[ GroovyGN ] Looping in Different Ways

來源自 這裡 
Preface: 
Looping in Groovy can be done in several ways. We can use the standard classic Java for loop or use the newer Java for-each loop. But Groovy adds more ways to loop several times and execute a piece of code. Groovy extends the Integer class with the step(), upto() and times() methods. These methods take a closure as a parameter. In the closure we define the piece of code we want to be executed several times. 

If we have a List in Groovy we can loop through the items of the list with the each() and eachWithIndex() methods. We also need to pass a closure as parameter to the methods. The closure is then executed for every item in the list. 

Sample Code: 

view plaincopy to clipboardprint?

1. // Result variable for storing loop results.  
2. def result = ''  
3. // Closure to fill result variable with value.  
4. def createResult = {   
5.     if (!it) {  // A bit of Groovy truth: it == 0 is false  
6.         result = '0'  
7.     } else {  
8.         result += it  
9.     }  
10. }  
11.   
12. // Classic for loop.  
13. for (i = 0; i < 5; i++) {  
14.     createResult(i)  
15. }  
16. assert '01234' == result  
17.   
18. // Using int.upto(max).  
19. 0.upto(4, createResult)  
20. assert '01234' == result  
21.   
22. // Using int.times.  
23. 5.times(createResult)  
24. assert '01234' == result  
25.   
26. // Using int.step(to, increment).  
27. 0.step 5, 1, createResult  
28. assert '01234' == result  
29.   
30. // Classic while loop.  
31. def z = 0  
32. while (z < 5) {  
33.     createResult(z)  
34.     z++  
35. }  
36. assert '01234' == result  
37.   
38. def list = [0, 1, 2, 3, 4]  
39.   
40. // Classic Java for-each loop.  
41. for (int i : list) {  
42.     createResult(i)  
43. }  
44. assert '01234' == result  
45.   
46. // Groovy for-each loop.  
47. for (i in list) {  
48.     createResult(i)  
49. }  
50. assert '01234' == result  
51.   
52. // Use each method to loop through list values.  
53. list.each(createResult)  
54. assert '01234' == result  
55.   
56. // Ranges are lists as well.  
57. (0..4).each(createResult)  
58. assert '01234' == result  
59.   
60. // eachWithIndex can be used with closure: first parameter is value, second is index.  
61. result = ''  
62. list.eachWithIndex { listValue, index -> result += "$index$listValue" }  
63. assert '0011223344' == result  

Supplement: 
* [ In Action ] Groovy control structures - Looping