Unexpected results using CEL function mod

Lance · August 12, 2015, 02:55

Hi, I just want to highlight some unexpected results I got from the CEL function mod(a,b). For example mod(0.3,0.05) does not return 0 as it should, but instead 0.05. Integers like mod(15,5) give the correct result. I also tested mod(0.15,0.05) in excel, which also gave 0.05. After doubting my math skills I tested it in Matlab, where the result was indeed 0.

So, what’s going on? It probably has to do with how CFX store decimal numbers, and that they cannot be not represented exactly. Solution? Do not use mod with decimal numbers. In my case it resulted in totally unexpected results which were difficult to debug. Maybe this is commonly known, but it was news to me.

Also, see the following link for a discussion on mod and excel.
http://answers.microsoft.com/en-us/o...d-087475cb7208

ghorrocks · August 12, 2015, 05:52

Sounds like a bug. Have you reported it to ANSYS?

Lance · August 13, 2015, 01:33

Yes, and a developer have now confirmed that it is a defect.

August 12, 2015, 02:55	Unexpected results using CEL function mod	#1
Lance Senior Member Lance Join Date: Mar 2009 Posts: 669 Rep Power: 22	Hi, I just want to highlight some unexpected results I got from the CEL function mod(a,b). For example mod(0.3,0.05) does not return 0 as it should, but instead 0.05. Integers like mod(15,5) give the correct result. I also tested mod(0.15,0.05) in excel, which also gave 0.05. After doubting my math skills I tested it in Matlab, where the result was indeed 0. So, what’s going on? It probably has to do with how CFX store decimal numbers, and that they cannot be not represented exactly. Solution? Do not use mod with decimal numbers. In my case it resulted in totally unexpected results which were difficult to debug. Maybe this is commonly known, but it was news to me. Also, see the following link for a discussion on mod and excel. http://answers.microsoft.com/en-us/o...d-087475cb7208

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August 12, 2015, 05:52		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Sounds like a bug. Have you reported it to ANSYS?

August 13, 2015, 01:33		#3
Lance Senior Member Lance Join Date: Mar 2009 Posts: 669 Rep Power: 22	Yes, and a developer have now confirmed that it is a defect.