How To Use Files

XYPLOT offers a unique graphical environment for working with sets of (x, y) data. The XYPLOT workspace treats each set as a single entity. You may think in terms of operations on sets, rather than on individual numbers. This structure allows you to use fast simple procedures to perform powerful operations. Sets of data and their visual representations constitute the workspace.

The Windows version of xyplot can accomodate arbitrarily large datasets, while the DOS version can hold up to 16,000 (x,y) pairs, partitioned into a maximum of 10 datasets. XYPLOT may read a file on disk and place the information in a dataset, or it may create a file and save the contents of a dataset on disk. The DOS version also reads and writes files that contain its entire workspace. The chapter describes the program's use of these files.

The Structure of Data Files

Data files contain numbers arrayed in one or more columns preceeded by an optional header.

1. A single column of data represents the y values with x, understood, a running index from one to the number of points.
   
   
2. A two column file represents (x, y) pairs. The first column is x.
   
   
3. With files of three or more columns, you are prompted to specify from the keyboard which column represents x and which column represents y.

If lines of text are included at the beginning of the file and enclosed between the /* and */ symbols, XYPLOT interprets the information as a descriptive header. For example, the data might be measurements of thermocouple voltages at different times. The header could be used to save whatever information you want to record about how the measurements were made. When you apply a calibration to convert the voltages to temperatures, a record of the calibration will be inserted automatically into the header. You may save the calibrated measurements in a new file together with the modified header.

The example below illustrates a data file with two columns. Use your favorite text editor to create this or other data files or write them with other programs. A valid data file has the following characteristics:

• A header may be present, but is not required.
  
• The first line of a header, if present, must be /*
  
• After the header text, the last line of the header must be */
  
• Numbers may be written in any format:
  
  • integer (20)
  • floating point (20.0)
  • exponential (2.0e1)
  
  
• Columns may be separated by spaces, commas, or tabs.

In the example the numbers are in floating point format, and the column delimiter is a space.