The ValueMap Train method assumes that keys are on the first column, and values on the second, rather than storing the columns and using those

[sfilipi]

If the lookupMap IDataView in the MapValue API is constructed so that the first column contains the keys, and the second one contains the values, this API will work as expected; if the lookup idv is constructed in any other order, the API won't work, because the Train method of the transformer has hardcoded the index = 0 to be the keys of the idv, and index = 1 to be the values .

The ValueMapping Train should not assume that the first column is the keys, and the second is the values; it should instead save the columns passed to it from the API (and not just the column names, like it is doing now) and get the cursor based on the columns, rather than indices.

Easy repro: : swap the order of the members in the LookupMap class below, and see how it fails.

 // Type for the IDataView that will be serving as the map
private class LookupMap
 {
     public float Value { get; set; }
     public string Category { get; set; }
 }

using System;
using System.Collections.Generic;

namespace Microsoft.ML.Samples.Dynamic
{
    public static class MapValueIdvLookup
    {
        /// This example demonstrates the use of MapValue by mapping floats to strings, looking up the mapping in an IDataView. 
        /// This is useful to map types to a grouping. 
        public static void Example()
        {
            // Create a new ML context, for ML.NET operations. It can be used for exception tracking and logging, 
            // as well as the source of randomness.
            var mlContext = new MLContext();

            // Get a small dataset as an IEnumerable.
            var rawData = new[] {
                new DataPoint() { Price = 3.14f },
                new DataPoint() { Price = 2000f },
                new DataPoint() { Price = 1.19f },
                new DataPoint() { Price = 2.17f },
                new DataPoint() { Price = 33.784f },

            };

            // Convert to IDataView
            var data = mlContext.Data.LoadFromEnumerable(rawData);

            // Create the lookup map data IEnumerable.   
            var lookupData = new[] {
                new LookupMap { Value = 3.14f, Category = "Low" },
                new LookupMap { Value = 1.19f , Category = "Low" },
                new LookupMap { Value = 2.17f , Category = "Low" },
                new LookupMap { Value = 33.784f, Category = "Medium" },
                new LookupMap { Value = 2000f, Category = "High"}

            };

            // Convert to IDataView
            var lookupIdvMap = mlContext.Data.LoadFromEnumerable(lookupData);

            // Constructs the ValueMappingEstimator making the ML.NET pipeline
            var pipeline = mlContext.Transforms.Conversion.MapValue("PriceCategory", lookupIdvMap, lookupIdvMap.Schema["Value"], lookupIdvMap.Schema["Category"], "Price");

            // Fits the ValueMappingEstimator and transforms the data converting the Price to PriceCategory.
            IDataView transformedData = pipeline.Fit(data).Transform(data);

            // Getting the resulting data as an IEnumerable.
            IEnumerable<TransformedData> features = mlContext.Data.CreateEnumerable<TransformedData>(transformedData, reuseRowObject: false);

            Console.WriteLine($" Price   PriceCategory");
            foreach (var featureRow in features)
                Console.WriteLine($"{featureRow.Price}\t\t{featureRow.PriceCategory}");

            // TransformedData obtained post-transformation.
            //
            // Price        PriceCategory
            // 3.14            Low
            // 2000            High
            // 1.19            Low
            // 2.17            Low
            // 33.784          Medium
        }

        // Type for the IDataView that will be serving as the map
        private class LookupMap
        {
            public float Value { get; set; }
            public string Category { get; set; }
        }

        private class DataPoint
        {
            public float Price { get; set; }
        }

        private class TransformedData : DataPoint
        {
            public string PriceCategory { get; set; }
        }
    }
}