View source for Looping with for and the Ruby Looping Methods

<table border="0" cellspacing="0">
<tr>
<td width="20%">[[Ruby While and Until Loops|Previous]]<td align="center">[[Ruby Essentials|Table of Contents]]<td width="20%" align="right">[[Ruby Strings - Creation and Basics|Next]]</td>
<tr>
<td width="20%">Ruby While and Until Loops<td align="center"><td width="20%" align="right">Ruby Strings - Creation and Basics</td>
</table>
<hr>


<htmlet>ruby</htmlet>


In the previous chapter we looked at [[Ruby While and Until Loops]] as a way to repeat a task until a particular expression evaluated to ''true'' or ''false''. In this chapter we will look at some other mechanisms for looping in a Ruby program, specifically ''for loops'' and a number of built-in methods designed for looping, specifically the ''loop'', ''upto'', ''downto'' and ''times'' methods.

== The Ruby for Loop ==

The ''for'' loop is a classic looping construct that exists in numerous other programming and scripting languages. It allows a task to be repeated a specific number of times. Ruby differs in that it is used in conjunction with ranges (see [[Ruby Ranges]] for more details). For example, we can repeat a task 8 times using the following ''for'' statement:

<pre>
for i in 1..8 do
    puts i
end
</pre>

The above loop will result in the following output:

<pre>
1
2
3
4
5
6
7
8
</pre>

The ''do'' in the ''for'' statement is optional, unless the code is placed on a single line:

<pre>
for i in 1..8 do puts i end
</pre>

Ruby ''for'' loops can be nested:

<pre>
for j in 1..5 do
     for i in 1..5 do
         print i,  " "
     end
puts
end
</pre>


The above code will result in the following output:

<pre>
1 2 3 4 5
1 2 3 4 5
1 2 3 4 5
1 2 3 4 5
1 2 3 4 5
</pre>

Also, the ''break if'' statement can be used to break out of a ''for loop'' (note that only the inner for loop is exited, if the loop is nested the outer loop will continue the looping run):

<pre>
for j in 1..5 do
     for i in 1..5 do
         print i,  " "
         break if i == 2
     end
end
</pre>

This causes the inner loop to break each time i equals 2, thereby returning control to the outer loop which in turn calls the inner loop:

<pre>
1 2
1 2
1 2
1 2
1 2
</pre>

== The Ruby times Method ==

The ''times'' method provides an extremely convenient alternative to the ''for'' loop. The ''times'' method is provided by the Integer class and allows a task to be performed a specific number of times:

<pre>
5.times { |i| puts i }

0
1
2
3
4
</pre>
<htmlet>adsdaqbox_flow</htmlet>
The above statement is identical to the following for loop, although moderately easier to type:

<pre>
for i in 0..4
    puts i
end

0
1
2
3
4
</pre>

== The Ruby upto Method ==

The ''upto'' method can be called on Integer, String and Date classes and provides similar functionality to the ''for'' loop described earlier in this chapter. 

For example, we might express a for loop as follows:

<pre>
for i in 1..5 do
   puts i
end
</pre>

Alternatively, we could achieve identical behavior using the ''upto'' method, passing the value to which we want the loop to run as an argument to the method:

<pre>
5.upto(10) do
   puts "hello"
end
</pre>

If we so desired, we could shorten this to a single line of code:

<pre>
1.upto(5) { |i| puts i }
</pre>

== The Ruby downto Method ==

The ''downto'' method is similar to the ''upto'' method with the exception that it starts at a value and counts down, rather than up. For example, we might want a loop to execute for numbers between 15 and 10:

<pre>
15.downto(10) {|i| puts i }
15
14
13
12
11
10
</pre>


<htmlet>ruby</htmlet>


<hr>
<table border="0" cellspacing="0">
<tr>
<td width="20%">[[Ruby While and Until Loops|Previous]]<td align="center">[[Ruby Essentials|Table of Contents]]<td width="20%" align="right">[[Ruby Strings - Creation and Basics|Next]]</td>
<tr>
<td width="20%">Ruby While and Until Loops<td align="center"><td width="20%" align="right">Ruby Strings - Creation and Basics</td>
</table>