C basics

Hello world

#include <stdio.h>
#include <stdlib.h>

int main()
{
    printf("Hello World !\n");
    return 0;
} 

Variables

#include <stdio.h>
#include <stdlib.h>

int main()
{
    char Name[] = "Protyro";
    int Age = 24;

    printf("My name is %s\n", Name);
    printf("I'm %d years old", Age);

    Age = 29;

    printf("In five years, my age will be %d years old", Age);
    
    return 0;
} 

Data types

    int age = 40;
    double gpa = 3.6;
    char grade = 'A';
    char phrase[] = "Hello guys.";

Printf

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int anotherFavNum = 499;
    char favLetter = 'z';

    printf("%s guys, my favorite number is %d but I also like the number %d.\n","Hello", 500, anotherFavNum);
    printf("And I'm %f years old now\n", 24.5);
    printf("My favorite letter of the alphabet is %c", favLetter);

    return 0;
} 

Math

#include <stdio.h>
#include <math.h>

int main()
{
    float result,value;
    printf("Input a float value: ");
    scanf("%f",&value);
    result = sqrt(value);
    printf("The square root of %.2f is %.2fn",value,result);
    result = pow(value,3);
    printf("%.2f to the 3rd power is %.2fn",value,result);
    result = floor(value);
    printf("The floor of %.2f is %.2fn",value,result);
    result = ceil(value);
    printf("And the ceiling of %.2f is %.2fn",value,result);
    return(0);
}

Comments

/* This is a comment
on several lines */

// This is a comment on one line

Constants

#include <stdio.h>
#include <stdlib.h>

int main()
{
    
    // const can't be modify, and use uppercases please :)
    const int NUM = 5;
    printf("%d \n", NUM);

    // also considered as a constance
    printf("Hello\n");
    printf("%d", 70);
    return 0;
} 

User input

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int age;
    printf("Enter your age: ");
    scanf("%d", &age);
    printf("You are %d years old\n", age);

    double gpa;
    printf("Enter your gpa: ");
    scanf("%lf", &gpa); // use %lf for double
    printf("You gpa is %f\n", gpa);

    char grade;
    printf("Enter your grade: ");
    scanf(" %c", &grade); // leave a blank before %c
    printf("Your grade is %c\n", grade);

    char name[20]; // define the number of characters expected
    printf("Enter your name: ");
    scanf("%s", name); // scanf take only characters (the first word) before a space
    printf("Your name is %s\n", name);

    char name[20]; // define the number of characters expected
    printf("Enter your name: ");
    fgets(name, 20, stdin); // use for string of characters
    printf("Your name is %s\n", name);

    return 0;
} 

Arrays

#include <stdio.h>
#include <stdlib.h>

int main()
{
    double unluckyNumbers[10]; // can store 10 double
    unluckyNumbers[0] = 5.32; // assign 5.32 to the first double
    printf("%f\n", unluckyNumbers[0]); // print 5.32

    int luckyNumbers[] = {4, 8, 15, 16, 23, 42};
    printf("%d\n", luckyNumbers[0]); //print 4
    luckyNumbers[2] = 50; // replace 15 by 50
    printf("%d\n", luckyNumbers[2]); //print 50
    return 0;
}

Functions

#include <stdio.h>
#include <stdlib.h>

int main()
{
    sayHi("Protyro", 24); // call function sayHi
    sayHi("Toto", 44);
    return 0;
}

void sayHi(char name[], int age) { // return anything, take char name[] and int age as parameter
    printf("Hello %s, you are %d years old\n", name, age);
}

Return statements

#include <stdio.h>
#include <stdlib.h>

double cube(double num) // take num as parameter
{
    double result = num * num * num; // cube the number
    return result; // return the result with double format, exit the function
}

int main()
{
    printf("Answer: %f\n", cube(3.0)); // call the function cube
    return 0;
}

If statements

#include <stdio.h>
#include <stdlib.h>

int max(int num1, int num2, int num3)
{
     int result;
     if(num1 >= num2 && num1 >= num3){
       result = num1;
     } else if(num2 >= num1 && num2 >= num3) {
       result = num2;
     } else {
       result = num3;
     }
     return result;
}

int main()
{
    printf("The biggest number is: %d\n", max(13,9,15));
    return 0;
}

