Feature request: variables and mathematical expressions

[ftessier]

It would prove useful in EGSnrc input files to be able to define variables (without having to create a loop with a loop count = 1). We could use the key variable = but otherwise the same syntax as for loop variables. While at it, we should add tokens integer, float, string instead of integers 0, 1, 2 on the value side to specify the variable type.

Patrick Saull suggested to add math for variables, so that an input expression such as

evaluate a*$(var1)+b*$(var2)

would be parsed and replaced by its value. This is not trivial, as it amounts to coding an expression parser; but it would be extremely useful. To this end, we should consider the Shunting Yard and Recursive Descent Parser Algorithms:

https://en.wikipedia.org/wiki/Shunting-yard_algorithm
https://en.wikipedia.org/wiki/Recursive_descent_parser

Patrick Saull also suggested that variables should be allowed in loop variable definitions, so as to create nested loops where the second loop variable depends on the first one, as in:

loop variable = 0 var2 $(var1) 1

Currently this does not work in EGSnrc input files and results in the following error message:

Processing input loop
Failed reading vmin vdelta from  0 var2 $(var1) 1