C cheatsheet

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

Best Practices

const int BIRTHYEAR = 1980;

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

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

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

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

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 day = 4;

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

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 i = 0;

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

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

int i = 0;

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

  i++;
}

int i = 0;

while (i < 10) {
  i++;

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

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 Thurs: puts("Thursday"); break;
  default: puts("Error!");
}

// 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);

// 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);

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 (&)

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);

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

int x = 5;
int y = 3;

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

Comparison operators are used to compare two values

// 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);

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);

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 \

#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 #

#include <stdio.h>

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

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

  return 0;
}

#include <stdio.h>

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

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

  return 0;
}

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;
}

// 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!"

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

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

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

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.

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;
  }
}

#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 down (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

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;
}

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

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

struct myStructure s2;
s2 = s1;

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

// 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);

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

#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()

#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
}

#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);
}

#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
}

#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();
}

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

void main() {
  FILE *fp;

  clrscr();

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

  getch();
}

#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();
}

#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

#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!

#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