# IMSL_CONSTANT

The IMSL_CONSTANT function returns the value of various mathematical and physical constants.

Note: This routine requires an IDL Analyst license. For more information, contact your Exelis VIS sales or technical support representative.

The names allowed are listed in the table that follows. Values marked with (mp) are exact (to machine precision). The references in the right-hand column are indicated by code numbers: (1) for Cohen and Taylor (1986), (2) for Liepman (1964), and (3) for precomputed mathematical constants. The supported units are listed below.

Name | Description | Value | Ref.
--- | --- | --- | ---
amu | atomic mass unit | 1.6605655 x 10–27 kg | 1
ATM | standard atm. pressure | 1.01325 x 105 N/m2 (mp) | 2
AU | astronomical unit | 1.496 x 1011 m |
Avogadro | Avogadro’s number, N | 6.022045 x 1023 1/mole | 1
Boltzman | Boltzman’s constant, k | 1.380662 x 10–23 J / K | 1
C | speed of light, c | 2.997924580 x 108 m/s | 1
Catalan | Catalan’s constant | 0.915965... (mp) | 3
E | base of natural logs, e | 2.718... (mp) | 3
ElectronCharge | electron charge, e | 1.6021892 x 10–19 C | 1
ElectronMass | electron mass, me | 9.109534 x 10–31 kg | 1
ElectronVolt | electron volt, ev | 1.6021892 x 10–19 J | 1
Euler | Euler’s constant, γ | 0.577... (mp) | 3
Faraday | Faraday constant, F | 9.648456 x 104 C/mole | 1
FineStructure | fine structure, α | 7.2973506 x 10–3 | 1
Gamma | Euler’s constant, γ | 0.577... (mp) | 3
Gas | gas constant, R0 | 8.31441 J/mole/K | 1
Gravity | gravitational constant, G | 6.6720 x 10–11 N m2 / kg2 | 1
Hbar | Planck’s constant / 2π | 1.0545887 x 10–34 J s | 1
PerfectGasVolume | std. vol. ideal gas | 2.241383 x 10–2 m3 / mole | 1
Pi | Pi, π | 3.141... (mp) | 3
Planck | Planck’s constant, h | 6.626176 x 10–34 J s | 1
ProtonMass | proton mass, Mp | 1.6726485 x 10–27 kg | 1
Rydberg | Rydberg’s constant, Rinfinity | 1.097373177 x 107 /m | 1
Speedlight | speed of light, c | 2.997924580 x 108 m/s | 1
StandardGravity | standard g | 9.80665 m/s2 (mp) | 2
StandardPressure | standard atm. pressure | 1.01325 x 105 N/m2 (mp) | 2
StefanBoltzman | Stefan-Boltzman, σ | 5.67032 x 10–8 W/K4 /m2 | 1
WaterTriple | triple point of water | 2.7316 x 102 K | 2

The units allowed are as follows:

Unit | Description
--- | ---
time | day, hour = hr, min = minute, s = sec = second, year
frequency | Hertz = Hz
mass | AMU, g = gram, lb = pound, ounce = oz, slug
distance | Angstrom, AU, feet = foot, in = inch, m = meter = metre, micron, mile, mill, parsec, yard
area | acre
volume | l = liter = litre
force | dyne, N = Newton
energy | BTU, Erg, J = Joule
work | W = watt
pressure | ATM = atmosphere, bar
temperature | degC = Celsius, degF = Fahrenheit, degK = Kelvin
viscosity | poise, stoke
charge | Abcoulomb, C = Coulomb, statcoulomb
current | A = ampere, abampere, statampere
voltage | Abvolt, V = volt
magnetic induction | T = Tesla, Wb = Weber
other units | l, farad, mole, Gauss, Henry, Maxwell, Ohm

The metric prefixes listed in the next table can be used with the previous units. The one- or two-letter prefixes can only be used with one-letter unit abbreviations.

Prefix | Definition | Value
a | atto | 10–18
f | femto | 10–15
p | pico | 10–12
n | nano | 10–9
u | micro | 10–6
m | milli | 10–3
c | centi | 10–2
d | deci | 10–1
dk | deca | 102
k | kilo | 103
| myria | 104
| mega | 106
g | giga | 109
t | tera | 1012

There is no one-letter unit abbreviation for myria or mega since m means milli.

## Examples

For additional information on using IMSL_CONSTANT, see Additional Examples.

### Example 1

In this example, Euler’s constant γ is obtained and printed. Euler’s constant is defined to be as follows:

PM, IMSL_CONSTANT('gamma')

0.577216

## Syntax

Result = IMSL_CONSTANT(name[, units] [, /DOUBLE])

## Return Value

By default, returns the desired constant. If no value can be computed, NaN (Not a Number) is returned.

## Arguments

### name

Scalar string specifying the name of the desired constant. The case of the characters is not relevant when specifying name, i.e., character strings “PI”, “Pi”, “pI”, and “pi” are equivalent. Spaces and underscores are allowed and ignored.

### units

Scalar string specifying the units of the desired constant. If empty, then Systeme International d’Unites (SI) units are assumed. The case of the characters is not relevant when specifying units, i.e., character strings “METER”, “Meter”, and “meter” are equivalent. Parameter units has the form “U1*U2*...*Um/V1/.../Vn,” where Ui and Vi are the names of basic units or the names of basic units raised to a power. Basic units must be separated by * or /. Powers are indicated by ^, as in “m^2” for m2. Examples are “METER*KILOGRAM/SECOND”, “M*KG/S”, “METER”, or “M/KG^2”.

## Keywords

### DOUBLE

If present and nonzero, double precision is used.

## Errors

### Warning Errors

MATH_MASS_TO_FORCE—Conversion of units-of-mass to units-of-force required for consistency.

## Additional Examples

### Example 2

In this example, the speed of light is obtained using several different units.

c1 = IMSL_CONSTANT('SpeedLight', 'meter/second')

c2 = IMSL_CONSTANT('SpeedLight', 'mile/second')

c3 = IMSL_CONSTANT('SpeedLight', 'cm/ns')

PM, 'speed of light = ', c1, c2, c3, $

Title =' meters/second ' + $

'miles/second cm/ns'

meters/second miles/second cm/ns

speed of light = 2.99792e+008 186282. 29.9792

## Version History

6.4 | Introduced

## See Also