Reputation
1,615
Top tag
Next privilege 2,000 Rep.
Edit questions and answers
Badges
7 28
Impact
~37k people reached

  • 0 posts edited
  • 0 helpful flags
  • 102 votes cast
May
17
accepted Two questions about drawing tool
May
17
accepted why set values in this way doesn't work?
May
7
accepted why there is a small imaginary part
May
7
comment why there is a small imaginary part
@bills thank you bill s. OK, I understand.
May
7
comment why there is a small imaginary part
@bills it's not the problem of calculating eigenvalues.
May
7
asked why there is a small imaginary part
May
6
comment direct assignment to the element in a Table
But why boost2 doesn't have to be set to HoldAll?Of course, the code is more complicated than holdtable. It is a upsetdelayed and there is pattern match. But I didn't find the key to explain why the red in the definition of boost2 is valid. I found that if I don't let func to be Set when using boost2, for example, I write Join[boost2[test[1],tmp],{111,222}] then the tmp passed to Block has been evaluated. This confused me a lot. I seems boost2 is not perfect, right?
May
6
comment direct assignment to the element in a Table
Thank you Mr.Wizard! I maybe a little anoying. But I have to make things clear, otherwise I will be very uncomfortable. Now I see I have to set HoldAll attribute to tobeheld, because otherwise it will be evaluated before being passed to Block. And I think that's why MMA make tobeheld red, the red is quite reasonable to prevent error.
May
5
revised A matrix-vector cross product
added 274 characters in body
May
5
comment A matrix-vector cross product
Ok, rule replacement works. But I still don't know where does it gone wrong? I looked into the Trace result, but it is unexpectedly long! I can't figure out why.
May
5
comment Confused about Unevaluated
I think I understand. Clear explanation, Thank you very much!
May
5
comment A matrix-vector cross product
OhOh, Sorry J.M. I found peculier things. Your solution is flawed. Try Cross[Unevaluated/@PauliMatrix[Range[3]],{0,1,0}]
May
5
comment Confused about Unevaluated
What does inert mean? Are there any other head that are inert too?
May
5
comment Confused about Unevaluated
Well, you mean the result of evaluating the head Unevaluated is just strip it, right?
May
5
comment A matrix-vector cross product
I finally choose your answer. easier and won't have to worry about the order and negtive sign when using LeviCivitaTensor
May
5
comment Confused about Unevaluated
yeah! I notice it from Trace. Mathematica just simply strip Unevaluated away in the Table in the first step. But why?
May
5
comment Confused about Unevaluated
@m_goldberg thank you for recommending the very good book.
May
5
comment Confused about Unevaluated
@MarkMcClure thank you very much
May
5
comment Confused about Unevaluated
@MarkMcClure You're telling me that the step "then evaluate it" in Map and the step 'substitutes values ....explicitly evaluates the first argument at those values' in Table is two kind of evaluate?!! I still don't understand, Now that Table has the attributes HoldAll, it should hold the Unevaluated. It seems that "expr, shift+Enter" and "Evaluate[expr]" is different ? And Map use the first one Table use the second?
May
5
comment Confused about Unevaluated
@MarkMcClure But according to the docs of Map. Map always effectively constructs a complete new expression and then evaluates it. And use Trace, I found in the last three steps, mathematica actually remove the Unevaluated, and finally bring back the Unevaluated head, why?