How to function a sum

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Andre Luis
Andre Luis 2013-2-23
Hello. I need to create a function, say Req, that is made by summing a function defined by:
function R = resist(T)
R = rand_R0(100,1000).*exp((rand_T0(12,17)./T).^(1/4));
and then
Req = 1./(1./resist(T)+1./resist(T)+...)
How can I do this recursively? I need to sum 1000 terms. Actually I need to plot Req versus T.
Thank you in advance.
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Andre Luis
Andre Luis 2013-2-24
Walter, I really appreciate your help. Like I said in the comment below, I'm trying to write a function Req that is made by summing the function R which is function of T (so Req is also function of T). It is a problem of parallel resistor and they depend on T (temperature). My functions are
function R = resist(T)
R = rand_R0(100,1000).*exp((rand_T0(12,17)./T).^(1/4));
and
function R0 = rand_R0(R01,R02)
RR0 = R01 + (R02-R01).*rand(1);
R0 = ceil(RR0);
(the same for T0)
And then, plot Req(T) versus T. Do you have any idea how vectorize it?

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回答（2 个）

Muthu Annamalai
Muthu Annamalai 2013-2-23
编辑：Muthu Annamalai 2013-2-23
You can try something like,
function val = Req( fhandle, arg, N )
val = 1;
for itr = N:-1:1 %build from smallest value term upwards to term 1.
val = fhandle(arg,itr) + 1/val;
end
end
While a recursive solution would work most of the times, since MATLAB has a limited stack size, and doesn't convert a tail-recursive call to an iterative solution, you want to write out the iteration explicitly.
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Andre Luis
Andre Luis 2013-2-24
Sorry, but what is fhandle? My resist function depends only on T.

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Youssef Khmou
Youssef Khmou 2013-2-24
hi Andre,
are you trying to write a program that sums Parallel resistors , well you did nt define the whole parameters such rand_R0 and rand_T0 ,
You can use one single function with nested Req function in :
try :
%-----------------------------------------------------------
function Req=Sum_resistors(N)
for T=1:N
Req(T) =1./(1./resist(T));
end
T=1:N;
figure, plot(T,Req), title(' Your funcrion : Resistors vs some Physical param')
function R = resist(T)
R = rand(1).*exp((rand(1)./T).^(1/4));
return
%--------------------------------------------------------------------
I adjusted Rand_R0 and Rand_To so as Req returns a scalar .
You said that you want plot Req vs T, summing wil only return a scalar, i suggest to use cumulative summation:
>>R =@(T) rand(1).*exp((rand(1)./T).^(1/4));
>>plot(cumsum(Req(1:1000)))
>>plot(Req(1:1000))
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Youssef Khmou
Youssef Khmou 2013-2-24
Andre, ok, So you want verify if the serie converges when N->Inf ? i think cumsum will not show you the convergence, lets try a simple prototype :
>>R=50; %50Ohm with identical elements then :
>>Req=R./(1:1000);
>>plot(Req) % It plots directly how the total resistance is when we %increment them 1:N : Decreasing function.
>>figure,plot(cumsum(Req)) % Log like function
which one seems logical to your system ?
Andre Luis
Andre Luis 2013-2-24
Actually, what I'm trying to do is write a function Req that is made by summing the function R which is function of T (so Req is also function of T). Then plot Req versus T.

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