”!

Do/While Loop

int i = 0;

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

For Loop

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

Break out of the loop Break/Continue

for (int i = 0; i < 10; i++) {
  if (i == 4) {
    break;
  }
  printf("%d\n", i);
}

break out of the loop when i is equal to 4

for (int i = 0; i < 10; i++) {
  if (i == 4) {
    continue;
  }
  printf("%d\n", i);
}

Example to skip the value of 4

While Break Example

int i = 0;

while (i < 10) {
  if (i == 4) {
    break;
  }
  printf("%d\n", i);

  i++;
}

While continue example

int i = 0;

while (i < 10) {
  i++;

  if (i == 4) {
    continue;
  }
  printf("%d\n", i);
}

Arrays

int myNumbers[] = {25, 50, 75, 100};

printf("%d", myNumbers[0]);
// output 25

change array elements

int myNumbers[] = {25, 50, 75, 100};
myNumbers[0] = 33;

printf("%d", myNumbers[0]);

Loop through the array

int myNumbers[] = {25, 50, 75, 100};
int i;

for (i = 0; i < 4; i++) {
  printf("%d\n", myNumbers[i]);
}

set array size

// Declare an array of four integers:
int myNumbers[4];

// add element
myNumbers[0] = 25;
myNumbers[1] = 50;
myNumbers[2] = 75;
myNumbers[3] = 100;

Enumeration Enum

enum week { Mon = 1, Tues, Wed, Thurs, Fri, Sat, Sun };

define enum variable

enum week a, b, c;
enum week { Mon = 1, Tues, Wed, Thurs, Fri, Sat, Sun } a, b, c;

With an enumeration variable, you can assign the value in the list to it

enum week { Mon = 1, Tues, Wed, Thurs, Fri, Sat, Sun };
enum week a = Mon, b = Wed, c = Sat;
// or
enum week{ Mon = 1, Tues, Wed, Thurs, Fri, Sat, Sun } a = Mon, b = Wed, c = Sat;

Enumerate sample applications

enum week {Mon = 1, Tues, Wed, Thurs} day;

scanf("%d", &day);

switch(day) {
  case Mon: puts("Monday"); break;
  case Tues: puts("Tuesday"); break;
  case Wed: puts("Wednesday"); break;
  case Thursday: puts("Thursday"); break;
  default: puts("Error!");
}

User input

// Create an integer variable to store the number we got from the user
int myNum;

// Ask the user to enter a number
printf("Please enter a number: \n");

// Get and save the number entered by the user
scanf("%d", &myNum);

// Output the number entered by the user
printf("The number you entered: %d", myNum);

User input string

// create a string
char firstName[30];
// Ask the user to enter some text
printf("Enter your name: \n");
// get and save the text
scanf("%s", &firstName);
// output text
printf("Hello %s.", firstName);

memory address

When a variable is created, it is assigned a memory address

int myAge = 43;

printf("%p", &myAge);
// Output: 0x7ffe5367e044

To access it, use the reference operator (&)

create pointer

int myAge = 43; // an int variable
printf("%d", myAge); // output the value of myAge(43)

// Output the memory address of myAge (0x7ffe5367e044)
printf("%p", &myAge);

pointer variable

int myAge = 43; // an int variable
int*ptr = &myAge; // pointer variable named ptr, used to store the address of myAge

printf("%d\n", myAge); // print the value of myAge (43)

printf("%p\n", &myAge); // output the memory address of myAge (0x7ffe5367e044)
printf("%p\n", ptr); // use the pointer (0x7ffe5367e044) to output the memory address of myAge

Dereference

int myAge = 43; // variable declaration
int*ptr = &myAge; // pointer declaration

// Reference: output myAge with a pointer
// memory address (0x7ffe5367e044)
printf("%p\n", ptr);
// dereference: output the value of myAge with a pointer (43)
printf("%d\n", *ptr);

Operators

Arithmetic Operators

int myNum = 100 + 50;
int sum1 = 100 + 50; // 150 (100 + 50)
int sum2 = sum1 + 250; // 400 (150 + 250)
int sum3 = sum2 + sum2; // 800 (400 + 400)

