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Is there a function which applied to a quantity returns its numerical value so I for instance can fill it into my Predictorfunction?

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  • $\begingroup$ you maybe want to accept an answer to this $\endgroup$
    – george2079
    Commented Oct 20, 2017 at 16:17

4 Answers 4

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Oops - found it! QuantityMagnitude[quantity] does the job. For example,

In[1]:= QuantityMagnitude[Quantity[1, "Feet"]]

Out[1]= 1
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    $\begingroup$ If you want to remove the units in an expression regardless of what level that unit resides on, you can tack /. Quantity :> Composition[QuantityMagnitude, Quantity] (/. Quantity :> QuantityMagnitude@*Quantity in version 10) or /. Quantity[x_, y_] :> x to the end of your expression. Examples. $\endgroup$
    – khanhnguyendata
    Commented Aug 4, 2014 at 18:18
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As already mentioned, the function you're looking for is QuantityMagnitude. However, I think single argument QuantityMagnitude is error prone. It is much better to give it a second argument:

QuantityMagnitude[Quantity[1, "Feet"]]
QuantityMagnitude[Quantity[1, "Feet"], "Meters"]
QuantityMagnitude[Quantity[1, "Feet"], "Seconds"]

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381/1250

Quantity::compat: Feet and Seconds are incompatible units

QuantityMagnitude[Quantity[1, "Feet"], "Seconds"]

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To temporary disable Quantity for all of your functions,

SetSystemOptions["DataOptions" -> "ReturnQuantities" -> False];

In[5]:= DateDifference[DateList[{2017, 11, 11}], DateList[]]
Out[5]= Quantity[-21.5912, "Days"]
In[6]:= oldSystemOptions = SystemOptions["DataOptions"]; 
        SetSystemOptions["DataOptions" -> "ReturnQuantities" -> False];
In[7]:= DateDifference[DateList[{2017, 11, 11}], DateList[]]
Out[7]= -21.5908

This is a neat trick to use. This works on Mathematica 10 & 11. Or simply use First.

In[12]:= First@DateDifference[DateList[{2017, 11, 11}], DateList[]]
Out[12]= -21.5869
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  • $\begingroup$ First@Quantity[5,"Meters"] and Quantity[5,"Meters"] // First are the same thing. It works on Mathematica 9, 10, 11. $\endgroup$
    – Ku Zijie
    Commented Oct 20, 2017 at 20:36
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First does work — try use // First after the expression to apply the First function such as below. But QuantityMagnitude is more proper so you can use either. First is for arrays and lists.

Quantity[5,"Meters"] // First

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Quantity[5,"Meters"] // QuantityMagnitude

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