<defmethod factorial n> <if> n.<is 1/> 1 else n.<times <factorial n.<minus 1/> /> /> </> </defmethod>

Transcription

1 Control in Water [1] <if> Water s <if> tag is similar to if statements in other languages. Note that is a keyword, not an XML tag. <defmethod factorial n> <if> n.<is 1/> 1 n.<times <factorial n.<minus 1/> /> /> </defmethod> The structure of an <if> statement can be misleading. The actual structure of the statement is as follows: <if> (test_expression single_statement)* ( single_statement)? Each <if> statement is a set of zero or more test_expressions followed by single_statements and an optional final /single_statment pair. These expressions act like if statements in other languages. For example: <defmethod menu selection> <if> selection.<is "a"/> <echo "Menu A"/> selection.<is "b"/> <echo "Menu B"/> selection.<is "c"/> <echo "Menu C"/> <echo "Invalid Menu Selection!"/> Lecture 22 Object-Oriented Languages and Systems 1

2 This structure means that the following <if> statement is invalid: <defmethod bump n> <if> n.<more 2/> n.<plus 1/> n.<plus 1/> n.<minus 1/> The bump method is meant to add two to numbers strictly greater than two and subtract one from numbers less than or equal to two. The bump method is not syntactically correct because there is more than one statement between the test_expression and the keyword. We can have multiple statements in an <if> statement, but we need to wrap the statements within a <do> block. <do> is like a statement block in other languages. Each member of the <do> statement is executed, and the result of the last statement is the returned. For example: <do <set msg="a message"/> msg.<to_uppercase/> /> We can use <do> to execute multiple lines in an <if> statement: <defmethod bump n> <if> n.<more 2/> <do n.<plus 1/> n.<plus 1/> /> n.<minus 1/> CSC/ECE 517 Lecture Notes 2005, Edward F. Gehringer 2

3 More uses of <do> <do> can also be used as a while loop structure. For example, the following code generates a string of periods based on the current time: <do <set dots="["/> > <while date.<current/>.millisecond.<less 500/> > dots.<concat "] ended."/> <set dots=dots.<concat "."/> /> The rules that <do> uses to execute statements are as follows: 1. Each argument in the opening tag is executed. For example, <set dots="["/> in the example above. These arguments are only executed once and they cannot include <while/>. 2. Each argument in the body is executed. N.B. that the body of the <while> statement is not executed. 3. If a call to <while> is encountered: <do> will loop through its content until <while> returns false. When <while> returns false, the contents of <while> are executed. The last statement of the <while> body is returned from <do>. What vector results from the following <do> statement? <do <set result=<vector/> /> result.<insert "A"/> > result.<insert "B" /> <while result.<length/>.<less 2/> > result.<insert "C" /> result.<insert "D" /> Lecture 22 Object-Oriented Languages and Systems 3

4 The <while> statement has a counterpart <until>, which stops the progress of the <do> loop when it evaluates to true. For example, <defmethod read_telegraph msg=required=string> <do <set words=msg.<split " "/> /> <set results=<vector/> /> > <until words.0.<is "STOP"/> > results.<join " "/> results.<insert words.<remove 0/> /> <read_telegraph "Here is my message STOP Sent on 06/08/1904" /> The read_telegraph message accepts a string and turns it into a <vector/> of blank-delimited words using the <split " "/> method. <until> a word in the message <is/> "STOP", read_telegraph moves words from the input vector to the output vector using the <insert/> and <remove/> vector methods. When "STOP" is reached, the method <join/>s the result vector back into a space delimited string and returns the result. <for_each> <for_each> loops over the elements of a collection (or the fields of an object). <for_each> is a useful construct for languages, such as Water, that do not force the concept of fixed-length arrays. Rather than having a keyword such as for, simply treat everything as a collection and allow iteration over all objects in the collection. CSC/ECE 517 Lecture Notes 2005, Edward F. Gehringer 4