---

Operator	Name	Example
+	Add	x + y
-	Subtract	x - y
*	Multiply	x * y
/	Divide	x / y
%	Modulo	x % y
++	Increment	++x
--	Decrement	--x

Assignment operator

example	as
x = 5	x = 5
x += 3	x = x + 3
x -= 3	x = x - 3
x *= 3	x = x * 3
x /= 3	x = x / 3
x %= 3	x = x % 3
x &= 3	x = x & 3
x |= 3	x = x | 3
x ^= 3	x = x ^ 3
x >>= 3	x = x >> 3
x <<= 3	x = x << 3

Comparison Operators

int x = 5;
int y = 3;

printf("%d", x > y);
// returns 1 (true) because 5 is greater than 3

---

Symbol	Name	Example
==	equals	x == y
!=	not equal to	x != y
>	greater than	x > y
<	less than	x < y
>=	greater than or equal to	x >= y
<=	less than or equal to	x <= y

Comparison operators are used to compare two values

Logical Operators

Symbol	Name	Description	Example
&&	and logical	returns true if both statements are true	x < 5 && x < 10
||	or logical	returns true if one of the statements is true	x < 5 || x < 4
!	not logical	Invert result, return false if true	!(x < 5 && x < 10)

Operator Examples

unsigned int a = 60; /*60 = 0011 1100 */
unsigned int b = 13; /*13 = 0000 1101 */
int c = 0;

c = a & b; /*12 = 0000 1100 */
printf("Line 1 -the value of c is %d\n", c);

c = a | b; /*61 = 0011 1101 */
printf("Line 2 -the value of c is %d\n", c);
c = a ^ b; /*49 = 0011 0001 */
printf("Line 3 -the value of c is %d\n", c);
c = ~a; /*-61 = 1100 0011 */
printf("Line 4 -The value of c is %d\n", c);
c = a << 2; /*240 = 1111 0000 */
printf("Line 5 -the value of c is %d\n", c);
c = a >> 2; /*15 = 0000 1111 */
printf("Line 6 -The value of c is %d\n", c);

Bitwise operators

operator	description	instance
&	Bitwise AND operation, “AND” operation by binary digits	(A & B) will get 12 which is 0000 1100
|	Bitwise OR operator, “or” operation by binary digit	(A | B) will get 61 which is 0011 1101
^	XOR operator, perform “XOR” operation by binary digits	(A ^ B) will get 49 which is 0011 0001
~	Inversion operator, perform “inversion” operation by binary bit	(~A) will get -61 which is 1100 0011
<<	binary left shift operator	A << 2 will get 240 which is 1111 0000
>>	binary right shift operator	A >> 2 will get 15 which is 0000 1111

Data Types

Basic data types

Data Type	Size Size	Range Range	Description Description
char	1 byte	−128 ~ 127	single character/alphanumeric/ASCII
signed char	1 byte	−128 ~ 127	-
unsigned char	1 byte	0 ~ 255	-
int	2 to 4 bytes	−32,768 ~ 32,767	store integers
signed int	2 bytes	−32,768 ~ 32,767
unsigned int	2 bytes	0 ~ 65,535
short int	2 bytes	−32,768 ~ 32,767
signed short int	2 bytes	−32,768 ~ 32,767
unsigned short int	2 bytes	0 ~ 65,535
long int	4 bytes	-2,147,483,648 ~ 2,147,483,647
signed long int	4 bytes	-2,147,483,648 ~ 2,147,483,647
unsigned long int	4 bytes	0 ~ 4,294,967,295
float	4 bytes	3.4E-38 ~ 3.4E+38
double	8 bytes	1.7E-308 ~ 1.7E+308
long double	10 bytes	3.4E-4932 ~ 1.1E+4932

Data types

// create variables
int myNum = 5; // integer
float myFloatNum = 5.99; // floating point number
char myLetter = 'D'; // string
// High precision floating point data or numbers
double myDouble = 3.2325467;
// print output variables
printf("%d\n", myNum);
printf("%f\n", myFloatNum);
printf("%c\n", myLetter);
printf("%lf\n", myDouble);

