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# predeclared variables

`float_epsilon`
  Syntax:
    `float_epsilon = 1e-11`
  Description:

    The default value is 1e-11

    Allows somewhat safer logical comparisons and integer truncation for floating point numbers. Most comparisons are treated as true if they are within float_epsilon of being true. eg.

        for (i = 0; i < 1; i += .1) {
            print i, int(10*i)
        }
        float_epsilon = 0
        // two bugs due to roundoff
        for (i = 0; i < 1; i += .1) {
            print i, int(10*i)
        }

  Warning

  I certainly haven’t gotten every floating comparison in the program to use `float_epsilon` but I have most of them including all interpreter logical operations, int, array indices, and Vector logic methods.

`hoc_ac_`

  A variable used by the graphical interface to communicate with the interpreter. It is very volatile. It sometimes holds a value on a function call. If this value is needed by the user it should be copied to another variable prior to any other function call.

`hoc_obj_`
  Syntax:

    `hoc_obj_[0]`

    `hoc_obj_[1]`

  Description:
    When a line on a `Graph` is picked with the PickVector tool two new `Vector`‘s are created containing the y and x coordinates of the line. The y vector is referenced by hoc_obj_[0] and the x vector is referenced by hoc_obj_[1].

`hoc_cross_x_`
  Syntax:
    `hoc_cross_x_`
  Description:
    X coordinate value of the last Crosshair manipulation.

  See also

  Crosshair

`hoc_cross_y_`
  Description:
    Y coordinate value of the last Crosshair manipulation.

  See also

  Crosshair

## constants

The following mathematical and physical constants are built-in:

            "PI",   3.14159265358979323846,
            "E",    2.71828182845904523536,
            "GAMMA",0.57721566490153286060, /* Euler */
            "DEG", 57.29577951308232087680, /* deg/radian */
            "PHI",  1.61803398874989484820, /* golden ratio */
            "FARADAY", 96484.56,    /*coulombs/mole*/
            "R", 8.31441,           /*molar gas constant, joules/mole/deg-K*/

Warning

Constants are not treated specially by the interpreter and may be changed with assignment statements.

The FARADAY is a bit different than the faraday of the units database. The faraday in a `.mod` mechanism is 96520.

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