code: public DelimitedTextParser setResponseIndex(Attribute response, int index) {
          if (response.getType() != Attribute.Type.NOMINAL && response.getType() != Attribute.Type.NUMERIC) {
              throw new IllegalArgumentException("The response variable is not numeric or nominal.");
          }
          
          this.response = response;
          this.responseIndex = index;
          return this;
      }
docs: Sets the attribute and column index (starting at 0) of dependent/response variable.