The algorithms do three types of operations:

- entry/exit

- attributing

- decision

The algorithm makes this operations in order to transform the given data into requested data.

Generally, algorithms are made by the exclusive utilization of certain structures ( linear, alternative, repetitive ).

A structure is a way of combining the operations an algorithm is working with.

The linear structure

A linear structure is a chain of operations that are executed in the order of their apparition:

op1

op2

op3

.

.

opn

The alternative structure

The alternative structure is a structure that permits the execution of a certain instruction if a condition is satisfied.

If S1 and S2 are structures and E is a logical expression, the following is an alternative structure:

if E then S1

else S2

endif

The execution mechanism is:

- evaluating the logical expression

-if the value of the expression is true, the structure S1 is being executed, else the structure S2 is being executed.

Example: Given an integer number n, find out if it is a multiple of 2.

Pascal version:

var n : integer;

begin

if n mod 2=0 then writeln(n, ' is a multiple of 2 ')

else writeln(n,' is not a multiple of 2 ')

end.

C++ version:

#include <iostream.h>

void main()

{

int n;

cin>>n;

if (n%2= =0){cout<<n<<" is a multiple of 2"<<endl;}

else {cout<<n<<"is not a multiple of 2"<<endl;}

}

The repetitive structure

There are two types of repetitive structures:

1. with initial condition:

while <condition> do <operation>

2. with final condition:

repeat <operation> until <condition>

Example: Write the first n natural numbers.

Pascal version:

var i,n:integer;

begin

for i:=0 to n-1 do

write(i,' ');

writeln;

end.

or

var i,n:integer;

begin

i=0;

while i<>n do

begin

write(i,' ');

inc(i);

end;

writeln;

end.

or

var n,i:integer;

begin

i:=0;

repeat

write(i,' ');

inc(i);

until i=n;

end.

C++ version:

#include<iostream.h>

void main()

{

int n,i;

cin>>n;

i=0;

while (i<n)

{

cout>>i>>endl;

i+=1;

}

}

or

#include<iostream.h>

void main()

{

int n,i;

cin>>n;

for (i=0;i<n;i++)

cout<<i<<endl;

}

or

#include<iostream.h>

void main()

{

int n,i;

cin>>n;

i=0;

do

{

cout>>i>>endl;

i+=1;

}

while (i!=n)

}