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 ^= 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 |
|
! |
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 |
^ |
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 valueacos(x)
arc cosine valueasin(x)
arc sineatan(x)
arc tangentcbrt(x)
cube rootcos(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 modeIf 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