CODEWITHSUNDEEP
idl-programming-languageenvi
Nico
Nico

Reputation: 643

Why IDL have different value in same expression?

I'm doing some image processing with IDL, and it required a high-precision. But when I debug my colleague's program, I find some thing strange:

IDL> lat,y_res
   45.749001
   0.00026999999
IDL> lat - findgen(10)*y_res + y_res * 0.5 + findgen(10)*y_res + y_res * 0.5
   45.749268       45.749268       45.749268       45.749268 ...      45.749268
IDL> lat - (findgen(10)*y_res + y_res * 0.5) + (findgen(10)*y_res + y_res * 0.5)
   45.749001       45.749001       45.749001       45.749001 ...

Just as code above, I don't know why the two results have different value? My IDL version is 8.3 with ENVI package.

Upvotes: 0

Views: 117

Answers (1)

Chris Torrence
Chris Torrence

Reputation: 452

TriskalJM is correct. If you look at your parentheses in the second expression, you are grouping your terms differently. This will always happen with floating-point arithmetic in any computer language, just due to roundoff errors. If you want a more information, you could consult: http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html

In the meantime, I would recommend that you switch to double-precision:

lat - dindgen(10)*y_res + y_res * 0.5 + dindgen(10)*y_res + y_res * 0.5

Upvotes: 2

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