Extract columns from array of string-CodePudding

I have an array of string

String[] outLines = ['1 2 3', '1 2 3', '1 2 3'];

What I want to do is extract all values of a certain column to a new array. For example, the first column array will contain [1, 1, 1]. The second column array will contain [2, 2, 2] etc.

My first though was to loop the string array, split every string using spaces and then add each value to its relevant column array.

I was wondering, is there a better way to do so? both performance and syntax wise.

CodePudding user response：

It sounds like you want to map the column values from the String array. Java provides several implementations of Map. The most applicable (imo) is LinkedHashMap since it preserves insertion order. Split the line on whitespace, iterate; for each column retrieve the existing List from the Map (or add a new List if one does not already exist). Something like,

String[] outLines = { "1 2 3", "1 2 3", "1 2 3" };
Map<Integer, List<String>> alMap = new LinkedHashMap<>();
List.of(outLines).forEach(s -> {
    String[] line = s.split("\\s ");
    for (int i = 0; i < line.length; i  ) {
        List<String> al = alMap.getOrDefault(i, new ArrayList<>());
        al.add(line[i]);
        alMap.put(i, al);
    }
});
for (int key : alMap.keySet()) {
    System.out.println(alMap.get(key));
}

Which outputs

[1, 1, 1]
[2, 2, 2]
[3, 3, 3]

CodePudding user response：

Wow ! It been while since last time I found this kind of questions!, there are many ways you can achieve your answer, consider one of them as follow with only four line of code.

String[] outLines = {"1 2 3", "1 2 3", "1 2 3"};
List<String[]> accumulator = new ArrayList<>();
Arrays.stream(outLines).map(o -> o.split(" ")).forEach(accumulator::add);
String[][] result = accumulator.toArray(String[][]::new);

//Validate the result
Arrays.stream(result).map(Arrays::stream).forEach(o -> {
  System.out.println();
  o.forEach(System.out::print);
});

You can also create a Collector and use the accumulator and combiner quit similar to what I wrote for you and reduce the answer to two line.

(Don't forget to Woot !)

CodePudding user response：

Try this.

static int[][] extract(String[] array) {
    int rows = array.length, cols = array[0].split("\\s ").length;
    int[][] matrix = new int[cols][rows];
    for (int i = 0; i < rows;   i) {
        String[] s = array[i].split("\\s ");
        for (int j = 0; j < cols;   j)
            matrix[j][i] = Integer.parseInt(s[j]);
    }
    return matrix;
}

public static void main(String[] args) {
    String[] outLines = {"1 2 3", "1 2 3", "1 2 3"};
    int[][] output = extract(outLines);
    for (int[] row : output)
        System.out.println(Arrays.toString(row));
}

output:

[1, 1, 1]
[2, 2, 2]
[3, 3, 3]