I want to turn a sum like this
sum =a-b+c+d
Into a List like this:
sumToList[sum]={a,-b,c,d}
How can I achieve this?
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2$\begingroup$ I am a professor of "vectorial analysis" in Mexico. I try to teach the subjects with the use of "Mathematica". I solved a problem seeing this page and also this: mathematica.stackexchange.com/questions/100758/… ... in which I was not allowed to comment. This was what I did to find the "scalar potential function". i.sstatic.net/ugIlP.png $\endgroup$– David GardeaCommented Feb 6, 2017 at 17:43
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$\begingroup$ @David I converted your answer to a comment, as it seems more appropriate as such. I know that new users do not have the "privilege" to comment everywhere (a "spam" control measure I believe) but if you continue to participate you will find that you soon do. $\endgroup$– Mr.WizardCommented Feb 6, 2017 at 19:12
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3 Answers
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List @@ sum
{a, -b, c, d}
From the docs on Apply (@@):
f@@expr replaces the head of expr by f.
So List@@sum replaces Head[sum] (that is, Plus) with List.
You can also get the same result by changing 0th Part of sum (which is its Head) to List:
sum[[0]] = List; sum
{a, -b, c, d}
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2$\begingroup$ There is a slight hiccup that you can encounter using this. If you're not explicitly doing a sum, you will list over whatever the head of the function is. e.g.
List @@ 2*3will result in{2,3}$\endgroup$– Alex DBCommented Feb 10, 2017 at 17:10 -
$\begingroup$ @AlexDB, you are right;
List@@exprdoes change the head ofexpr, whatever it is, toList. $\endgroup$– kglrCommented Feb 10, 2017 at 17:16 -
2$\begingroup$ So what's the most general way to make a list, so that we include both distinct cases where there is one and multiple terms? $\endgroup$– halCommented Aug 7, 2020 at 18:30
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Try :
a - b + c + d /. Plus -> List
You can have a look at a - b + c + d //FullForm to see why this works.
answered Oct 22, 2012 at 10:27
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3$\begingroup$ Good one. And maybe
sum[[0]] = List; sum$\endgroup$– user1066Commented Oct 22, 2012 at 19:32 -
1$\begingroup$ @TomD that is the one I like to use :) $\endgroup$ Commented Oct 22, 2012 at 22:24
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Still another route:
Last[CoefficientArrays[#]] Variables[#] &[a - b + c + d]
{a, -b, c, d}
answered Oct 22, 2012 at 11:25
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