Error

using UnityEngine;
using System.Collections;
using System.Linq; // used for Sum of array

public class AssignSplatMap : MonoBehaviour {

    void Start () {
        // Get the attached terrain component
                Terrain terrain = GetComponent<Terrain>();

        // Get a reference to the terrain data
            TerrainData terrainData = terrain.terrainData;

        // Splatmap data is stored internally as a 3d array of floats, so declare a new empty array ready for your custom splatmap data:
        float[, ,] splatmapData = new float[terrainData.alphamapWidth, terrainData.alphamapHeight, terrainData.alphamapLayers];

        for (int y = 0; y < terrainData.alphamapHeight; y++)
            {
            for (int x = 0; x < terrainData.alphamapWidth; x++)
             {
                // Normalise x/y coordinates to range 0-1
                float y_01 = (float)y/(float)terrainData.alphamapHeight;
                float x_01 = (float)x/(float)terrainData.alphamapWidth;

                // Sample the height at this location (note GetHeight expects int coordinates corresponding to locations in the heightmap array)
                        float height = terrainData.GetHeight(Mathf.RoundToInt(y_01 * terrainData.heightmapHeight),Mathf.RoundToInt(x_01 * terrainData.heightmapWidth) );

                // Calculate the normal of the terrain (note this is in normalised coordinates relative to the overall terrain dimensions)
                Vector3 normal = terrainData.GetInterpolatedNormal(y_01,x_01);

                // Calculate the steepness of the terrain
                float steepness = terrainData.GetSteepness(y_01,x_01);

                // Setup an array to record the mix of texture weights at this point
                float[] splatWeights = new float[terrainData.alphamapLayers];

                // CHANGE THE RULES BELOW TO SET THE WEIGHTS OF EACH TEXTURE ON WHATEVER RULES YOU WANT

                // Texture[0] has constant influence
                splatWeights[0] = 0.5f;

                // Texture[1] is stronger at lower altitudes
                splatWeights[1] = Mathf.Clamp01((terrainData.heightmapHeight - height));

                // Texture[2] stronger on flatter terrain
                // Note "steepness" is unbounded, so we "normalise" it by dividing by the extent of heightmap height and scale factor
                // Subtract result from 1.0 to give greater weighting to flat surfaces
                splatWeights[2] = 1.0f - Mathf.Clamp01(steepness*steepness/(terrainData.heightmapHeight/5.0f));

                // Texture[3] increases with height but only on surfaces facing positive Z axis
                splatWeights[3] = height * Mathf.Clamp01(normal.z);

                // Sum of all textures weights must add to 1, so calculate normalization factor from sum of weights
                float z = splatWeights.Sum();

                // Loop through each terrain texture
                for(int i = 0; i<terrainData.alphamapLayers; i++){

                    // Normalize so that sum of all texture weights = 1
                    splatWeights[i] /= z;

                    // Assign this point to the splatmap array
                    splatmapData[x, y, i] = splatWeights[i];
                }
            }
        }

        // Finally assign the new splatmap to the terrainData:
        terrainData.SetAlphamaps(0, 0, splatmapData);
    }
}