Removing string from arraylist

Question

I need to remove a string from an array list if it does not start with 13. I have read in the data file and each line is read into the ArrayList as a string. The whole line needs to be removed if it does not start with 13. Below shows an example of the data file read in:

13 0 150801 00133990 04 04 1 078 2160 999 1 0 1 0 16 04 07 058 0320 061 0127 068 0546 033 0129 048 0586 042 0133 033
13 0 150801 00140480 04 04 1 086 1730 999 1 0 1 0 13 04 06 047 0309 039 0143 032 0772 017 0131 029 0140 037
13 0 150801 00141320 04 04 1 075 1660 999 1 0 1 0 13 03 06 053 0319 036 0132 037 0733 023 0130 026 0134 029
13 0 150801 00144430 01 01 1 071 1570 999 2 0 1 0 04 03 02 040 0567 065
30 0 150801 0015 15 01 0000 0000 0004 0000 0001 0002 0000 0000 0000 0000 0000 0000 0001 0001 0000 0000 0008 0000
30 0 150801 0015 15 02 0000 0000 0005 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
30 0 150801 0015 15 03 0000 0000 0013 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
30 0 150801 0015 15 04 0000 0000 0021 0000 0000 0001 0001 0000 0000 0001 0000 0000 0000 0006 0000 0000 0007 0001
21 0 150801 0015 15 01 0002 0001 0005 0004 0003 0001 0001 0000 0000 0000 000973 002 001 010 0001 0001
21 0 150801 0015 15 02 0000 0000 0000 0001 0001 0002 0000 0001 0000 0000 000000 000 000 000 0001 0001
21 0 150801 0015 15 03 0000 0001 0000 0000 0001 0004 0003 0001 0002 0001 000000 000 000 000 0003 0003
21 0 150801 0015 15 04 0000 0002 0009 0008 0007 0002 0006 0004 0000 0000 001252 000 004 012 0001 0003
70 0 150801 0015 15 01 0004 0000 0012 0001 00001400 00000000 00005956 00000476 000412 000000 000897 000076 000042542 000000000 000068665 000005776
70 0 150801 0015 15 02 0005 0000 0000 0000 00001790 00000000 00000000 00000000 000511 000000 000000 000000 000053047 000000000 000000000 000000000
70 0 150801 0015 15 03 0011 0002 0000 0000 00003648 00000625 00000000 00000000 001240 000231 000000 000000 000144378 000026705 000000000 000000000
70 0 150801 0015 15 04 0021 0001 0015 0001 00007334 00000417 00006991 00000449 002203 000086 001172 000080 000235235 000007396 000092708 000006400
13 0 150801 00151590 04 04 1 078 2240 999 1 0 1 0 16 04 07 064 0318 073 0137 071 0545 083 0137 079 0604 066 0133 067
13 0 150801 00155070 01 01 1 078 1760 999 1 0 1 0 13 04 06 059 0324 054 0135 060 0643 022 0133 028 0127 049
13 0 150801 00160780 01 01 1 080 2230 999 1 0 1 0 16 04 07 057 0374 059 0132 059 0724 064 0138 066 0456 052 0138 063
13 0 150801 00162180 04 04 1 063 1820 999 1 0 1 0 16 04 07 058 0264 082 0113 081 0474 079 0113 080 0501 069 0113 070

This is what I have written but it doesn't seem like any items are deleted:

public class WIMdataReader {
    public static void main(String[] args) throws IOException {

        //Opens the file
        FileInputStream fis = new FileInputStream("DATA1month.txt");
        BufferedReader br = new BufferedReader(new InputStreamReader(fis));

        String readLine;

        //Creates an ArrayList to store the data line by line
        //Using an ArrayList, which handles dynamic sizing, whereas an array will require a defined size up front, which may not be known.
        List<String> WIMdataList = new ArrayList<String>();

        //Reads file line by line
        while ((readLine = br.readLine()) != null) {
            WIMdataList.add(readLine);
        }

