| confused multiplying arrays [message #73482] |
Thu, 11 November 2010 13:11  |
paulartcoelho
Messages: 30
Registered: March 2007
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Member |
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hello,
i've got two arrays:
IDL> help,area,fluxes
AREA FLOAT = Array[2000]
FLUXES FLOAT = Array[2000, 112864]
I want the i-th element in area AREA[i] to multiply all elements in
the corresponding 2nd dimension in fluxes FLUXES[i,*] and then sum up
all the 2000 resulting fluxes, i.e., using loop:
for i = 0,1999 do begin
TOTAL_FLUX = TOTAL_FLUX + AREA[i]*reform(FLUXES[i,*])
endfor
i've been trying to figure out (without success) how to compute
TOTAL_FLUX without the loop using TOTAL(). ideas?
thx
p.
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| Re: confused multiplying arrays [message #73550 is a reply to message #73482] |
Fri, 12 November 2010 08:21  |
penteado
Messages: 866
Registered: February 2018
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Senior Member
Administrator |
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On Nov 12, 2:10 pm, Paula <paulartcoe...@gmail.com> wrote:
> living and learning... :)
> thanks for the tips about the matrix multiplications operators, they
> were new to me.
That is indeed the best option. I should have noticed that array
operation was equivalent to a product.
By the way, for cases when operands must be transposed before the
product, there is the matrix_multiply() function, which has keywords
to specify that one or both operands must be transposed, to save the
memory and time that would be consumed by transposing large things.
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| Re: confused multiplying arrays [message #73558 is a reply to message #73482] |
Fri, 12 November 2010 06:30 |
David Gell
Messages: 29
Registered: January 2009
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Junior Member |
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On Nov 11, 3:11 pm, Paula <paulartcoe...@gmail.com> wrote:
> hello,
>
> i've got two arrays:
>
> IDL> help,area,fluxes
> AREA FLOAT = Array[2000]
> FLUXES FLOAT = Array[2000, 112864]
>
> I want the i-th element in area AREA[i] to multiply all elements in
> the corresponding 2nd dimension in fluxes FLUXES[i,*] and then sum up
> all the 2000 resulting fluxes, i.e., using loop:
>
> for i = 0,1999 do begin
> TOTAL_FLUX = TOTAL_FLUX + AREA[i]*reform(FLUXES[i,*])
> endfor
>
> i've been trying to figure out (without success) how to compute
> TOTAL_FLUX without the loop using TOTAL(). ideas?
>
> thx
> p.
Use the matrix multiply operator, TOTAL_FLUX=TOTAL(AREA # FLUXES).
IDL can be a little confusing about matrix multiplication. It has two
operators # and ## that perform the operation.
A#B = B##A.
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