Lesson 3: Loops
Loops are used to repeat a
block of code. You should understand the concept of C++'s true and false, because it will
be necessary when working with loops.There are three types of loops: for, while, (and) do
while. Each of them has their specific uses. They are all outlined below.
FOR - for loops are the most useful type. The layout is for(variable initialization,
conditional expression, modification of variable) The variable initialization allows you
to either declare a variable and give it a value or give a value to an already declared
variable. Second, the conditional expression tells the program that while the conditional
expression is true the loop should continue to repeat itself. The variable modification
section is the easiest way for a for loop to handle changing of the variable. It is
possible to do things like x++, x=x+10, or even x=random(5);, and if you really wanted to,
you could call other functions that do nothing to the variable. That would be something
ridiculous probably.
Ex.
#include <iostream.h> //We only need one header file
int main() //We always need this
{
//The loop goes while
x<100, and x increases by one every loop
for(int x=0;x<100;x++) //Keep in mind that
the loop condition checks the x value before it
{}
//Actually repeats, or loops, again. So if x==100 the loop will end.
cout<<x<<endl; //Outputting x
return 0;
}
This program is a very simple
example of a for loop. x is set to zero, while x is less than 100 it calls
cout<<x<<endl; and it adds 1 to x until the loop ends. Keep in mind also that
the variable is incremented after the code in the loop is run for the first time. WHILE -
WHILE loops are very simple. The basic structure is...WHILE(true) then execute all the
code in the loop. The true represents a boolean expression which could be x==1 or
while(x!=7) (x does not equal 7). It can be any combination of boolean statements that are
legal. Even, (while x==5 || v==7) which says execute the code while x equals five or while
v equals 7..
Ex.
#include <iostream.h> //We only need this header file
int main() //Of course...
{
int x=0; //Don't forget
to declare variables
while(x<100) //While x is
less than 100 do
{
cout<<x<<endl;
//Same output as the above loop
x++; //Adds 1 to x every time
it repeats, in for loops the
//loop structure allows this to be done in the
structure
}
return 0;
}
This was another simple example, but it is longer than the above FOR
loop. The easiest way to think of the loop is to think the code executes, when it reaches
the brace at the end it goes all the way back up to while, which checks the boolean
expression.
DO WHILE - DO WHILE loops are useful for only things that want to loop at least once. The
structure is DO {THIS} WHILE (TRUE);
For example:
#include <iostream.h>
int main()
{
int x;
x=0;
do
{
cout<<"Hello world!";
}while(x!=0);
return 0;
}
Keep in mind that you must include a trailing semi-colon after while
in the above example. Notice that this loop will also execute once, because it
automatically executes before checking the truth statement.