The historical parameter notation using & as the first character of a line as equivalence to .param. is deprecated and will be removed in a coming release.

Confusion may arise in ngspice because of its multiple numerical expression features. The .param lines and the brace expressions (see Chapt. 2.9) are evaluated in the front-end, that is, just after the subcircuit expansion. (Technically, the X lines are kept as comments in the expanded circuit so that the actual parameters can be correctly substituted). Therefore, after the netlist expansion and before the internal data setup, all number attributes in the circuit are known constants. However, there are circuit elements in Spice that accept arithmetic expressions not evaluated at this point, but only later during circuit analysis. These are the arbitrary current and voltage sources (B-sources, 5), as well as E- and G-sources and R-, L-, or C-devices. The syntactic difference is that `compile-time' expressions are within braces, but `run-time' expressions have no braces. To make things more complicated, the back-end ngspice scripting language accepts arithmetic/logic expressions that operate only on its own scalar or vector data sets (17.2). Please see Chapt. 2.13.

It would be desirable to have the same expression syntax, operator and function set, and precedence rules, for the three contexts mentioned above. In the current Numparam implementation, that goal is not achieved.