---

Data Type	Description
char	character type
short	short integer
int	integer type
long	long integer
float	single-precision floating-point type
double	double-precision floating-point type
void	no type

Basic format specifiers

format specifier	data type
%d or %i	int integer
%f	float single-precision decimal type
%lf	double high precision floating point data or number
%c	char character
%s	for strings strings

Basic format specifiers

	short	int	long
Octal	%ho	%o	%lo
Decimal	%hd	%d	%ld
Hexadecimal	%hx /%hX	%x /%X	%lx /%lX

Data format example

int myNum = 5;
float myFloatNum = 5.99; // floating point number
char myLetter = 'D';     // string
// print output variables
printf("%d\n", myNum);
printf("%f\n", myFloatNum);
printf("%c\n", myLetter);

C Preprocessor

Preprocessor Directives

command	description
#define	define a macro
#include	include a source code file
#undef	undefined macro
#ifdef	Returns true if the macro is defined
#ifndef	Returns true if the macro is not defined
#if	Compile the following code if the given condition is true
#else	Alternative to #if
#elif	If the #if condition is false, the current condition is true
#endif	End a #if...#else conditional compilation block
#error	Print an error message when standard error is encountered
#pragma	Issue special commands to the compiler using the standardized method

// replace all MAX_ARRAY_LENGTH with 20
#define MAX_ARRAY_LENGTH 20
// Get stdio.h from the system library
#include <stdio.h>
// Get myheader.h in the local directory
#include "myheader.h"
#undef FILE_SIZE
#define FILE_SIZE 42 // undefine and define to 42

Predefined macros

macro | description —-| —- __DATE__ | The current date, a character constant in the format “MMM DD YYYY” __TIME__ | The current time, a character constant in the format “HH:MM:SS” __FILE__ | This will contain the current filename, a string constant __LINE__ | This will contain the current line number, a decimal constant __STDC__ | Defined as 1 when the compiler compiles against the ANSI standard

ANSI C defines a number of macros that you can use, but you cannot directly modify these predefined macros

Predefined macro example

#include <stdio.h>

int main() {
  printf("File :%s\n", __FILE__);
  printf("Date :%s\n", __DATE__);
  printf("Time :%s\n", __TIME__);
  printf("Line :%d\n", __LINE__);
  printf("ANSI :%d\n", __STDC__);
}

Macro continuation operator ()

A macro is usually written on a single line.

#define message_for(a, b) \
    printf(#a " and " #b ": We love you!\n")

If the macro is too long to fit on a single line, use the macro continuation operator \

String Constantization Operator (#)

#include <stdio.h>

#define message_for(a, b) \
  printf(#a " and " #b ": We love you!\n")

int main(void) {
  message_for(Carole, Debra);

  return 0;
}

When the above code is compiled and executed, it produces the following result:

Carole and Debra: We love you!

When you need to convert a macro parameter to a string constant, use the string constant operator #

tag paste operator (##)

#include <stdio.h>

#define tokenpaster(n) printf ("token" #n " = %d", token##n)

int main(void) {
  int token34 = 40;
  tokenpaster(34);

  return 0;
}

defined() operator

#include <stdio.h>

#if !defined (MESSAGE)
   #define MESSAGE "You wish!"
#endif

int main(void) {
  printf("Here is the message: %s\n", MESSAGE);

  return 0;
}

Parameterized macros

int square(int x) {
  return x * x;
}

The macro rewrites the above code as follows:

#define square(x) ( (x) * (x) )

No spaces are allowed between the macro name and the opening parenthesis

#include <stdio.h>
#define MAX(x,y) ( (x) > (y) ? (x) : (y) )

int main(void) {
  printf("Max between 20 and 10 is %d\n", MAX(10, 20));

  return 0;
}

C Function

Function declaration and definition

int main(void) {
  printf("Hello World!");

  return 0;
}

The function consists of two parts

void myFunction() { // declaration declaration
  // function body (code to be executed) (definition)
}

---

• Declaration declares the function name, return type and parameters (if any)
• Definition function body (code to execute)

---

// function declaration
void myFunction();
// main method
int main() {
  myFunction(); // --> call the function

  return 0;
}

void myFunction() {// Function definition
  printf("Good evening!");
}

Call function

// create function
void myFunction() {
  printf("Good evening!");
}

int main() {
  myFunction(); // call the function
  myFunction(); // can be called multiple times

  return 0;
}
// Output -> "Good evening!"
// Output -> "Good evening!"

