Getting Started

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);

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

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);

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 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);

// 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);

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!");
}

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;

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;

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

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

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

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

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

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"!

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

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

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!"

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

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

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

// this is a comment
printf("Hello World!"); // Can comment anywhere in file
/\*Multi-line comment, print Hello World!
to the screen, it's awesome \*/

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

Best Practices

const int BIRTHYEAR = 1980;

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;

#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!