Projection.java

package com.github.lwhite1.tablesaw.table;

import com.github.lwhite1.tablesaw.api.Table;
import com.github.lwhite1.tablesaw.columns.Column;
import com.github.lwhite1.tablesaw.filtering.Filter;
import com.github.lwhite1.tablesaw.util.Selection;

/**
 * A table projection, i.e. the subset of columns in a table that should be returned in a query
 */
public class Projection {

    private final Table table;
    private final Column[] columns;

    public Projection(Table table, String[] columnNames) {
        this.table = table;
        columns = new Column[columnNames.length];
        for (int i = 0; i < columnNames.length; i++) {
            String name = columnNames[i];
            columns[i] = table.column(name);
        }
    }

    public Table where(Filter filter) {
        Table projectedTable = Table.create(table.name(), columns);
        Table newTable = projectedTable.emptyCopy();
        Selection map = filter.apply(table);
        Rows.copyRowsToTable(map, projectedTable, newTable);
        return newTable;
    }
}