5 With respect to <for_each>, an integer n is a collection from 0 to n, a string is a collection of characters, an object is a collection of its key- pairs, etc. One of the simplest uses of <for_each> is to return all the s in a vector: <vector "a " "b " "c "/>.<for_each combiner=insert> The combiner=insert tells Water to insert each subsequent in the result that is returned. Without it, it would <vector "a " "b " "c "/>.<for_each> The "" refers to the of each element that is iterated over. We could easily substitute the key: <vector "a " "b " "c "/>.<for_each> key What do you think this returns? It is easy to do various kinds of reduction operations on vectors. For example, this code adds all elements of the vector: <vector />.<for_each combiner=plus> Boolean operations are also permitted: <vector "true" 5.<is 6/> "false"/>.<for_each combiner=and> Lecture 22 Object-Oriented Languages and Systems 5

6 Similarly, one can concatenate s in vectors: <vector "not" "with" "standing"/>.<for_each combiner=concat> The object that is iterated over does not have to be a vector; any object will do: <defclass person first last/> <person "Ed" "Gehringer"/> How can we return a string of the s of all fields in this object? We can restrict <for_each> to a specific number of iterations: 3.<for_each> <do "Hello there! "/> (Will this work?) We don t have to restrict <for_each> to a finite number. Of course, in this case, some other code must stop it <for_each forever=true> <set rand_val=3.<random_number/> /> <if> rand_val.<is 2/> <return "Loop exited." from="for_each"/> Let s see what s happening here. What numbers can be returned by <random_number/>? Another example is counting repeatedly from 1 to n, until the limit specified by the subject is reached. After that, the count starts over again. CSC/ECE 517 Lecture Notes 2005, Edward F. Gehringer 6

7 <set mycount=0/> 2.<for_each combiner=insert forever=true> <if> mycount.<is 7/> <return for_each_result from="for_each"/> <do <set mycount=mycount.<plus 1/> /> /> The result of each <for_each> iteration is stored in the for_each_result variable. This can be seen in the above example in the <return> call. By default, the initial of for_each_result is null, but we can supply it with a starting by setting the initial_result field of <for_each>. <vector 3 4/>.<for_each combiner=insert initial_result=<vector/> >.<times /> This example stores the square of each in the input vector into a new output vector. A Fibonacci Example Suppose we want to compute a Fibonacci sequence a sequence beginning with 0, 1 where each term is the sum of the two preceding terms. For our first example, we will check how many result items have been produced so far. If zero results have been produced, then we will return 0. If one result has been produced, then we will return 1. If more than one result has been produced, we will return the sum of the previous two results. We will also terminate when ten results have been produced. <for_each combiner=insert forever=true> Lecture 22 Object-Oriented Languages and Systems 7

8 <set len=for_each_result.<length/> /> <if> len.<is 0/> 0 len.<is 1/> 1 len.<is 5/> <return for_each_result from="for_each"/> Next we supply an initial_result of <vector 0 1/> to handle our first and second "special cases." This leaves us with only 8 iterations, making our code faster as well as smaller. <for_each combiner=insert forever=true initial_result=<vector 0 1 /> > <set len=for_each_result.<length/>/> <if> len.<less 10 /> <return for_each_result from="for_each" /> The next algorithm is quite different. Here we make the length the subject and test whether the is less than 2: 10.<for_each combiner=insert> <if>.<less 2/> CSC/ECE 517 Lecture Notes 2005, Edward F. Gehringer 8

9 We can combine this code with an initial result: 10.<for_each combiner=insert initial_result=<vector 0 1/>> <if>.<less 2/> skip But we don t need to skip the of the first two iterations if we don t perform them! Let s just start by initializing the vector to <vector 0 1/> and go from there. Summary 10.<minus 2/>.<for_each combiner=insert initial_result=<vector 0 1/> > In this lecture we ve covered the basics of Water s control structures: <if>, <do>, and <for_each>. References [1] Plusch, Mike. Water: Simplified Web Services and XML Programming. Lecture 22 Object-Oriented Languages and Systems 9