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Operations on Tables

In this section we describe several of the mathematical operations that can be performed on table data. More specialised topics are described in Chapter 8, "Fitting of Data", and in Chapter 11 Vol B, "Multivariate Analysis Methods".

Arithmetic operations between columns in a table are done with the command
COMPUTE/TABLE. The selection flag is reset by this command. Special functions, named according to the FORTRAN mathematical library, are also available.

Simple statistical descriptors are displayed with the command STATISTICS/ TABLE. These descriptors are stored in output keywords for further usage in a procedure.

A set of histogram-related commands, with qualifier HISTOGRAM, allow the graphic display of the histogram of a column (PLOT and OVERPLOT commands), the printout of histogram values (READ and PRINT commands), or the generation of a 1D image with the histogram of a column (command COMPUTE/HISTOGRAM).

Linear or polynomial fits in one or two dimensions can be performed on table columns with the command REGRESSION, qualifiers LINEAR and POLY respectively. The coefficients and error estimations are kept in output keywords that can be stored as table descriptors with the command SAVE/REGRESSION. Fitted values are calculated with COMPUTE/REGRESSION.

A topic of special interest is the generation of table data from an image and vice versa. A transformation was already described in section 5.4.3, and consists in copying the data from one format to the other, using the commands COPY/IT and COPY/TI. Tabular data can be converted into 1D or 2D image data with the command CONVERT/TABLE. This command works in several modes controlled by a parameter. In all cases the sampling domain of the result is defined by a reference image. The modes currently available are : POLY (polynomial fit to table data), SPLINE (spline approximation), PLOT (scattergram of the data in the table) and FREQ (2D histogram of the data in the table).

For more specific resampling and interpolating algorithms, the commands REBIN and INTERPOLATE will provide full conversion between image and table formats (qualifiers TT, TI, IT and II).

Table 5.8: Operations on Table Data
Command Description
COMPUTE/TABLE Compute numeric expression of columns.
COMPUTE/HISTOGRAM Compute column histogram, result in table or image format.
REBIN Resampling data in table/image formats.
INTERPOLATE Spline interpolation of data.
REGRESSION/LINEAR Compute linear regression.
REGRESSION/POLY Compute polynomial fit.
SAVE/REGRESSION Store regression coefficients as table descriptors.
COMPUTE/REGRESSION Compute fitted values using the regression coefficients.
STATISTICS/TABLE Simple statistics on a table column.

next up previous contents index
Next: Command Overview Up: Table File System Previous: Interactive Editing of Tables
Petra Nass