Switch statements

#include <stdio.h>
#include <stdlib.h>

int main()
{
    char grade = 'A';

    switch(grade){
      case 'A' :
        printf("Well done !\n");
        break;
      case 'B' :
        printf("You did alright\n");
        break;
      case 'C' :
        printf("You did poorly\n");
        break;
      case 'D' :
        printf("You did very bad\n");
        break;
      case 'F' :
        printf("You failed\n");
        break;
      default :
        printf("Invalid grade :(\n");
    }

    return 0;
}

Structs

#include <stdio.h>
#include <stdlib.h>

struct Student{     //struct definiton
    char name[50];
    char major[50];
    int age;
    double gpa;
};

int main()
{
    struct Student student1; //create a container with student struct data
    student1.age = 22;
    student1.gpa = 3.2;
    strcpy(student1.name, "Protyro"); // to set a string use strcpy
    strcpy(student1.name, "Business");

    printf("%f\n", student1.gpa);

    struct Student student2; //create a container with student struct data
    student2.age = 32;
    student2.gpa = 2.5;
    strcpy(student2.name, "James"); // to set a string use strcpy
    strcpy(student2.name, "Art");

    printf("%f\n", student2.name);

    return 0;
}

While loops

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int index = 1;

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

    do {  // do while loop execute do instructions before check the while condition
      printf("%d\n", index);
    }while(index <= 5);

    return 0;
}

For loops

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int luckyNumbers[] = {4,8,15,16,23,42};
    int i;

    for(i = 0; i < 6; i++){
      printf("%d\n", luckyNumbers[i]);
    }
    
    return 0;
}

2D Arrays & Nested Loops

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int i, j;
    int nums[3][2] = { // 3 array elements and each one have 2 array elements inside
                    {1,2},
                    {3,4},
                    {5,6}
                    }; // 2 dimensions arrays

    printf("%d\n", nums[0][0]); // print 1
    printf("%d\n", nums[2][1]); // print 6

    for (i = 0; i < 3; i++){ // 3 arrays elements --> i < 3
      for (j = 0; j < 2; j++){ // each one have 2 values inside --> i < 2
        printf("%d,", nums[i][j]);
      }
      printf("\n");
    }
    return 0;
}

Memory Addresses

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int age = 30;
    double gpa = 3.4;
    char grade = 'A';

    printf("age: %p\ngpa: %p\ngrade: %p\n", &age, &gpa, &grade); // %p for pointers, displays the memory address of the variables

    return 0;
}

Pointers

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int age = 30;
    int * pAge = &age; // this pointer varibale is storing the memory address of the age variable
    double gpa = 3.4;
    double * pGpa = &gpa; // this pointer varibale is storing the memory address of the gpa variable
    char grade = 'A';
    char * aGrade = &grade;

    printf("age's memory address : %p\n", &age); // &age means we access to the memory address of the age variable
    printf("age's memory address : %p\n", pAge); // here we use the pointer variable directly

    return 0;
}

Dereferencing Pointers

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int age = 30;
    int * pAge = &age;

    printf("age's memory address : %p\n", &age); // print the memory address of age
    printf("age's memory address : %p\n", pAge); // print the memory address of age thanks to the pointer variable
    printf("age's memory value : %d\n", *pAge); // dereference of pointer, print the value of age
    printf("age's memory value : %d\n", *&age); // derenference of the &age = address memory value / print the value of age

    return 0;
}

Writing Files

#include <stdio.h>
#include <stdlib.h>

int main()
{
    FILE * fpointer = fopen("/tmp/test.txt","w"); // if the path is not specified, it will use the current directory
    fprintf(fpointer, "This is a test !\n");
    fclose(fpointer);

    fpointer = fopen("/tmp/test.txt","a"); // use 'a' to append and don't rewrite the file
    fprintf(fpointer, "This is my second test ! :)\n");
    fclose(fpointer);

    return 0;
}

Reading Files

#include <stdio.h>
#include <stdlib.h>

int main()
{
    char lines[250]; // we can read up to 250 chars
    FILE * fpointer = fopen("/tmp/test.txt","r");
    
    fgets(lines, 250, fpointer); // it will read fpointer and store the first line contain into the lines variable
    printf("%s", lines);

    fgets(lines, 250, fpointer); //read second line
    printf("%s", lines);

    fclose(fpointer);
    return 0;
}