Best method to define a linear piece-wise function given limits and line equations?

2012 1 2
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I am writing a function(also new to matlab) that is fed the following:
1. the x limits of each piece
2. the slope and intercept values of each piece
3. the y values for which I am trying to find corresponding x values
How can I write a function that can take any number of pieces, and create a piece-wise function? I then would like to be able to feed it y values and return the corresponding x values. I currently can only write the function for a given number of pieces, not any number..?
My code thus far:
%%define the piecewise function
function x = peicewisehot(y,Hbreaks,slope)
%y; input from user
%Hbreaks; vertical breaks
%slope; slope of equation
%x = (y-b)/m
for i = 1:length(y)
if y(i) >= Hbreaks(1) & y(i) < Hbreaks(2)
x(i) = (y(i) - b(1))./slope(1);
elseif y(i) >= Hbreaks(2) & y(i) < Hbreaks(3)
x(i) = (y(i) - b(2))./slope(2);
else
x(i) = nan;
end
end

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Walter Roberson
Walter Roberson 2012-1-2

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David Hagan
David Hagan 2012-1-3
Thank you very much. I tried doing this and using interp1 seems to work great, however I am having issues using ppval to evaluate points. I am getting an error that says: "The input array does not seem to describe a pp function." The following is my code:
load HXData
B= unique(sort(TH(:)));
numsecB = size(B) - 1;
slopeB = zeros(1,numsecB);
s = [length(TC) length(TH)];
for i=1:numsecB
tester = (B(i)+B(i+1))/2;
for j=1:s(2)
if (tester >= TH(j,2) & tester <= TH(j,1))
checkB(i,j) = 1;
slopeB(i) = slopeB(i)+ KH(j);
else
checkB(i,j) = 0;
end
end
end
%Calculates the total enthalpy needed for each stream
Hhot=entHC1(B,slopeB); %enthalpy needed for each hot stream section
Hhotpt(1)=0; %calculates the y value of the hot stream at each point
for i=2:(length(Hhot)+1)
Hhotpt(i) = (Hhot(i-1)+Hhotpt(i-1));
end
u = linspace(B(1), B(end), 1000)';
[k,k] = histc(u,B);
n = length(B);
k(k == n) = n-1;
yinterp1 = interp1(B,Hhotpt,u, 'linear')';
I also was wondering if this is how i would correctly evaluate a point on the piecewise curve:
check = ppval(yinterp1, 40);
Walter Roberson
Walter Roberson 2012-1-3
yinterp1 = interp1(B, Hhotpt, 'linear', 'pp');
Notice the "u" is not passed, as you are not interpolating at specific points when you create the pp.

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