Function parameters

void myFunction(char name[]) {
  printf("Hello %s\n", name);
}

int main() {
  myFunction("Liam");
  myFunction("Jenny");

  return 0;
}
// Hello Liam
// Hello Jenny

Multiple parameters

void myFunction(char name[], int age) {
  printf("Hi %s, you are %d years old.\n",name,age);
}
int main() {
  myFunction("Liam", 3);
  myFunction("Jenny", 14);

  return 0;
}
// Hi Liam you are 3 years old.
// Hi Jenny you are 14 years old.

Return value

int myFunction(int x) {
  return 5 + x;
}

int main() {
  printf("Result: %d", myFunction(3));
  return 0;
}
// output 8 (5 + 3)

two parameters

int myFunction(int x, int y) {
  return x + y;
}

int main() {
  printf("Result: %d", myFunction(5, 3));
  // store the result in a variable
  int result = myFunction(5, 3);
  printf("Result = %d", result);

  return 0;
}
// result: 8 (5 + 3)
// result = 8 (5 + 3)

Recursive example

int sum(int k);

int main() {
  int result = sum(10);
  printf("%d", result);

  return 0;
}

int sum(int k) {
  if (k > 0) {
    return k + sum(k -1);
  } else {
    return 0;
  }
}

Mathematical functions

#include <math.h>

void main(void) {
  printf("%f", sqrt(16)); // square root
  printf("%f", ceil(1.4)); // round up (round)
  printf("%f", floor(1.4)); // round up (round)
  printf("%f", pow(4, 3)); // x(4) to the power of y(3)
}

---

• abs(x) absolute value
• acos(x) arc cosine value
• asin(x) arc sine
• atan(x) arc tangent
• cbrt(x) cube root
• cos(x) cosine
• the value of exp(x) Ex
• sin(x) the sine of x
• tangent of tan(x) angle

C Structures

Create structure

struct MyStructure { // structure declaration
  int myNum; // member (int variable)
  char myLetter; // member (char variable)
}; // end the structure with a semicolon

Create a struct variable called s1

struct myStructure {
  int myNum;
  char myLetter;
};

int main() {
  struct myStructure s1;

  return 0;
}

Strings in the structure

struct myStructure {
  int myNum;
  char myLetter;
  char myString[30]; // String
};

int main() {
  struct myStructure s1;
  strcpy(s1. myString, "Some text");
  // print value
  printf("my string: %s", s1.myString);

  return 0;
}

Assigning values ​​to strings using the strcpy function

Accessing structure members

// create a structure called myStructure
struct myStructure {
  int myNum;
  char myLetter;
};

int main() {
  // Create a structure variable called myStructure called s1
  struct myStructure s1;
  // Assign values ​​to the members of s1
  s1.myNum = 13;
  s1.myLetter = 'B';

  // Create a structure variable of myStructure called s2
  // and assign it a value
  struct myStructure s2 = {13, 'B'};
  // print value
  printf("My number: %d\n", s1.myNum);
  printf("My letter: %c\n", s1.myLetter);

  return 0;
}

Create different structure variables

struct myStructure s1;
struct myStructure s2;
// Assign values ​​to different structure variables
s1.myNum = 13;
s1.myLetter = 'B';

s2.myNum = 20;
s2.myLetter = 'C';

Copy structure

struct myStructure s1 = {
  13, 'B', "Some text"
};

struct myStructure s2;
s2 = s1;

In the example, the value of s1 is copied to s2

Modify value

// Create a struct variable and assign it a value
struct myStructure s1 = {
  13, 'B'
};
// modify the value
s1.myNum = 30;
s1.myLetter = 'C';
// print value
printf("%d %c %s",
    s1.myNum,
    s1.myLetter);

file processing

File processing function

function	description Description
fopen()	open a new or existing file
fprintf()	write data to file
fscanf()	read data from a file
fputc()	write a character to file
fgetc()	read a character from a file
fclose()	close the file
fseek()	set the file pointer to the given position
fputw()	Write an integer to a file
fgetw()	read an integer from a file
ftell()	returns the current position
rewind()	set the file pointer to the beginning of the file

