# Hackmyfortress - Hacking Training Center - En/Fr

83.4K views ## Alghorithm

Alghorithmia is an operation or instruction sequence that allows a problem to be solved or a result to be obtained.

Let us take as an example a basic language, C.

### Variable

To start doing the operation you will need variables, example:

int A;

I get a variable of type int named A. Now I will assign a value to the variable that I just declared, for example:

int    A;
A = 42;


I can also assign a value to the variable on the line where I declare it, for example:

int A = 42;

There are different types of variables, an int can store integer values ​​-2147483648 to 2147483647. To store a larger value you will need to change the type of variable.

Now that we have variables, let's talk about the next tools.

### while

The 'while' loop will repeat operations as long as ... Example:

int    var = 0;

while (var < 42)
{
var = var + 1;
}


Here while my variable var is strictly inferior to 42, we carry out the instructions in the loop. So here the loop will be repeated 42 times. Once the condition is fulfilled the code reading resumes under the loop.

### For

The "for" loop allows you to repeat one or more statements as long as a test is verified. The most used "for" loop consists of having a variable, "i", initialized to 0 and repeating an instruction as long as "i" is less than a number such as 3. The instruction contained in the "for" loop will be repeated 3 times. (Once at 0, once at 1, then at 2, then "i" is equal to 3, then "i" does not check the inferiority condition at 3). Example:

{
console.log(i); // 0, 1, 2
}


### if, else if, else

The if is a condition, for example:

int     var = 2;

if (var == 2)
{
printf("var = 2");
}


Here the statements in the if (between the brackets) will only be executed if the condition is true, printf () is a function that allows to display text. So in this case my program will show "var = 2". These are examples of conditions that can be used in the following conditions:

• > , < , >=, <= (Upper, lower, higher or equal, lower or equal)
• !=, == ( Different, equal)

If I add the else, else if, for example:

int    var = 42;

if (var == 4)
{
printf("1");
}
else if (var == 21)
{
printf("2")
}
else
{
printf("3")
}

Which message will be displayed ?

You understood ? Perfect !

You have just seen the basic tools to create c algorithm, it's tools are common to many languages. Whatever loop example in other language:

Ruby :

while (compteur <= 4)
print compteur
compteur += 1
end


Java

while (a < b)
{
System.out.println("coucou " +a+ " fois !!");
}


Python

while i < 10
print("i")
i++;


As you saw, they're all the same.

Do an "Hello World"

It was simple :) Go to a more complex ! I ask you to produce code that will display the alphabet. "printf (" abcdefghijklmnopqrstuvwxyz "); You say ? Interdiction to use the printf function. You will use :

• while loop
• a variable
• the my_putchar () (function that displays the parameter you pass to it between the parentheses.)

To succeed at this exercise you'll need to look at the ascii table. In this table you will see that decimal value ​​representing characters. Take a look MAN ASCII

Which decimal value is the character 'a' ?
Write a function that will display the alphabet
A clue ?

Suppose that I declare a variable of type int which is named i, I say that "i = 97".

int i;
i = 97;
my_putchar(i);


The variable i contains a decimal value which is equal to the character 'a' so I will display 'a'

Solution
	int i = 'a';
while(i <= 'z')
{
my_putchar(i);
i++;
}   