Ngspice and ngnutmeg store data in the form of vectors: time, voltage, etc. Each vector has a type, and vectors can be operated on and combined algebraically in ways consistent with their types. Vectors are normally created as the output of a simulation, or when a data file (output raw file) is read in again (ngspice, ngnutmeg, see the load command 17.5.40), or when the initial data-file is loaded directly into ngnutmeg. They can also be created with the let command 817.5.37).

An expression is an algebraic formula involving vectors and scalars (a scalar is a vector of length 1) and the following operations:

+ - * / ^ % ,

% is the modulo operator, and the comma operator has two meanings: if it is present in the argument list of a user definable function, it serves to separate the arguments. Otherwise, the term x , y is synonymous with x + j(y). Also available are the logical operations & (and), | (or), ! (not), and the relational operations <, >, >=, <=, =, and <> (not equal). If used in an algebraic expression they work like they would in C, producing values of 0 or 1. The relational operators have the following synonyms:

The operators are useful when < and > might be confused with the internal IO redirection (see 17.4, which is almost always happening). It is however safe to use < and > with the define command (17.5.15).

The following functions are available:

Several functions offering statistical procedures are listed in the following table:

An input vector may be either the name of a vector already defined or a floating-point number (a scalar). A scalar will result in an output vector of length 1. A number may be written in any format acceptable to ngspice, such as 14.6Meg or -1.231e-4. Note that you can either use scientific notation or one of the abbreviations like MEG or G, but not both. As with ngspice, a number may have trailing alphabetic characters.

The notation expr [num] denotes the num'th element of expr. For multi-dimensional vectors, a vector of one less dimension is returned. Also for multi-dimensional vectors, the notation expr[m][n] will return the nth element of the mth subvector. To get a subrange of a vector, use the form expr[lower, upper]. To reference vectors in a plot that is not the current plot (see the setplot command, below), the notation plotname.vecname can be used. Either a plotname or a vector name may be the wildcard all. If the plotname is all, matching vectors from all plots are specified, and if the vector name is all, all vectors in the specified plots are referenced. Note that you may not use binary operations on expressions involving wildcards - it is not obvious what all + all should denote, for instance. Thus some (contrived) examples of expressions are:

Expressions examples:

cos(TIME) + db(v(3))
sin(cos(log([1 2 3 4 5 6 7 8 9 10])))
TIME * rnd(v(9)) - 15 * cos(vin#branch) ^ [7.9e5 8]
not ((ac3.FREQ[32] & tran1.TIME[10]) gt 3)
(sunif(0) ge 0) ? 1.0 : 2.0
mag(fft(v(18)))

Vector names in ngspice may look like @dname[param], where dname is either the name of a device instance or of a device model. The vector contains the value of the parameter of the device or model. See Appendix, Chapt. 31 for details of which parameters are available. The returned value is a vector of length 1. Please note that finding the value of device and device model parameters can also be done with the show command (e.g. show v1 : dc).

There are a number of pre-defined constants in ngspice, which you may use by their name. They are stored in plot (17.3) const and are listed in the table below:

These constants are all given in MKS units. If you define another variable with a name that conflicts with one of these then it takes precedence.

Additional constants may be generated during circuit setup (see .csparam, 2.10).