zxing.net qr code reader WORKING WITH LISTS AND RECORDS in Objective-C

Creator QR Code JIS X 0510 in Objective-C WORKING WITH LISTS AND RECORDS

CHAPTER 6 WORKING WITH LISTS AND RECORDS
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The main challenge in both writing and understanding the script is understanding the different list variables. The script will create and handle four list variables: master_file_list types_of_files_in_master_list unique_types_list list_of_file_lists I ll go over the different lists this script creates and manipulates, explaining each one in turn. The first list is master_file_list. This is simply a list where each item is the name of a file in the chosen folder. The number of items then will be the same as the number of files in the folder. The list types_of_files_in_master_list will be a parallel list to master_file_list. It ll have the same number of items, but in this list, each item will have a file type. The two lists correspond in that if you take the same item number, one list will contain a file s name, and the other list will contain the file s type. Synchronized lists such as this are a simple way to build database functionality for use at runtime. Figure 6-12 shows the two lists side by side. The windows you re looking at are taken from Late Night Software s Script Debugger, which has some great views of lists, records, and other compound values.
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Figure 6-12. Comparing the lists master_file_list and types_of_files_in_master_list In this script you use two separate lists to hold related filename and type data. Another option is to keep related values together by creating a single list of records, where each record holds both the name and type for one file. You ll look at records in the Working with Records section of this chapter. So what is unique_types_list responsible for doing The unique_types_list variable holds a list that will have a single item for each unique file type. Since the folder I chose contains files of four different types, the unique_types_list list will contain four items by the end. These three list variables collect their values in the first repeat loop (line 9 to line 17). Line 19 introduces a new list variable: list_of_file_lists. This list will have as many items as unique_types_list. Each of the items will be a list in itself, containing the names of the files whose file type is equal to the corresponding item in the unique_types_list variable. Figure 6-13 illustrates how the script is designed with corresponding lists.
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CHAPTER 6 WORKING WITH LISTS AND RECORDS
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Figure 6-13. The synchronization between the unique_types_list and list_of_file_lists variables A key to understanding the structure shown previously is that AppleScript isn t aware of any link between the two lists, and it isn t responsible for maintaining their synchronicity. This job is up to you, and it takes planning and confidence; because you are unleashing your script to gather together two separate lists, you are expecting these two lists to match up with each other! From line 20 to line 31 you actually have a double repeat loop. The outer loop loops through the list of unique file types. The inner loop loops through the entire master_file_list. What happens in the loop is that the type from the type list, which is assigned in line 21 to the variable unique_file_type, is compared with the type of the file, which is assigned in line 25 to the variable the_file_type. The names of all the files that match the current comparison file type are collected and appended to the list in variable file_names_with_this_type in line 27. Notice the conditional statement between lines 26 and 28. Line 29 is key. In that line, the list list_of_files_of_that_type is tacked onto the end of the list_of_file_lists variable. Since this happens once in the repeat loop starting on line 20 and since this loop repeats a number of times equal to the number of items in types_list, the result is that the variable list_of_file_lists will have, by the end, as many items as the unique_types_list variable; each item is a list by itself. In the next portion of the script, a string variable called report_text is created (line 33). This variable slowly gathers the textual data to include in the report. After all, the report isn t a list; it s a string of text. Following the introduction of the report_text variable, you start another pair of nested repeat loops, this time to extract the data assembled by the loops in the previous section. Here, however, instead of creating the list_of_file_lists variable, you re using the values in it to generate the report. Now, whenever you look at your script and see that two parts of it are almost identical, you will probably start wondering whether you could have done the job with only one of the parts, slightly rewritten to deal with both jobs. Surely, if you collect the data you need to use in the report in one part of the script and in the other part of the script you use the data you collected, couldn t you have just used the data the first time Couldn t you just generate the report as you re collecting the information Wouldn t it be faster and more efficient Yes and yes, but this has a catch! Yes, you probably could have just generated the report without creating the elaborate lists. Yes, doing so would have created a faster script. But it s not always better to create faster, smaller, and more efficient scripts. The same lessons are true for the scripts you write. Sometimes you have to spread them out a little to make it better. The construction of the script clearly distinguishes between data collecting/ structuring and the final output of that data, making the code easier to debug and change at a later
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