        //removes items from the arrayList 
        for (int i = 0; i < WIMdataList.size(); i++) {

            if (WIMdataList.get(i).contains("13")) {

            } else {
                WIMdataList.remove(i);
            }
        }
    }
}

If you're removing things from the list, and iterating by index, iterate backwards. — Andy Turner
– Andy Turner, Commented Sep 6, 2017 at 13:29
Another thing you can do instead of removing from a List is to create a new List and add only valid entries to it. — SedJ601
– SedJ601, Commented Sep 6, 2017 at 13:32
Thanks guys, iterated backwards and it works now. Appreciate all the help! — Gilmore Munro
– Gilmore Munro, Commented Sep 6, 2017 at 13:38

Andy Turner · Accepted Answer · 2017-09-06 13:53:28Z

5

If you're removing things from the list, and iterating by index, iterate backwards. Otherwise, you never check the item immediately after the one you've just removed, and thus you won't remove it, even if it meets your criteria.

for (int i = WIMdataList.size() - 1; i >= 0; --i) {
  if (!WIMdataList.get(i).contains("13")) {
    WIMdataList.remove(i);
  }
}

Note that repeatedly removing from the middle of an ArrayList is inefficient, because all of the subsequent elements have to be moved along each time. You can avoid this by iterating forwards, and shifting the elements yourself, and truncating the list at the end:

int dst = 0;
for (int src = 0; src < WIMdataList.size(); ++src) {
  if (WIMdataList.get(src).contains("13")) {
    WIMdataList.set(dst++, WIMdataList.get(src));
  }
}
WIMdataList.subList(dst, WIMdataList.size()).clear();

But it's easier just to use removeIf:

WIMdataList.removeIf(s -> !s.contains("13"));

Or a filter, to create a new list:

List<String> filtered = WIMdataList.stream().filter(s -> s.contains("13")).collect(Collectors.toList());

Or, of course, just don't add them to the list in the first place.

edited Sep 6, 2017 at 13:53

answered Sep 6, 2017 at 13:31

Andy Turner

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2 Comments

f1sh Over a year ago

but use it with startsWith, not contains.

Andy Turner Over a year ago

@f1sh in OP's code: "WIMdataList.get(i).contains("13")". The specific criterion is unimportant.

Evin1_ · Accepted Answer · 2017-09-06 13:37:03Z

You can just avoid adding the Strings that don't start with a 13 the first time you fill your ArrayList adding:

if(readLine.startsWith("13")){
    WIMdataList.add(readLine);
}

Inside the while that loops through the contents of the file.

public class WIMdataReader {
    public static void main(String[] args) throws IOException {
        //Opens the file
        FileInputStream fis = new FileInputStream("DATA1month.txt");
        BufferedReader br = new BufferedReader(new InputStreamReader(fis));

        String readLine;

        //Creates an arrayList to store the data line by line
        //Using an ArrayList, which handles dynamic sizing, whereas an array will require a defined size up front, which may not be known.
        List<String> WIMdataList = new ArrayList<String>();

        //Reads file line by line
        while ((readLine = br.readLine()) != null) {
            if (readLine.startsWith("13")) {
                WIMdataList.add(readLine);
            }
        }

        System.out.println(WIMdataList);
    }
}

You won't iterate the loop twice if you check it beforehand.

Anton Balaniuc · Accepted Answer · 2017-11-01 12:38:08Z

1

You can do it in a shorter manner using Files and Stream:

Files.lines(Paths.get("data.txt"))
           .filter(line -> line.startsWith("13"))
           .collect(Collectors.toList());

edited Nov 1, 2017 at 12:38

answered Sep 6, 2017 at 13:48

Anton Balaniuc

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1 Comment

jrtapsell Over a year ago

The original answer collects to a list, so you probably want to use .collect(Collectors.toList()) at the end

SHALong · Accepted Answer · 2017-09-06 13:39:20Z

0

You should put the if statement in the while loop so that you don't need to iterate through the list again.

//Reads file line by line
while ((readLine = br.readLine()) != null) {
    // regex to dertermine if the first "num" is 13
    if (!readLine.matches("^13 .*")) {
        // add to the list if doesn't start with 13
        WIMdataList.add(readLine);
    }
}

answered Sep 6, 2017 at 13:39

SHALong

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