Vertical sum using java stream api-CodePudding

Imagine that I have a matrix of integer elements List<List<BigDecimal>>:

-------------
| 1 | 2 | 3 |
-------------
| 2 | 3 | 4 |
-------------
| 3 | 4 | 5 |
-------------
| 4 | 5 | 6 |
-------------

I want to vertically sum the elements of a matrix and get a List<BigDecimal>
[10, 14, 18]

I suppose first I have to transpose the matrix and then it's easy to get the sum of the elements. But how to perfectly transpose a matrix using stream api??

CodePudding user response：

One way to transform the 2D list is to map to an object and by collecting that list we can find the transpose of the 2D list as below:

List<List<BigDecimal>> result = IntStream.range(0, list.get(0).size())
                        .mapToObj(i -> list.stream().map(l -> l.get(i)).collect(Collectors.toList()))
                        .collect(Collectors.toList());

And Without transposing, you can directly find the sum of the list using reduce function below:

List<BigDecimal> result = IntStream.range(0, list.get(0).size())
                        .mapToObj(i -> list.stream().map(l -> l.get(i)).reduce(BigDecimal.ZERO, BigDecimal::add))
                        .collect(Collectors.toList());

CodePudding user response：

DISCLAIMER: This isn't the efficient way with streams or something fancy that you could do in just a couple lines, but it is a simple easy-to-follow way. Why not just do something simple like this? Iterate over the list length and sum that column via the index:

List<List<BigDecimal>> matrix = <<your matrix here>>
List<BigDecimal> columnSum= new List<BigDecimal>;
// For each column
for(int i = 0; i < matrix.get(0).size()-1; i  ){
      int sum = 0;
      for(List<BigDecimal> row : matrix){
            sum  = row.get(i);
      }
      columnSum.add(BigDecimal(sum));
}