Author:halw

Date:2010-02-04T22:27:19.000000Z


git-svn-id: https://svn.eiffel.com/eiffel-org/trunk@437 abb3cda0-5349-4a8f-a601-0c33ac3a8c38

documentation/current/method/eiffel-tutorial-et/et-instructions.wiki

@@ -60,7 +60,7 @@ First let's look at the loop in what is probably its most common usage. This is
 
 <code>Initialization</code> and <code>Loop_body</code> are sequences of zero or more instructions; <code>Exit_condition</code> is a boolean expression. 
 
-The effect is to execute <code>Initialization</code>, then, zero or more times until <code>Exit_condition</code> is satisfied, to execute <code>Loop_body</code>. (If after <code>Initialization</code> the value of <code>Exit_condition</code> is already true, <code>Loop_body</code> will not be executed at all.) 
+The effect is to execute <code>Initialization</code>, then, zero or more times until <code>Exit_condition</code> is satisfied, to execute <code>Loop_body</code>. (If after <code>Initialization</code> the value of <code>Exit_condition</code> is already true, <code>Loop_body</code> will not be executed at all.) So, at the risk of stating the obvious, the key to loops that always complete is to ensure that there is something in the loop body that will always cause the exit condition eventually to become true. 
 
 This form of the loop is used commonly to traverse data structures. For example, suppose that we wished to print every element in a linked list of strings. We can do so with this usage of the loop construct, as shown below. 
 
@@ -78,20 +78,42 @@ This form of the loop is used commonly to traverse data structures. For example,
                 my_list.forth
             end
 </code>
+Loop example 1.
+
 
 The <code>Initialization</code> part will attempt to set the cursor at the first item of the list. The loop will exit when there is no active list item. Then, in the <code>Loop_body</code>, the current list item will be printed, and the cursor advanced.
 
-So, this usage of the loop construct has been the traditional mechanism for traversing data structures. However, extensions to Eiffel's loop construct have provided a more concise way of expressing the same traversal:
+So, the usage of the loop construct in Loop example 1 above has been the traditional mechanism for traversing data structures. However, extensions to Eiffel's loop construct have provided a more concise way of expressing the same traversal:
 
 <code>
-            across my_list as c loop print (c.item) end 
+            across my_list as ic loop print (ic.item) end 
 </code>
+Loop example 2.
 
-Here the <code>across</code> indicates an iteration process across the structure <code>my_list</code>. The <code>as c</code> indicates that a cursor object referenced by the name <code>c</code>, and available only for the scope of the iteration, will be created to effect the iteration. The element of <code>my_list</code> which is currently referenced by the cursor <code>c</code> is <code>c.item</code> as you see in the call to <code>print (c.item)</code> in the loop body. The loop body does not contain the call to the structure's <code>forth</code> feature, as our more traditional example did. Neither do you see the call to <code>start</code> nor the check of <code>off</code> in the exit condition. The semantics of the iteration abstract these for you, relieving you of their burden ... while eliminating some opportunities for error.
 
-Concerning cursors, both ways of using the loop construct to traverse a structure employ a cursor. In the traditional usage, the cursor is internal to the structure object. In the case of the example, that would be the instance of <code>LINKED_LIST [STRING]</code> called <code>my_list</code>. Applying the feature <code>item</code> to <code>my_list</code> retrieves the list element currently referenced by the cursor. In the iteration version of traversal, the variable <code>c</code> holds the iteration cursor, external to the list object. So, you apply <code>c.item</code> to get the current list element. The advantage to the external cursor is that multiple traversals of the structure can occur simultaneously without interfering with one another. This is possible in the traditional usage, but only by saving and restoring the structure's cursor.
+Here the <code>across</code> indicates an iteration process across the structure <code>my_list</code>. The "<code>as ic</code>" indicates that an iteration cursor object referenced by the name <code>ic</code>, and available only for the scope of the iteration, will be created to effect the iteration. The element of <code>my_list</code> which is currently referenced by the cursor <code>ic</code> is <code>ic.item</code> as you see in the call to <code>print (ic.item)</code> in the loop body. The loop body does not contain the call to the structure's <code>forth</code> feature, as our more traditional example did. Neither do you see the call to <code>start</code> nor the check of <code>off</code> in the exit condition. The semantics of the iteration abstract these for you, relieving you of their burden ... while eliminating some opportunities for error.
 
-At first observation, it may not appear that both traversal examples are using the same language construct. But, indeed they are simply two different forms of a single language construct. In order to see this more clearly, it will help now to examine (almost) the entire anatomy of the loop construct.
+Concerning cursors, both ways of using the loop construct to traverse a structure employ a cursor. In the traditional usage, the cursor is internal to the structure object. In the case of the example, that would be the instance of <code>LINKED_LIST [STRING]</code> called <code>my_list</code>. Applying the feature <code>item</code> to <code>my_list</code> retrieves the list element currently referenced by the cursor. In the iteration version of traversal, the variable <code>ic</code> holds the iteration cursor, external to the list object. So, you apply <code>ic.item</code> to get the current list element. The advantage to the external cursor is that multiple traversals of the structure can occur simultaneously without interfering with one another. This is possible in the traditional usage, but only by saving and restoring the structure's cursor.
+
+At first observation, it may not appear that both traversal examples are using the same language construct. But, indeed they are simply two different forms of a single language construct. In order to see this more clearly, it will help now to examine the specific parts of the loop construct.
+
+
+====Two basic loop forms====
+
+The two basic forms of use of the Eiffel loop construct can be differentiated by the parts of the construct with which they begin. 
+
+The form shown in Loop example 1 above begins with an ''Initialization'' part ( <code>from my_list.start</code> ). Let's call this form the ''traditional'' form.
+
+The form shown in Loop example 2 above begins with an ''Iteration'' part ( <code>across my_list as c</code> ). We'll call this form the ''iteration'' form.
+
+The iteration form is special in the sense that it is designed to work with objects which are ''iterable'', usually data structures. The iteration form always targets a particular object, usually a data structure, based on a class that inherits, either directly or indirectly from the library class <code>ITERABLE</code>.  
+
+Every valid loop must have at least an ''initialization'' part or ''iteration'' part. An ''iteration'' part always precedes an ''initialization'' part. So, it is possible for loops in the iteration form (but not possible for traditional loops) to have both. Suppose we wanted to compute the sum of the lengths of all the strings in the list <code>my_list</code>. We could write an iteration loop with an initialization setting the sum to zero:
+
+<code>
+            across my_list as ic from sum := 0 loop sum := sum + ic.item.count end
+</code>
+Loop example 3.
 
 ===Debug===