I was not able to use a self defined (using Module and For loops) piecewise function to fit data. It seems that the problem is that I have to define a pure function. I would like to learn how to define pure functions in the following example using Modules and For loops. Please help.

So let's consider a problem - we have some data, and a self defined piecewise function f[z] that goes exactly through all the data points. LinearModelFit fitting data with f[z] should return value parameter equal to 1.

Disclaimer: it will return a constant instead (basically will not understand what is the f[z] provided and will use a constant to fit the data).

However, in the second part of the following example I will define a function g[z] (which will be equal to f[z]) using Piecewise[] and it will work perfectly (LinearModeFit will return 1).

So, once again, I would like to write f[z] using Module and For loops so that it works in LinearModelFit the way g[z] does.

x = Table[x + Random[], {x, 1, 5}]; y = 2 # + Random[] & /@ x; data = Transpose@{x, y}; 

Part 1: self defined function f[z]

fpiecewise[x_, xlist_, ylist_] := Module[{i = 1}, For[i = 1, i < Length[xlist], i++, If[x <= xlist[[i]], Break[]]]; Return[ylist[[i]]] ]; f[z_] := fpiecewise[z, x, y]; LinearModelFit[data, f[z], z]; ListPlot[{{x, % /@ x} // Transpose, data}, Joined -> True] 

Part 2: Piecewise[] defined function g[z]

x1 = Join[{0.}, x]; g[z_] := Piecewise[ Table[{y[[i]], x1[[i]] < z <= x1[[i + 1]]}, {i, 1, Length[x]}]] LinearModelFit[data, g[z], z]; ListPlot[{{x, % /@ x} // Transpose, data}, Joined -> True]