| Re: How do you access a field in a struct? [message #18322 is a reply to message #18321] |
Wed, 15 December 1999 00:00   |
m218003
Messages: 56
Registered: August 1999
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Member |
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In article <38573DF7.39E554F9@twins.dna.fi>,
Karri Kaksonen <karri@twins.dna.fi> writes:
> Hi,
>
> I have a small problem with accessing my structs.
>
> The complete program (that does not work) looks like:
> pro beer
> a = { $
> beer: 1, $
> icecream: 2 $
> }
>
> b = { $
> milk: 0.60, $
> beer: 1.25, $
> icecream: 0.75, $
> tea: 0.50 $
> }
>
> totalsum = 0.0
> selection = tag_names(a)
> for i=0,n_tags(a)-1 do totalsum=totalsum + a.(i) * b.(selection)
> print, totalsum
> end
>
> When I run this program I get:
> IDL> beer
> % Type conversion error: Unable to convert given STRING to Long.
> % Detected at: BEER 16 C:\RSI\IDL52\mri\beer.pro
> % Type conversion error: Unable to convert given STRING to Long.
> % Detected at: BEER 16 C:\RSI\IDL52\mri\beer.pro
> 1.80000
>
> The funny thing is that it obviously replaced "beer" and "icecream"
> with "milk" as it could not understand that b.('beer') is the same
> as b.beer
>
Sure enough, IDL complains. Insert a
HELP, selection
statement, and you'll see that selection is of type string.
The "tag selector" b.(i) requirs an integer, however. The solution to your
problem is the WHERE function. The core statement is
bind = WHERE(tag_names(b) eq selection[i])
However, for reasons of efficiency and user-tampering-protection, the code
would rather look like:
totalsum = 0.0
selection = tag_names(a)
bnames = tag_names(b)
for i=0,n_tags(a)-1 do begin
bind = WHERE(bnames eq selection[i],count)
if (count eq 1) then begin
totalsum=totalsum + a.(i) * b.(bind[0])
endif else begin
print,'No matching tag in b for ',selection[i]
endelse
endfor
print, totalsum
BTW: the answer is 2.75 ;-)
Cheers,
Martin
--
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[[ Bundesstr. 55, 20146 Hamburg [[
[[ phone: +49 40 41173-308 [[
[[ fax: +49 40 41173-298 [[
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