🆘C Tutorial

The main difference between C and C++ is that C++ support classes and objects, while C does not

Quick syntax review of a hello world:

#include <stdio.h>

int main(){
    printf("hello world");
    return 0;
}

#include <stdio.h> is a header file library that lets us work with input and output functions, such as printf()

Another thing that always appear in a C program is main(). This is called a function. Any code inside its curly brackets {} will be executed

printf() is a function used to output/print text to the screen.

Note that: Every C statement ends with a semicolon ;

To insert a new line, you can use the \n character:

#include <stdio.h>

int main() {
  printf("Hello World!\nI am learning C.\nAnd it is awesome!");
  return 0;
}

Single-line comments start with two forward slashes (//).

Multi-line comments start with /* and ends with */.

C Variables

different types of variables in C:

  • int - stores integers (whole numbers), without decimals, such as 123 or -123

  • float - stores floating point numbers, with decimals, such as 19.99 or -19.99

  • char - stores single characters, such as 'a' or 'B'. Characters are surrounded by single quotes

To create a variable, we must specify the type and assign it a value:

 type variableName = value;

Variable called theVarInt, type int, and it's value is 24:

int theVarInt = 24;

or we can assign value later:

//declare var:
int theVarInt;

//assign value:
theVarInt = 24;

if we wanted to print out the value assigned to theVarInt it would not be a printf(theVarInt);

C Format Specifiers

Format specifiers are used together with the printf() function to tell the compiler what type of data the variable is storing.

int would use the format specifier %d surrounded by double quotes (""), inside the printf() function:

int theVarInt = 24;
printf("%d", theVarInt);

(%c for char and %f for float)

To combine both text and a variable, separate them with a comma inside the printf() function:

printf("i was born on june: %d", theInt); // June 24 ;)

And to print different type in a same printf:

printf("i was born on june %d and pi is equal to %f ", theInt, theFloat);

Change Variable Values

If you assign a new value to an existing variable, it will overwrite the previous value:

int myNum = 15;  // myNum is 15
myNum = 10;  // Now myNum is now 10

you can also assign a value using another variable:

int myNum = 15;

int myOtherNum = 23;

// Assign the value of myOtherNum (23) to myNum
myNum = myOtherNum;

Add Variables Together

Using the + operator:

int x = 5;
int y = 6;
int sum = x + y;
printf("%d", sum);

Declare Multiple Variables

if we want to declare multiple (same type) variable, we can do it in one line with comma separated values:

int x = 5, y = 6, z = 50;
printf("%d", x + y + z);

or same value:

int x, y, z;
x = y = z = 50;
printf("%d", x + y + z);

Real life example: calculate area of rectangle:

#include <stdio.h>

int main(){
  int a = 2;
  int b = 13;
  float area;
  
  area = a * b;
  printf("the area is: %f", area );
  return 0;
}

//the area is: 26.000000

C Character Data Types

char myGrade = 'A';
printf("%c", myGrade);

But we can also use ASCII to print out char values:

char a = 65;
printf("%c", a)
//prints A

To store multiple characters or words, we need to use strings:

char myText[] = "Hello";
printf("%s", myText);

Numeric Types

Use int when you need to store a whole number without decimals, like 35 or 1000, and float or double when you need a floating point number

The precision of float is six or seven decimal digits, while double variables have a precision of about 15 digits. Therefore, it is often safer to use double for most calculations - but note that it takes up twice as much memory as float (8 bytes vs. 4 bytes).

If we want to be more precise with our outputs, we can use a dot (.) followed by a number that specifies how many digits that should be shown after the decimal point:

float myFloatNum = 3.5;

printf("%f\n", myFloatNum);   // Default will show 6 digits after the decimal point
printf("%.1f\n", myFloatNum); // Only show 1 digit
printf("%.2f\n", myFloatNum); // Only show 2 digits
printf("%.4f", myFloatNum);   // Only show 4 digits

Get the Memory Size

If we want to print out the memory size of a var → we need to use the sizeof operator.

it's better to use the %lu format specifier to print the result:

printf("%lu\n", sizeof(myInt));
printf("%lu\n", sizeof(myFloat));
printf("%lu\n", sizeof(myDouble));
printf("%lu\n", sizeof(myChar));

real-life example of using different data types, to calculate and output the total cost of a number of items:

#include <stdio.h>

int main(){
  int a = 50;
  float b = 5.5;
  float total = a * b;

  printf("the cost of the a item is %d\n", a);
  printf("the cost of the whole stock is %f\n", total);
  return 0;
}

Type Conversion

if we need to use 2 int but the result is a float (5/2=2.5), we can use implicit conversion:

if you assign an int value to a float type:

// Automatic conversion: int to float
float myFloat = 9;

printf("%f", myFloat); // 9.000000

and the other way around:

