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I am studying Mathematica myself and doing some exercises for improvement.

Question: Try to guess the internal forms of b/c and c/b. Verify your answer using FullForm.

Here is what I got by using FullForm:

FullForm[b/c] 

Times[b,Power[c,-1]] 

FullForm[c/b] 

Times[Power[b,-1],c]

As you can see from the full form of b/c and c/b, the order of two terms in function Times is different. I am wondering why does this happen? What is the purpose of this? For the full form of c/b, why not Times[c,Power[b,-1]]? Just curious, however this doesn't seem to be important.

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Times has the Attribute Orderless:

Attributes[Times]

{Flat, Listable, NumericFunction, OneIdentity, Orderless, Protected}

As the documentation states:

Orderless is an attribute that can be assigned to a symbol f to indicate that the elements ei in expressions of the form f[e1, e2, ...] should automatically be sorted into canonical order. This property is accounted for in pattern matching.

Observe it acting on an arbitrary user head:

Attributes[head] = {Orderless};
head[b, c]
head[c, b]

head[b, c]

head[b, c]

Among other things this helps to put expressions into a canonical form, allowing e.g. head[b, c] == head[c, b] to return True, as it should be for a commutative operator.

Recommended reading:

May also be of interest:

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  • $\begingroup$ Thanks. I still wonder why the form of b/c is sorted (canonical order) but c/b is not? $\endgroup$
    – emnha
    Sep 29 '17 at 8:01
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    $\begingroup$ @anhnha It is sorted according to the canonical ordering function of Mathematica, e.g. Sort[{Power[b, -1], a, c}] outputs {a, 1/b, c}. Sadly that ordering remains poorly documented. $\endgroup$
    – Mr.Wizard
    Sep 29 '17 at 8:07
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    $\begingroup$ Well, in this case, it's simple: What comes first in the alphabet? $\endgroup$
    – halirutan ♦
    Sep 29 '17 at 8:08
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    $\begingroup$ @anhnha Only for simple symbols. You can look up the documentation of sort and find in the details section: "Sort orders symbols by their names, and in the event of a tie, by their contexts." and "Sort usually orders expressions by putting shorter ones first, and then comparing parts in a depth-first manner. " and "Sort treats powers and products specially, ordering them to correspond to terms in a polynomial." As pointed out by Mr.Wizard, things get more involved as soon as you have complex expressions. $\endgroup$
    – halirutan ♦
    Sep 29 '17 at 8:28
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    $\begingroup$ @halirutan You and I have different definitions for nice. ;-p $\endgroup$
    – Mr.Wizard
    Sep 29 '17 at 9:22

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