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Working with external data : 2-dimensional arrays : Case 6

Case 6: providing values for 2-dimensional variables

A variable will need a 2-dimensional array of parameter values if it is itself a 2-dimensional array variable, or if it is inside a 2-dimensional multiple-instance submodel, or if it is a 1-dimensional array variable inside an ordinary multiple-instance submodel, or if it is inside 2 nested multiple-instance submodels.

There are two ways of providing two-dimensional data for such a variable. The first two are generalizations of methods already described, and can also be used for higher-dimensional variables. The third involves reading data directly from a file in a 2-dimensional format, and was introduced in Simile v4.8.

Values direct from scenario file

This method is limited to small arrays. You enter the data in the edit field in the file parameter value as for one-dimensional arrays, but in place of each value there is another list of index and value pairs, in curly brackets. For instance, the data for a 4x3 array of integers could be supplied as:

1 {1 4 2 9 3 1} 2 {1 8 2 5 3 6} 3 {1 9 2 11 3 6} 4 {1 0 2 7 3 9}

Note that after these values are accepted by the dialogue, it will show the indices with a hash and colon, e.g., #2: to distinguish them from the data items. They can in fact be entered either with or without these decorations, making it easier to enter them by cutting and pasting.

It is fairly easy to see how this mechanism can be extended to cope with higher-dimensional arrays.

Values from a column in a .csv (comma-separated value) file

The data file should be in standard csv (comma-separated value) format. It should have one header line, consisting of one or more labels, separated by commas, followed by n lines. Each line should have the same number of values as there are labels in the header line, and again the values should be separated by commas. The number (n) of data lines should be at least as great as the number of instances for the submodel. One column should contain the values required for the variable in the model. There should also be one column containing the outer index values, and one column containing the inner index values.

For instance, a .csv file to generate the same array as in the last subsection, might look like this:














Note that the row index (which would normally correspond to the outer index in a 2-D array within Simile) actually appears after the column index in the file. This does not matter; the columns can be in any order in the file. So can the rows, since their ordering in the model data is going to be specified by the values in the index columns, rather than their line numbers.

When you come to run the model

Simile will produce a File Parameter dialogue window, and this will contain an entry for this variable.

  1. Click on the Read table button for this variable

  2. Select the 'Data from column' tab in the Table Data dialogue, if it is not already selected

  3. Browse through your file system to find the .csv file containing the data values.

  4. Click on the OK button.

    You will now see a window listing the columns in the table, by the label associated with each column.

  5. Drag the header of the column containing the values for the outermost index across into the 'indices' listbox. In the example above, this column header is IndV.

  6. Repeat this action for the inner index. In the example above, this column header is IndH.

  7. Double-click on the label corresponding to the column of values you wish to assign to the variable. In the example, it is Val.

  8. Click on the OK button.

  9. Click Save if you wish to save this setting (along with any others), and specify a file to save it in.

  10. Click OK when you wish to proceed with the simulation.

Values from a grid in a .csv file

In many cases, the data for a 2-d array is available as a grid of values in a .csv file, typically one saved by a spreadsheet. In such cases it is difficult to convert it to a single column with separate columns for indices, so Simile allows this type of data to be read directly. The rows in the grid will correspond to the outermost index of the Simile array, and the columns to the innermost index. If the grid data is arranged the other way round from this, the spreadsheet's 'transpose' function can be used to swap the rows and columns.

After you click the 'get values from file' button in the File Parameters dialogue, the Table Data dialogue contains four tabs. Initially, the 'Data in Column' tab is raised, but for this case we use the 'Data in Grid' tab. So first click on that tab to raise it.

Now we see a file selector, the same as in the previous examples. Use it to open your .csv file containing the grid of values.

On opening the file, the four entry fields for start/finish at row/column will be filled with the numbers of the first and last rows and columns respectively, providing indication of the total size of the grid of data in the file. Note that any header rows and/or columns will be included in these dimensions, so the 'start at' values will always be 1. At this point you can adjust the values to correspond to the part of the grid you actually want to load. At any time you can hit the View/Edit button on the right of the dialogue, to display a grid of the values in the part of the file currently selected for loading. Ths is useful if you are unsure which part of the file contains headers and which the numerical values. However, note that the table display tool only works up to a maximum of 10,000 rows or columns.

The size of the area you eventually select must match the dimensions of the array you need to load into the model. These dimensions are displayed in the header of the Table Data dialogue for convenience. Once you have chosen the appropriate start and end points, you can use the View/Edit button again to actually change some of the values (e.g., if some were badly formatted in the file) but be aware that if you do this and then save the scenario, all the values will be saved in the parameter file, rather than just a reference to the .csv file.

Values from an image or other binary-format file

Loading a 2-D array of data from an image is very much like loading it from a grid of numerical values. Follow the same procedure except, when you open the Table Data dialogue, select the 'Data from Image' tab. You then choose the area of the image from which you want to load values, exactly as was done for the case of loading data from a .csv grid, except in this case there are no header rows or columns to worry about.

There is an important extra step when using an image file - each pixel in an image has a colour, and you must specify how these colours are to be converted into numerical values to use in the model. This has to be done before you can use the View/Edit button, since the table will display numerical values. Each pixel will correspond to a number between a lower and an upper bound. These bounds are entered as 'Value for black' and 'Value for white', and these are the values that will be generated for black, and white, pixels respectively. Value for black can be greater than or less than value for white. Pixels of other colours will generate values between these two values, as specified in the 'For other colours' pulldown menu. For instance, if you select 'Use green level', a green pixel will give the same value as a white one, since the green level in white is 100%, while a red pixel will give the same value as a black one, since the red level in green is 0%.

If the image file has 8-bit colourmap data, as is the case for most GIF files, then these values can be used directly, and by setting the values for black and for white to 0 and 255 respectively, the values themselves will be preserved. Finally, some images contain transparent areas; when using such an image you must set a separate value for transparent pixels in the 'Value for clear' entry field. This need not be in the range between the values for black and for white.

Other grid data formats

Several other data file formats, principally GeoTIFFs, can be loaded into Simile model parameters if you have the Geospatial Data Abstraction Library (GDAL) installed on your computer. This library is not included in Simile's installation, but is distributed free of charge, and is available for all operating systems for which Simile is supported. The procedure for loading these data files is the same as for loading image data, except that since the values in the files are already interpreted as numerical rather than colour, there is no need to specify how they are to be converted to numeric values. Use the last tab in the table values dialogue (Data from GeoTIFF, etc).

In: Contents >> Working with external data >> Using scenario files