// Automatic conversion: float to int
int myInt = 9.99;

printf("%d", myInt); // 9

The conversion is tricky:

float sum = 5 / 2;

printf("%f", sum); // 2.000000

the result is 2 and not 2.5 because we assigned the value of sum with 2 int, we need to manually convert the integer values to floating-point values

// Manual conversion: int to float
float sum = (float) 5 / 2;

printf("%f", sum); // 2.500000

or

int num1 = 5;
int num2 = 2;
float sum = (float) num1 / num2;

printf("%f", sum); // 2.500000 
//and if we wanted to make the output even cleaner:
printf("%.1f", sum); //2.5

Real life example

create a program to calculate the percentage of a user's score in relation to the maximum score in a game:

#include <stdio.h>

int main(){
  int maxScore = 1000;
  float userScore= 756;

  float pourcentScore= (userScore/maxScore)*100;
  printf("le pourcentage est de %.2f\n", pourcentScore);
  return 0;
}  //le pourcentage est de 75.60

C Constants

If we do not want anyone to be able to change a variable we can use the const keyword.

It will make the variable unchangeable and read-only:

When you declare a constant variable, it must be assigned with a value

//YES:
const int minutesPerHour = 60;

//NO
const int minutesPerHour;
minutesPerHour = 60; // error

C Operators

Operators are used to perform operations on variables and values.

you can have arithmetic operators:

assignment operators:

Comparisons operators:

Logical operators:

C Booleans

Good to know we must import booleans:

#include <stdbool.h>

A boolean variable is declared with the bool keyword and can only take the values true or false

It's good to know that boolean values are returned as integers (1=true, 0=false)

So to print it we must use %d

You can use bool to compare values:

printf("%d", 10 > 9);  // Returns 1 (true) because 10 is greater than 9
printf("%d", 10 == 10); // Returns 1 (true), because 10 is equal to 10

or 2 bool variables:

//if both variables are true
printf("%d", isHamburgerTasty == isPizzaTasty); 
//will return 1

Real life example:

Output "Old enough to vote!" if myAge is greater than or equal to 18. Otherwise output "Not old enough to vote.":

#include <stdio.h>

int main(){
  int myAge = 21;
  const int minAge= 25;

  if(myAge>=minAge){
    printf("You are old enough to vote");
  }else{
    printf("You are not old enough to vote, you are only %d years old", myAge);
  }
  return 0;
}

C If ... Else

Combined to the conditions we just saw, we can add conditional statements:

Short Hand If...Else (Ternary Operator)

As a one-liner enjoyer it's good to know that this exists:

to replace simple if else statements, we can use this syntax:

 variable = (condition) ? expressionTrue : expressionFalse;

Example:

int time = 20;
if (time < 18) {
  printf("Good day.");
} else {
  printf("Good evening.");
}

turns into:

int time = 20;
(time < 18) ? printf("Good day.") : printf("Good evening.");

Real life example:

use if..else to "open a door" if the user enters the correct code

#include <stdio.h>

int main(){
  int code = 5678;

  if(code !=5678){
    printf("Access Denied");
  }else{
    printf("Access Granted\n");
  }
  //or
  (code == 5679) ? printf("Access granted") : printf("Access denied");
  return 0;
} 
/*Access Granted
Access denied*/

C Switch

The switch statement selects one of many code blocks to be executed:

switch (expression) {
  case x:
    // code block
    break;
  case y:
    // code block
    break;
  default:
    // code block
}

Quick little switch condition to see the mood today:

#include <stdio.h>

int main(){
  int mood = 3;
  
  switch(mood){
    case 1:
      printf("I am happy");
      break;
    case 2:
      printf("I am sad");
      break;
    case 3:
      printf("I am grinding");
  }
  return 0;
}
// Output "I am grinding"

if the mood variable has some chances of not being in our switch case, we can add a default case that specifies some code to run if there is no case match:

...
case 2:
      printf("I am sad");
      break;
    case 3:
      printf("I am grinding");
    default:
      printf("none of them");
  }
  return 0;
}
// Output for mood > 3 --> "none of them"

C While Loop

Loops can execute a block of code as long as a specified condition is reached.

#include <stdio.h>

int main(){
  int mood = 12;
  int i = 0;
  
  while(i<mood){
    printf("%d is not yet equal to %d\n", i, mood);
    i++;
  }
  return 0;
}

Another variable of the while loop is the do/while loop:

#include <stdio.h>

int main(){
  int mood = 12;
  int i = 0;
  
  do{
    printf("%d is not yet equal to %d\n", i, mood);
    i++;
  }
    while(i < mood);
  return 0;
}

C For Loop

If we know how many occurrences we want to have, we can use the for loop that takes 3 expressions

Expression 1 is executed (one time) before the execution of the code block.

