使用正则表达式将字符串按列解析为列表，以模仿Java中的表

Question

What I have:

I have a string that looks like this

"a, b, c\nd, e, f\ng, h, i"

each new line represents a new row, each comma represents a different column

What I want to do:

I'm trying to pass this into a list so that it looks like this:

[ [a,d,g], [b,e,h], [c,f,i] ]

that way it mimics a table, a table that would look like this:

a   b   c
d   e   f
g   h   i

In other words, I'm making a list of each column.

What I know how to do:

I know if I were making a list of rows I could do this:

String rows[]= text.split("\\r?\\n");

and end up with an array of rows:

{"a, b, c", "d, e, f", "g, h, i"}

and then format each string as a list later but how would I accomplish what I want stated above using a regex with 1 or no loop? I'm trying for O(n) or O(nlogn) algorithm where n is the number of rows. Thank you :)

What I'm currently doing:

 //This is pseudocode so it's not valid

 new list of list called columnList
 String rows[]= text.split("\\r?\\n");
 for each row in rows:
     i=0
     new splitRow = row split by comma
     for each element in splitRow:
         columnList(i).add(element)
         i++

This is the algorithm I came up with but it's really slow and requires nested loops. I don't think it's very efficient.

Further Info:

I'm doing all this in Java

I can't use any Hash structures so no dictionaries sadly

Sorry if my formatting is oddly formal, I've had problems communicating my questions clearly in the past.

Answer 1

I think the steps should be:

1. Replace "\\n" in the original String
2. Split the "," to the String[]
3. Convert one dimension array to two dimension array.

Something looks like:

        String s = "a, b, c\nd, e, f\ng, h, i";
        String[] elements = s.replace("\n", ",").split(",");
        String[][] table = new String[3][3];
        for (int i = 0; i < 3; i++) {
            for(int j = 0; j < 3; j++){
                table[i][j] = elements[(j * 3) + i];
            }
        }

Hope this help.

Answer 2

Use your solution with the nested loops. If you're worried that it isn't O(n), don't be: it is O(n). The time it takes will always be exactly proportional to the number of input elements.

Concrete example #1 : In your 3-row, 3-column input, you have 9 elements total. The outer loop gets executed 3 times. The inner loop executes 3 times for each of those, a total of 9 times. 9 inputs = 9 inner loop executions.

Concrete example #2 : Now let's say you give it a 5-row, 7-column input. You have 35 items total. The outer loop executes five times. The inner loop execute seven times for each of those, a total of 5x7=35 times. 35 inputs = 35 inner loop executions.

Abstract : How about an input of r rows by c columns? Total input items = r x c . Outer loop executes r times, inner loop executes c times for each of those. Thus, r x c total inner loop iterations. r x c input items = r x c inner loop iterations.

Whatever the total number of input elements, the inner loop executes exactly that many times. That is virtually the definition of O(n) .

Answer 3

You can try this code

    String text = "a, b, c\nd, e, f\ng, h, i";
    Pattern pattern = Pattern.compile("(.+)(?:\r?\n|$)");
    Matcher matcher = pattern.matcher(text);
    List<List<String>> result = Lists.newArrayList();
    while (matcher.find()){
        List<String> row = Splitter.on(",").trimResults().splitToList(matcher.group(1));
        result.add(row);
    }

    System.out.println(result);

remember import:

import com.google.common.base.Splitter;
import com.google.common.collect.Lists;

Hope this help.