Fit function with use of a structure

// The structure definition
Structure expFitStruct
    Wave coefw // required coefficient wave
    Variable x  // required X value input
    Variable x0 // constant
EndStructure
 
    
// The fitting function
Function fitExpUsingStruct(s) : FitFunc
    Struct expFitStruct &s
    return s.coefw[0] + s.coefw[1]*exp(-(s.x-s.x0)/s.coefw[2])
End
// The driver function that calls FuncFit:
Function expStructFitDriver(pw, yw, xw, xOff)
    Wave pw // coefficient wave- pre-load it with initial guess Wave yw
    Wave xw
    Wave yw
    Variable xOff
    
    Variable doODR
    // An instance of the structure. We initialize the x0 constant only, // Igor (FuncFit) will initialize coefw and x as required.
    STRUCT expFitStruct fs
    fs.x0 = xOff // set the value of the X offset in the structure
    FuncFit fitExpUsingStruct, pw, yw /X=xw /D /STRC=fs
    // no history report for structure fit functions. We print our own // simple report here:
    print pw
    Wave W_sigma
    print W_sigma
End

Make/D/O/N=100 expData,expDataX
expDataX = enoise(0.5)+100.5
expData = 1.5+2*exp(-(expDataX-100)/0.2) + gnoise(.05)
Display expData vs expDataX
ModifyGraph mode=3,marker=8
    
Make/D/O expStructCoefs = {1.5, 2, .2}
expStructFitDriver(expStructCoefs, expData, expDataX, 100)