Expression 2 defines the condition for executing the code block.

Expression 3 is executed (every time) after the code block has been executed.

#include <stdio.h>

int main(){
  int mood = 4;
  
  for(int i = 0; i < mood; i++){
    printf("is %d equal to %d?\n", i, mood);
  }
  return 0;
}

We can also play with nested loops, but it will multiply by the number of occurrences of the first loop:

#include <stdio.h>

int main(){
  int mood = 4;
  int nested_loop=2;
  
  for(int i = 0; i < mood; i++){
    printf("is %d equal to %d?\n", i, mood);
    for(int b =0; b < nested_loop; b++){
      printf("the nested number %d is not equal to %d\n", b, nested_loop);
    }
  }
  return 0;
}

C Break and Continue

The break statement can also be used to jump out of a loop.

The continue statement breaks one iteration (in the loop), if a specified condition occurs, and continues with the next iteration in the loop.

C Arrays

C Strings

here is the syntax for a string in C:

 char greetings[] = "Hello World!";

for the printf output we need the format specifier %s for strings and %c format specifier to print a single character.

If we wanted to modify a single character in a string, we would refer to the index like in an array since strings are quite litterally char arrays, and we would use single quotes for the assignment:

char greetings[] = "Hello World!";
greetings[0] = 'J';
printf("%s", greetings);
// Outputs Jello World! instead of Hello World!

You can use the for loop just like any other way we learnt earlier:

char felix[] = "Felix";

  for(int i = 0; i < 5; i++){
    printf("%c is the %dth letter of my name\n", felix[i], i+1);
  }

another interestig way of printing out strings is to use {} and single quotes:

  char felix[] = {'f','e','l','i','x'};
  printf("%s is my name", felix);

and to finish the size of a string is pretty straightforward:

char felix[] = {'f','e','l','i','x'};

printf("%s is my name\n", felix);
printf("the number of chars of my name is %d", sizeof(felix));

If we want to print out " or ' or \ (special chars)in our strings without it breaking the function we need to put escape characters:

char txt[] = "We are the so-called "Vikings" from the north.";
//error
char txt[] = "We are the so-called \"Vikings\" from the north.";
//no error

String functions:

We can perform some interesting stuff on strings if we include <string.h> header file in our program ->

first we need to know the difference between strlen() and sizeof()

sizeof and strlen behaves differently, as sizeof also includes the \0 character when counting:

char alphabet[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
printf("%d", strlen(alphabet));   // 26
printf("%d", sizeof(alphabet));   // 27

and sizeof() will return the memory size in bytes and not the length as we understand it:

char alphabet[50] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
printf("%d", strlen(alphabet));   // 26
printf("%d", sizeof(alphabet));   // 50

If we want to concatenate strings, we can use the strcat() function but we need to be aware of buffer overflow:

In C, when you declare an array of characters like char string1[12], you're allocating space for 12 characters, including the null terminator '\0'. However, when you initialize string1 with "hello", it already consumes 6 characters (5 for "hello" and 1 for '\0'). So, string1 has only 6 characters left before reaching its maximum capacity.

Copy strings:

we need to use the strcpy() function if we want to assign the value of one string to another

char string1[6] = "hello";
char string2[6];
  
strcpy(string2, string1);
printf("%s", string2);

notice how while using strcpy we put the var that we want to assign something to in first

Compare strings:

If we want to compare 2 strings, we need to use the strcmp() function:

the output is either ->

0 == equal

!0 == not equal

char string1[6] = "hello";
  char string2[6] = "hello";
  char string3[6] = "hel";
  
  printf("%d\n", strcmp(string1, string2)); //0
  printf("%d", strcmp(string1, string3)); //108

C User Input

To capture user input, we need to use the scanf() function:

int myNum;

// Ask the user to type a number
printf("Type a number: \n");

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

// Output the number the user typed
printf("Your number is: %d", myNum);

Like this:

int myage;
printf("enter your age:\n");
scanf("%d", &myage);
printf("your age is %d", myage);

and for multiple inputs:

  int myage;
  char myname[10];
  printf("enter your age:\n");
  printf("enter your name:\n");

  scanf("%d", &myage);
  scanf("%s", &myname);
  printf("your age is %d and your name is %s", myage, myname);

scanf() considers space chars as escape chars so if you were to write your name "John Doe" it would ouput only John

Use the scanf() function to get a single word as input, and use fgets() for multiple words.

fgets() takes the following parameters: the name of the string variable, sizeof(string_name), and stdin:

char fullName[30];

printf("Type your full name: \n");
fgets(fullName, sizeof(fullName), stdin);

printf("Hello %s", fullName);

C Memory Address

To access memory addresses in C you have to put a & before the variable:

int myAge = 43;
printf("%p", &myAge); // Outputs 0x7ffe5367e044

the memory address is in hexadecimal form

C Pointers

A pointer is a variable that stores the memory address of another variable as its value.

