Variable changes to previously calculated value each iteration.

Here is the equation I am dealing with

p2 = p1./(exp((50)./(29.3.*((T)))));

T = 1:340

p1 = 99977

I need to calculate p2 for all T values and have p1 change to the previously calculate p2 value each iteration.

I wrote this:

for i = 1:length(T)
    p2(i) = p1./(exp((50)./(29.3.*((T)))));
    p1 = p2(i)
end

I keep getting an error "Unable to perform assignment because the left and right sides have a different number of elements."

Do I need an embedded for loop to do this and if so, how so?

Answer 1

[Stephen23]

This is very easy with a preallocated vector (giving exactly the values as you stated here):

X = 100:440;
N = numel(X);
Z = nan(1,N);
Z(1) = 99977;
for k = 2:N
    Z(k) = Z(k-1) ./ exp(50./(29.3*(X(1))));
end %                                 ^ only use first X value.

Giving:

>> Z(:)
ans =
   99977.00000
   98285.38250
   96622.38728
   94987.53004
   93380.33470
   91800.33322
   90247.06546
   88720.07910
... lots of values here
   340.39
   334.63
   328.97
   323.41
   317.93
   312.55
   307.27
   302.07

But if you really want to use all of those X values (and not ignore them):

X = 100:440;
N = numel(X);
V = nan(1,N);
V(1) = 99977;
for k = 2:N
    V(k) = V(k-1) ./ exp(50./(29.3*(X(k))));
end %                                 ^ use each X value.

Giving:

>> V(:)
ans =
   99977.00000
   98301.99009
   96671.05506
   95082.62195
   93535.18842
   92027.31890
   90557.64106
... lots of values here
   8285.1
   8252.5
   8220.1
   8188.0
   8156.0
   8124.2
   8092.6
   8061.2
   8030.0

Answer 2

[Erivelton Gualter]

Since T is an array and you are using a for loop to find each value of p2(T), you should you T(i) instead of T.

Check the correct code:

clear all

T = 1:340;
p1 = 99977;

for i = 1:length(T)
    p2(i) = p1/(exp(50/(29.3*T(i))));
    p1 = p2(i);
end

plot(T, p2);

I am also plotting the P2 vs T for further verification.

[Adam Kevin Francis Baker]

Does this still work if T = rand(1,340)?

[Erivelton Gualter]

Hi Adam,

It does work as well.

[Adam Kevin Francis Baker]

I think I asked this question wrong. I submitted another question to ask what I really needed and can't figure out how to delete this question. I think you answered the question I asked. I meant to ask this question.

https://www.mathworks.com/matlabcentral/answers/460184-use-a-loop-to-repeat-an-equation-using-the-previous-answer-as-the-new-variable

I would like to thank you for your time and effort...sorry I'm not proficient in matlab/the matlab website.