# \$ZARCTAN

Inverse (arc) tangent function.

## Synopsis

```\$ZARCTAN(n)
```

### Parameter

Argument Description
n Any positive or negative number.

## Description

\$ZARCTAN returns the trigonometric inverse (arc) tangent of n. Possible results range from 1.57079 (half of pi) through zero to –1.57079. The result is given in radians.

## Parameter

### n

Any positive or negative number. It can be specified as a value, a variable, or an expression. You can use the \$ZPI special variable to specify pi. A non-numeric string is evaluated as 0.

The following are arc tangent values returned by \$ZARCTAN:

 2 returns 1.107148717794090502 1 returns .7853981633974483098 0 returns 0 -1 returns -.7853981633974483098

## Examples

The following example permits you to calculate the arc tangent of a number:

```   READ "Input a number: ",num
WRITE !,"the arc tangent is: ",\$ZARCTAN(num)
QUIT```

The following example compares the results from InterSystems IRIS fractional numbers (\$DECIMAL numbers) and \$DOUBLE numbers. In both cases, the arc tangent of pi/2 is a fractional number (not 1), but the arc tangent of 0 is 0:

```  WRITE !,"the arc tangent is: ",\$ZARCTAN(0.0)
WRITE !,"the arc tangent is: ",\$ZARCTAN(\$DOUBLE(0.0))
WRITE !,"the arc tangent is: ",\$ZARCTAN(\$ZPI)
WRITE !,"the arc tangent is: ",\$ZARCTAN(\$DOUBLE(\$ZPI))
WRITE !,"the arc tangent is: ",\$ZARCTAN(\$ZPI/2)
WRITE !,"the arc tangent is: ",\$ZARCTAN(\$DOUBLE(\$ZPI)/2)```