A pointer variable points to a data type (like int) of the same type, and is created with the * operator.

int myAge = 43;     // An int variable
int* ptr = &myAge;  // A pointer variable, with the name ptr, that stores the address of myAge

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

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

// Output the memory address of myAge with the pointer (0x7ffe5367e044)
printf("%p\n", ptr);

you can dereference a pointer by calling it with a * character before ->

int myAge = 43;     // Variable declaration
int* ptr = &myAge;  // Pointer declaration

// Reference: Output the memory address of myAge with the pointer (0x7ffe5367e044)
printf("%p\n", ptr);

// Dereference: Output the value of myAge with the pointer (43)
printf("%d\n", *ptr);

You can also declare pointers thes 2 ways in C:

int* myNum;
int *myNum;

Pointers & Arrays

let's say we have the following array:

int myNumbers[4] = {25, 50, 75, 100};

Note that the last number of each of the elements' memory address is different, with an addition of 4.

It is because the size of an int type is typically 4 bytes ->

In c, the name of the array is a direct pointer to the first element of the array

The memory address of the first element is the same as the name of the array:

int myNumbers[4] = {25, 50, 75, 100};

// Get the memory address of the myNumbers array
printf("%p\n", myNumbers);

// Get the memory address of the first array element
printf("%p\n", &myNumbers[0]);

//0x7ffe70f9d8f0
//0x7ffe70f9d8f0

So if we want to access to an element of the array through a pointer, we have to take the array (which points to the first element) and add 1,2,3... ->

int myNumbers[4] = {25, 50, 75, 100};

// Get the value of the first element in myNumbers
printf("%d", *myNumbers); //25

// Get the value of the second element in myNumbers
printf("%d\n", *(myNumbers + 1)); //50

// Get the value of the third element in myNumbers
printf("%d", *(myNumbers + 2)); //75

Arrays are used to store multiple values in a single variable, instead of declaring separate variables for each value.

We first define the data type (like int) and specify the name of the array followed by

square brackets []

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

To access an array element, refer to its index number:

int myNumbers[] = {25, 50, 75, 100};
printf("%d", myNumbers[0]);

// Outputs 25

Now if we wanted to change the value assigned to a array value:

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

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

// Now outputs 33 instead of 25

And finally, we could use the for loop to iterate through an array:

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

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

Now if we wanted to know the size of an array:

int myNumbers[] = {10, 25, 50, 75, 100};
printf("%lu", sizeof(myNumbers)); // Prints 20

the output is equal to 20 because the output refers to the size of bytes and since an int is usually equal to 4 bytes, 4x5=20

if we wanted to output the size as we think of it (the number of variables in the array):

int myNumbers[] = {10, 25, 50, 75, 100};
int length = sizeof(myNumbers) / sizeof(myNumbers[0]);

printf("%d", length);  // Prints 5

  • sizeof(myNumbers) gives the total size in bytes of the myNumbers array. Since myNumbers is an array of integers (int), and there are five integers in the array, the total size would be 5 * sizeof(int).

  • sizeof(myNumbers[0]) gives the size in bytes of the first element of the myNumbers array, which is an integer (int).

  • By dividing the total size of the array (sizeof(myNumbers)) by the size of a single element (sizeof(myNumbers[0])), we get the number of elements in the array.

We can make our for loop better by specifying the length of our array in the for loop:

int myNumbers[] = {25, 50, 75, 100};
int length = sizeof(myNumbers) / sizeof(myNumbers[0]);
int i;

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

A real life example of a program that calculates the average of different users:

#include <stdio.h>

int main(){
  int age[] = {22,23,15,90};

  int length = sizeof(age) / sizeof(age[0]);
  int value =0;
  for(int i = 0; i < length; i++){
    value += age[i];
  }
  int finalMoyenne = value / length;
  printf("le total des ages reunis est %d\n", value);
  printf("La moyenne est de %d", finalMoyenne);
  return 0;
}
//le total des ages reunis est 150
//La moyenne est de 37

another useful thing good to know is multi dimensional arrays:

lets create a 2D array:

int matrix[2][3] = { {1, 4, 2}, {3, 6, 8} };

To access the values of the array, you first specify the which row you want then which column:

int matrix[2][3] = { {1, 4, 2}, {3, 6, 8} };

printf("%d", matrix[0][2]);  // Outputs 2, row number 0 and element 2 in that row

if you want to change the value of an element in the array it's the same, you first choose the row, then the element in the row:

  int twoDarray[2][3] = {{1,2,3},{4,5,6}};
  printf("%d\n", twoDarray[0][2]);
  twoDarray[0][2] = 4;
  printf("%d\n", twoDarray[0][2]);
//3
//4
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