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H2O Core
There is a newer version: 3.8.2.9
package water.rapids;

import water.DKV;
import water.H2O;
import water.fvec.Frame;
import water.fvec.Vec;
import water.util.VecUtils;

import java.util.ArrayList;
import java.util.Arrays;

/** A collection of Strings only.  This is a syntatic form only, and never
 *  executes and never gets on the execution stack.
 */
public class ASTStrList extends ASTParameter {
  public String[] _strs;
  ASTStrList( Exec e ) {
    ArrayList strs  = new ArrayList<>();
    while( true ) {
      char c = e.skipWS();
      if( c==']' ) break;
      if( e.isQuote(c) ) strs.add(e.match(c));
      else throw new IllegalArgumentException("Expecting the start of a string");
    }
    e.xpeek(']');
    _strs = strs.toArray(new String[strs.size()]);
  }
  // Strange count of args, due to custom parsing
  @Override int nargs() { return -1; }
  // This is a special syntatic form; the number-list never executes and hits
  // the execution stack
  @Override public Val exec(Env env) { throw H2O.fail(); }
  @Override public String str() { return Arrays.toString(_strs); }
  // Select columns by number or String.
  @Override int[] columns( String[] names ) { 
    int[] idxs = new int[_strs.length];
    for( int i=0; i < _strs.length; i++ ) {
      int idx = idxs[i] = water.util.ArrayUtils.find(names,_strs[i]);
      if( idx == -1 ) throw new IllegalArgumentException("Column "+_strs[i]+" not found");
    }
    return idxs;
  }
}

/** Assign column names */
class ASTColNames extends ASTPrim {
  @Override public String[] args() { return new String[]{"ary", "cols", "names"}; }
  @Override int nargs() { return 1+3; } // (colnames frame [#cols] ["names"])
  @Override public String str() { return "colnames="; }
  @Override
  public Val apply(Env env, Env.StackHelp stk, AST asts[]) {
    Frame fr = stk.track(asts[1].exec(env)).getFrame();
    if( asts[2] instanceof ASTNumList ) {
      if( !(asts[3] instanceof ASTStrList) )
        throw new IllegalArgumentException("Column naming requires a string-list, but found a "+asts[3].getClass());
      ASTNumList cols = ((ASTNumList)asts[2]);
      ASTStrList nams = ((ASTStrList)asts[3]);
      int d[] = cols.expand4();
      if( d.length != nams._strs.length ) 
        throw new IllegalArgumentException("Must have the same number of column choices as names");
      for( int i=0; i