Working with Structures in Java

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15 Working with Structures
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Figure 15-12: A structure with fruit prices by the pound and by the sack.
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So now that you have this set of nested structures, how do you sort by price per pound A fourth attribute of StructSort()describes a dot path to the sort value, as shown in Listing 15-4.
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Listing 15-4: Sorting by a nested key
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<cfset keyArray = StructSort(FruitCosts, numeric , asc , lb )> <cfoutput> <cfloop from= 1 to= #ArrayLen(keyArray)# index= i > #keyArray[i]#: #FruitCosts[keyArray[i]].lb# / lb.<br> </cfloop> </cfoutput>
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The good thing about this method is that you can very easily switch to sorting by price per sack, as shown in Listing 15-5.
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Listing 15-5: Sorting by a different nested key
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<cfset keyArray = StructSort(FruitCosts, numeric , asc , sack )> <cfoutput> <cfloop from= 1 to= #ArrayLen(keyArray)# index= i > #keyArray[i]#: #FruitCosts[keyArray[i]].sack# / sack.<br> </cfloop> </cfoutput>
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Part III The ColdFusion MX Language
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You can sort by a key any number of levels deep by adding elements to the dot path:
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<cfset keyArray = StructSort(FruitCosts, numeric , asc , sack.10lb.fresh )>
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Be aware that, if the specified subkey doesn t exist for every top-level element in the main structure, ColdFusion throws an error.
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Copying a Structure
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Now you come to one of the most confusing parts of dealing with structures: What happens after you copy a structure and then start modifying the copy The answer is not what you may think, so pay close attention to the details. Consider the following snippet:
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<cfset myVar = 1> <cfset yourVar = myVar> <cfset myVar = 2>
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In these three lines, you have two simple variables: myVar, containing 1, and yourVar, containing 2. If you set yourVar equal to myVar, you make a copy of myVar, so setting myVar to a different value doesn t affect yourVar. This is true for almost every variable type in ColdFusion assigning one variable to another makes a copy and then divorces the two variables so that making a change in one doesn t make a change in the other. Structures, however, are the one variable type where such simple copying doesn t apply. Say that you call the following snippet, which creates a structure named myStruct, populates it with a couple values, and then copies myStruct into a new structure named yourStruct:
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<cfset <cfset <cfset <cfset <cfset myStruct = StructNew()> myStruct[ aKey ] = 1> myStruct[ bKey ] = 2> yourStruct = myStruct> myStruct[ aKey ] = 3>
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You would expect to end up with myStruct.aKey equal to 3 and yourStruct.aKey still equal to the original value of 1, but Figure 15-13 shows what really happens. Figure 15-13: Two structures after attempting to copy and modify one.
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What s going on here Both structures show the same value for aKey even though you changed it only in myStruct. This happens because of the way that you attempted to copy myStruct, as follows:
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<cfset yourStruct = myStruct>
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15 Working with Structures
Unlike any other ColdFusion data type, structures are accessed by reference and not by value. If you access a variable by value, the variable is the actual value. If you access a variable by reference, the variable is a pointer to the value, which is stored somewhere else in memory. Suppose that you call the following:
<cfset yourStruct = myStruct>
You aren t copying the value you re copying the reference. yourStruct, therefore, is now pointing to the same object as myStruct, so changing something about myStruct changes yourStruct as well because they re really the same structure. In fact, the opposite holds true as well: Changing the value of yourStruct.aKey changes the value of myStruct.aKey to the same value. If you want to copy myStruct to yourStruct so that the values in each structure are no longer linked to one another, use StructCopy(), as follows:
<cfset yourStruct = StructCopy(myStruct)>
Now, making a modification to myStruct doesn t affect the contents of yourStruct. Well, not exactly. Things get a little confusing here, so hold on tight . . . Say that you change the contents of myStruct to include a nested structure, as follows:
<cfset <cfset <cfset <cfset <cfset <cfset <cfset <cfset myStruct = StructNew()> myStruct[ aKey ] = 1> myStruct[ bKey ] = 2> myStruct[ aSubStruct ] = StructNew()> myStruct.aSubStruct[ subKey ] = 4> yourStruct = StructCopy(myStruct)> myStruct[ aKey ] = 3> myStruct.aSubStruct[ subKey ] = 3>
You would expect that, because you copied the structure by using StructCopy() instead of just copying the reference, modifying myStruct.aKey wouldn t affect yourStruct.aKey, and you are correct. The change to myStruct.aSubStruct.subKey, however, does modify yourStruct s value. This situation is shown in Figure 15-14. Figure 15-14: Copying structures by using StructCopy().