There are some standard operations on arrays that are used a lot. But Java lacks much support for doing these easily.

Transcription

1 New Section 4 Page 1 Operations on Arrays 2:51 PM There are some standard operations on arrays that are used a lot. But Java lacks much support for doing these easily. This set of slides provides some help in how to do these: Manage partially empty arrays Output an array. Add a value into an array Remove a value from within an array Check whether a value occurs IN an array Find where a value APPEARS in an array Concatenating two arrays. Input an array

2 New Section 4 Page 2 Partially Empty 2:56 PM Java arrays have their size LOCKED at birth. Once you say: int[] myary = new int[5]; Your array [named myary] is ALWAYS going to have EXACTLY 5 values stored inside it. This is a problem if you want to add a sixth element or to remove one of the five. A traditional way to manage this is to start with a pretty big array and only use the FRONT part of the array. The price for this is the need for another variable whose only purpose is to keep track of how much of the FRONT part of the array you really want to use.

3 New Section 4 Page 3 Partially Empty 2 2:59 PM So here is an example of an array that starts off with three elements [actually in use] but can "grow" or "shrink" as you go along. Suppose we want the array to start off as: [3,27,5]: int[] myary = new int[] { 3,27,5}; But we want to enlarge it to contain 50 values for later expansion: myary = Arrays.copyOf(myary,50); int myarylen = 3; You could use some other name for the myarylen variable. So wherever we want to use myary, we need to keep myarylen fairly close nearby so we know how many of the 50 spaces in the myary are actually intended [by us] to be "in use".

4 New Section 4 Page 4 Print out Array 3:11 PM If you just want to nicely print out ALL of any array named myary just use the string: Arrays.toString(myary) For example: out.println("mine="+arrays.tostring(myary)); However, if myary is partially empty and myarylen gives how much of myary is "in use" then you can write: out.println("mine="+arrays.tostring( Arrays.copyOf(myary,myarylen) ));

5 New Section 4 Page 5 Extend Array 3:16 PM If you want to extend an array and ALL of it is "in use" SORRY!!!!! You CANNOT DO that. However, if your array is named myary and myarylen counts how much is "in use" and that is LESS than the actual size of myary, then you can add an element value named myvalue as follows: myary[myarylen++] = myvalue;

6 New Section 4 Page 6 Remove Element 3:21 PM If your array named myary can be partially empty and you keep track of how much is actually "in use" using the variable myarylen, then removing the last [rightmost] element of those that are "in use" is very easy: --myarylen; HOWEVER, if you wish to remove an element from WITHIN the array - for example the element at index position named x. Then it is a little trickier: --myarylen; System.arraycopy(myary,x+1, // part following x myary,x, // over onto part starting at x myarylen-x; // how long that was );

7 New Section 4 Page 7 Membership 3:28 PM Suppose you have a partially empty array named myary and keep how much is "in use" in the variable myarylen. Suppose you want to know [true or false] whether the value named findthis appears in the "in use" part of your array. The following code places that true or false in the boolean variable named foundit: boolean foundit = false; // assume failure for(int k=0;k<myarylen;++k) { if(myary[k] == findthis) { foundit = true; break; // found it } }

8 New Section 4 Page 8 Membership 2 3:28 PM Suppose you have an array named myary and are "using" ALL of it. Suppose you want to know [true or false] whether the value named findthis appears ANYWHERE in your array. The following code places that true or false in the boolean variable named foundit: boolean foundit = false; // assume failure for(int x : myary) if(x==findthis) { foundit = true; break; // found it }

9 New Section 4 Page 9 Location 3:28 PM Suppose you have a partially empty array named myary and keep how much is "in use" in the variable myarylen. Suppose you want to know WHERE the value named findthis appears in the "in use" part of your array - namely you want the index of it. The following code places that index in the int variable named where or places a -1 [impossible answer] into where when the findthis value CANNOT be found in the "in use" part of myary. int where = -1; // assume failure for(int k=0;k<myarylen;++k) { if(myary[k] == findthis) { where = k; break; // found it } }

10 New Section 4 Page 10 Location 2 3:28 PM Suppose you have an array named myary and are "using" ALL of it. Suppose you want to know WHERE the value named findthis appears in ALL of your array - namely you want the index of it. The following code places that index in the int variable named where or places a -1 [impossible answer] into where when the findthis value CANNOT be found in the "in use" part of myary. int k=0, where = -1; // assume failure for(int x : myary) if(x==findthis) { where = k; break; // found it } else ++k;

11 New Section 4 Page 11 Concatenate 3:42 PM Suppose you have two arrays named myary and yourary and are using ALL of both of them. Suppose you wish to create a new array named bothary that is the concatenation of myary followed by yourary. Here is how to do that: int bothlen = myary.length + yourary.length; int[] bothary = Arrays.copyOf(myary,bothlen); System.arraycopy(yourary,0, // source of copy bothary,myary.length, // destination yourary.length );

12 New Section 4 Page 12 Concatenate 2 3:47 PM Suppose you have a partially empty array named myary and myarylen says how much of the array is "in use". Suppose similarly for another array named yourary and using yourarylen. Suppose myary has enough "unused" space in it to concatenate yourary [we mean the "in use" part of it] onto the end of myary [again,we mean the "in use" part of it.] Here is the code: System.arraycopy(yourary,0, // source of copy myary,myarylen, // destination yourarylen // how much to copy ); myarylen += yourarylen; // increase of "in use" part

13 New Section 4 Page 13 Input One per Line 3:53 PM Suppose the user will type a series of numbers that should go into a new array named myary. The length of that series is NOT known in advance - but the user will type to indicate that enough array elements have been entered. However the is NOT supposed to go into the array. Clearly, we need an array that is pretty big and will end up "partially" empty. Let us use myarylen to keep track of how much of this array named myary is actually "in use". Suppose we have some kind input scanner named cin that produces a user input via a nextint() method. int[] myary = new myary[200]; int myarylen = 0; while(true) { int usernbr = cin.nextint(); if(usernbr==99999) break myary[myarylen++] = usernbr; }

14 New Section 4 Page 14 Input ALL on ONE Line 4:03 PM Suppose the user will enter a SINGLE line and the numbers on that line are separated by commas. Suppose that cin is some kind of scanner with a nextline() method that will read in an entire user line as one String. This code will make an array named myary which is ALL in use and has just the correct size for the length of the user's input [ = how many integers the user placed on the one line] String line = cin.nextline(); String[] data = line.split(","); int[] myary = new int[data.length]; for(int k=0;k<data.length;++k) myary[k] = Integer.parseInt(data[k].trim()); If you need to make myary much larger - and thus partially empty do this: int myarylen = myary.length; myary = Arrays.copyOf(myary,300);