User's Guide: Evaluation of Mathematical Expressions

Both psrstat and psrplot make use of the same interpreter for the evaluation of mathematical expressions. psrstat uses this interpreter to print useful information to stdout, and psrplot can use it to label plots. This page documents the features and syntax of the interpreter that are common to both programs.

Variable substitution

Any word preceded by a dollar sign ($) will be treated as a variable name, and its value will be queried from the archive and substituted. For example: 
psrplot -c below:r='DM: $dm' [...]
would print a string like "DM: 2.64510488510132" just below the upper right hand corner of the plot frame. (To set the number of significant digits printed, see the notes on numerical precision below.)

For a full list of available variable names, run psrstat <filename>.

Mathematical expressions

Any string of characters enclosed in braces ({ }) will be evaluated as a mathematical expression and the result will be substituted into the string. For example: 
psrstat -c '{fabs($bw)/$freq}' [...]
would print the fractional bandwidth for each file specified on the command line.

Any mathematical expression that can be parsed by the Parsifal evaluateExpression function may be used inside the braces.

Vector functions

A number of functions that operate on arrays of values are also available, including
  • min - minimum value
  • max - maximum value
  • med - median value
  • avg - mean value
  • rms - standard deviation
  • sum - sum of all values
The function name is specified before the opening brace bracket, and the vector expression is included inside the braces. For example, to print the mean value of the reduced chi-squared statistic stored in the polarization calibrator solution extension, averaged over frequency channels 16 through 121: 
psrstat -c '{sum{$pcal:eqn[16-121]:chisq}/sum{$pcal:eqn[16-121]:nfree}}' [...]
Note that the outer braces are required to invoke evaluation of the final division operation.

Numerical precision

The numerical precision of floating point values may be set by appending a precision specifier to either the variable name or mathematical expression. This specifier is a percent sign followed by an integer (%N) where N is the total number of significant digits to be printed. For example: 
psrplot -c below:r='DM: $dm%4' [...]
would add a string like "DM: 2.645" below the upper right hand corner of each plot. Likewise: 
psrstat -c 'Fractional BW: {fabs($bw)/$freq}%2' [...]
would print a string like "Fractional BW: 0.048" for each file.