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Screensize: 640 x 480 800 x 600 1024 x 768 1280 x 1024 UserName: Password: Anti-Spam Code: Format Mode: Basic  Help  Prompt  Format: Font Andale Mono Arial Arial Black Book Antiqua Century Gothic Comic Sans MS Courier New Georgia Impact Lucida Console Script MT Bold Stencil Tahoma Times New Roman Trebuchet MS Verdana   Size 1 2 3 4 5 6   Color Black Red Yellow Pink Green Orange Purple Blue Beige Brown Teal Navy Maroon LimeGreen Forum: Forum for Origin C Subject: Message: * HTML is OFF * Forum Code is ON Smilies Origin Ver. and Service Release (Select Help-->About Origin): 2020b Operating System:win10 64bit Hi, I have some troubles fixing parameters with PWL3 fit function. Here's my code: [code]bool ASP_VO2_fit_PWL3(vector vX, vector vY, vector& vParam, double dXval, int fix) { NLFitSession nlfS; nlfS.SetFunction("PWL3"); nlfS.SetData(vY, vX); // Initialisation vector vParamValues(6); vector<int> vnParamsOffsets(6); // the begin index of one group of parameters, normally one group for one dataset nlfS.ParamsInitValues(); nlfS.GetParamValuesAndOffsets(vParamValues, vnParamsOffsets); if (fix > 0) // if fix xi1 or xi2 { if (fix == 1) // fix xi1 { vParamValues[2] = dXval; nlfS.SetParamFix(2, true); } if (fix == 2) // fix xi2 { vParamValues[4] = dXval; nlfS.SetParamFix(4, true); } nlfS.SetParamValues(vParamValues); } // Fit vector vErr(6); if (nlfS.Fit()) { nlfS.GetFitResultsParams(vParam, vErr); return true; } return false; }[/code] if fix = 0, fitting should be free if fix = 1, I want to fix xi1 (3rd parameter) as dXval if fix = 2, I want to fix xi2 (4th parameter) as dXval When fix > 0, I get different result than fix = 0, but xi1 or xi2 are not fixed to dXval What am I doing wrong ? Bonus question: I tried NLFit Class and got results 10 times faster. But I couldn't find how to fix parameters. Is it possible ?   Check here to subscribe to this topic.