There are many functions in the C library to open/read/write/search and close files

Open mode parameter

Mode Mode	Description Description
r	Open a text file in read mode, allowing the file to be read
w	Open a text file in write mode, allowing writing to the file
a	Open a text file in append mode If the file does not exist, a new one will be created
r+	Open a text file in read-write mode, allowing reading and writing of the file
w+	Open a text file in read-write mode, allowing reading and writing of the file
a+	Open a text file in read-write mode, allowing reading and writing of the file
rb	Open a binary file in read mode
wb	Open binary file in write mode
ab	Open a binary file in append mode
rb+	open binary file in read-write mode
wb+	Open binary file in read-write mode
ab+	open binary file in read-write mode

Open the file: fopen()

#include <stdio.h>

void main() {
  FILE *fp;
  char ch;

  fp = fopen("file_handle.c", "r");

  while (1) {
    ch = fgetc(fp);
    if (ch == EOF)
      break;
    printf("%c", ch);
  }
  fclose(fp);
}

After performing all operations on the file, the file must be closed with fclose()

Write to file: fprintf()

#include <stdio.h>

void main() {
  FILE *fp;
  fp = fopen("file.txt", "w"); // open the file

  // write data to file
  fprintf(fp, "Hello file for fprintf..\n");
  fclose(fp); // close the file
}

Read the file: fscanf()

#include <stdio.h>

void main() {
  FILE *fp;

  char buff[255]; // Create a char array to store file data
  fp = fopen("file.txt", "r");

  while(fscanf(fp, "%s", buff) != EOF) {
    printf("%s ", buff);
  }
  fclose(fp);
}

Write to file: fputc()

#include <stdio.h>

void main() {
  FILE *fp;
  fp = fopen("file1.txt", "w"); // open the file
  fputc('a',fp); // write a single character to the file
  fclose(fp); // close the file
}

Read the file: fgetc()

#include <stdio.h>
#include <conio.h>

void main() {
  FILE *fp;
  char c;

  clrscr();

  fp = fopen("myfile.txt", "r");

  while( (c = fgetc(fp) ) != EOF) {
    printf("%c", c);
  }
  fclose(fp);

  getch();
}

Write to file: fputs()

#include<stdio.h>
#include<conio.h>

void main() {
  FILE *fp;

  clrscr();

  fp = fopen("myfile2.txt","w");
  fputs("hello c programming",fp);
  fclose(fp);

  getch();
}

Read files: fgets()

#include<stdio.h>
#include<conio.h>

void main() {
  FILE *fp;
  char text[300];

  clrscr();

  fp = fopen("myfile2.txt", "r");
  printf("%s", fgets(text, 200, fp));
  fclose(fp);

  getch();
}

fseek()

#include <stdio.h>

void main(void) {
  FILE *fp;

  fp = fopen("myfile.txt","w+");
  fputs("This is Book", fp);

  // Set file pointer to the given position
  fseek(fp, 7, SEEK_SET);

  fputs("Kenny Wong", fp);
  fclose(fp);
}

set the file pointer to the given position

rewind()

#include <stdio.h>
#include <conio.h>

void main() {
  FILE *fp;
  char c;

  clrscr();

  fp = fopen("file.txt", "r");

  while( (c = fgetc(fp) ) != EOF) {
    printf("%c", c);
  }

  rewind(fp); // move the file pointer to the beginning of the file

  while( (c = fgetc(fp) ) != EOF) {
    printf("%c", c);
  }
  fclose(fp);

  getch();
}
// output
// Hello World! Hello World!

ftell()

#include <stdio.h>
#include <conio.h>

void main () {
   FILE *fp;
   int length;

   clrscr();

   fp = fopen("file.txt", "r");

   fseek(fp, 0, SEEK_END);
   length = ftell(fp); // return current position
   fclose(fp);

   printf("File size: %d bytes", length);

   getch();
}
// output
// file size: 18 bytes