# Parameter expressions

Gnucap supports expressions for almost all parameters.

Ideally, the usual syntax is supported. Actually, in some cases you must enclose the expression in quotes or curly braces.

Expressions use late evaluation and dynamic scoping, like a functional language.

The “eval” command evaluates and prints the present value of an expression.

The “measure” command evaluates and assigns the present value of an expression. For now, it only takes simple function calls used in measurements.

## Example

```gnucap> param a=5
gnucap> param b=a+3
gnucap> eval a
a= 5.
gnucap> eval b
b= 8.```

Now change the value of “a”.

```gnucap> param a=9
gnucap> eval b
b= 12.```

Thanks to late evaluation, the value of “b” also changed.

This is so you can do things like this:

```r1 (in out) resistor r={rtotal*ratio}
r2 (out 0)  resistor r={rtotal*(1-ratio)}```
```param rtotal=100k
param ratio=.5```

r1 and r2 are both 50k.

`param rtotal=10k`

r1 and r2 are both 5k.

`param ratio = .3`

r1 is 3k. r2 is 7k.

## Operators

The usual binary operators (+ - * /) are supported in the usual way with the usual precedence.

The usual comparison operators ( < > <= >= == != ) are supported. The result is 1 if it evaluates to true, 0 if it evaluates to false.

The binary operators (&& ||) are supported, and evaluate to 1 or 0.

## Functions

These functions are included by default. More functions can be added as plugins.

 abs(x) Magnitude of x. cos(x) Cosine of x. cosh(x) Hyperbolic cosine of x. eval(x) Evaluate x. exp(x) e raised to the x power. int(x) Integer part of x. log(x) Natural log of x. na(x) “Not available”. Returns “NA” regardless of its argument. pow(x,y) x raised to the y power. sin(x) Sine of x. sinh(x) Hyperbolic sine of x. sqrt(x) Square root of x. tan(x) Tangent of x. tanh(x) Hyperbolic tangent of x. 