3 - Functional Programming/1.2 - Control Statements.texView File
@ -173,7 +173,7 @@ In this example we also have left out the default case, this case is optional, s
\subsection{For Loops}
We have seen some statements that will help the direction of our code, but what about repeating code?
Lets say that we need to manually determine what the square of a number is using multiplication (rather than the exponential operator \^).
Lets say that we need to manually determine what the square of a number is using multiplication (rather than the exponential operator \verb|^|).
This can be expressed fairly easily.
\begin{lstlisting}[caption={Square Without Loop}]
@ -251,8 +251,90 @@ The same would be true if we changed the Condition to \pigVar{true} or \pigVar{1
\subsection{While Loops}
So we have just seen how For loops are used to loop based on a condition and a counter for a set interval, but what if we wanted to just loop forever until a condition was met?
Well, we have While loops!
While loops are great for things such as iterating over a file or a result set from a database query or when the duration of loop is unknown.
\par
While loops contain two parts, the conditional and the code block.
The conditional is checked for each iteration of the loop, if it evaluates to \pigVal{true} then the code block is executed.
The main difference between a While loop and a For loop is that a For loop is usually designed so that it runs at least once or for a set number of times,
but a While loop has the potential to run the code block 0 times.
Let us jump into an example.
\begin{lstlisting}[caption={While Loop}]
num = 0
while num < 25:
print ``Loop''
num += 5
\end{lstlisting}
This program will print \pigOut{Loop} 5 times.
When the program gets to the While loop it first evaluates the conditional to see if the code block should be run once.
In this case \pigVar{num} is less than \pigVal{25} so the code block is executed, which prints \pigOut{Loop} and then increments \pigVar{num} by \pigVal{5}.
This loop continues until \pigVar{num} is incremented to \pigVal{25}.
\par
This example is fairly simple and even in some resembles how a For loop works.
I contains an initialization of a counter variable, \pigVar{num} to \pigVal{0}.
The conditional ensures that \pigVar{num} is below \pigVal{25}.
Finally the update is when we increment \pigVar{num} by \pigVal{5}.
So let us take a look at an example that does not use numbers to see how the While loop can be useful.
\begin{lstlisting}[caption={While Loop Over File}]
file = OpenFile(``example.txt'')
line = ReadLineFromFile(file)
while line != EndOfFile:
print line
line = ReadLineFromFile(file)
CloseFile(file)
\end{lstlisting}
As you can guess from this program, a file \pigVal{example.txt} is opened and the first line is read into the variable \pigVar{line}.
When the While loop is reached the conditional checks to see if the end of the file has been reached.
It is possible for this conditional to evaluate to \pigVal{false} the first check (if the file is empty).
For each iteration of the loop the line read is printed out.
Lastly another line is read from \pigVar{file} into \pigVar{line}; without line 6 {pigVar{line = ReadLineFromFile(file)} then the loop would continue forever as \pigVar{line} would
not update and the conditional will always evaluate to the \pigVal{true}.
\par
Although the above example uses some concepts you might not be familiar with (functions and file input/output), it should illustrate the usefulness of the While loop and
how it can differ from a For loop.
\subsection{Do-While Loops}
A Do-While loop is very similar to a While loop except in a single regard; the code block is guaranteed to run at least once.
So as we are familiar with While loops lets jump right into an example.
\begin{lstlisting}[caption={Do-While Loop}]
num = 0
do:
print ``Loop''
num += 5
while num < 25
\end{lstlisting}
This example is just like the first While loop example we looked at and will run exactly the same number of times.
The only difference is that the \pigOut{Loop} is printed and \pigVar{num} is incremented by \pigVal{5} before the conditional is checked for the first time.
Now let us take a look at an example where the Do-While loop is useful.
