# Numeric Quantities

NumberQ[expr] | test whether expr is explicitly a number |

NumericQ[expr] | test whether expr has a numerical value |

Testing for numeric quantities.

Pi is a symbol, so

Pi+3 is not explicitly a number.

Out[1]= | |

It does however have a numerical value.

Out[2]= | |

This finds the explicit numerical value of

Pi+3.

Out[3]= | |

*Mathematica* knows that constants such as Pi are numeric quantities. It also knows that standard mathematical functions such as Log and Sin have numerical values when their arguments are numerical.

Log[2+x] contains

, and is therefore not a numeric quantity.

Out[4]= | |

Many functions implicitly use the numerical values of numeric quantities.

Out[5]= | |

In general, *Mathematica* assumes that any function which has the attribute NumericFunction will yield numerical values when its arguments are numerical. All standard mathematical functions in *Mathematica* already have this attribute. But when you define your own functions, you can explicitly set the attribute to tell *Mathematica* to assume that these functions will have numerical values when their arguments are numerical.