I need help with fixing the error in my for loop equation

Please how do I fix the error message: Attempted to access ID(3); index out of bounds because numel(ID)=2.

I am trying to get a sum of all the iteration (H) from the code below: The error is somewhere in the formula for H but I am not able to fix it.

   i=              input('Number of Layers = ');     % Say i=4
  for n=1:i
   ID(n)=          input(['Layer ' , num2str(n) , ' Internal Diameter (m)= ']);
   OD(n)=          input(['Layer ' , num2str(n) , ' Outer Diameter (m)= ']);
   k_L(n)=         input(['Thermal Conductivity of Layer ' , num2str(i) , '(W/(m*K))=  ']);
   *H=   ID(1)*sum((log(ID(n+1)/ID(n))/(2*k_L(n))))* ;  
                                               %D(1) is the innermost pipe layer which is ID at n=1
  end

The formula for H is given below.

Answer 1

[Image Analyst]

When you're on the n'th iteration of your loop, why do you think ID(n+1) will have a value yet? You have not assigned a value to that element yet - it's the next one. You won't get the user to specify a value for it until the next iteration.

[Image Analyst]

If this is homework, make sure you "Tag" it as homework above.

What does the sum go over? If i (which would be better named numberOfLayers) is 4, then does n in the sum go from 1 to 3, so that ID(n+1) will finish at ID(4) and not ID(5) which does not exist?

Alright, let's get you a little closer. Try this code:

numberOfLayers = input('Number of Layers = ');     % Say i=4
for n = 1 : numberOfLayers
  % Create prompts
  prompt1 = sprintf('Layer #%d Internal Diameter (m) = ', n);
  prompt2 = sprintf('Layer #%d  Outer Diameter (m) = ', n);
  prompt3 = sprintf('Thermal Conductivity of Layer %d (W/(m*K)) =  ', n);

  % Ask user for three floating point numbers.
  defaultValue = {'1', '2', '3'};
  titleBar = 'Enter the values';
  userPrompt = {prompt1, prompt2, prompt3};
  caUserInput = inputdlg(userPrompt, titleBar, 1, defaultValue);
  if isempty(caUserInput),break,end; % Bail out if they clicked Cancel.
  % Convert to floating point from string.
  ID(n) = str2double(caUserInput{1})
  OD(n) = str2double(caUserInput{2})  % Not used!!!!!!!
  k_L(n) = str2double(caUserInput{3})
end
% Compute H
ratios = ID(2:numberOfLayers) ./ ID(1 : (numberOfLayers-1))
theFormula = log(ratios) ./ (2*k_L(n))
% Not sure the formula for H is correct since
% it refers to ID*, not ID(1)*, but anyway....
H = ID(1) * theFormula
% ID(1) is the innermost pipe layer which is ID at n=1

I was wondering why your k_L always asked for the value for the last layer - the i'th layer, instead of the n'th layer. Is this just a typo because you did not use descriptive enough variable names? I also don't quite understand that H seems to want the whole ID array multiplied by the sum, which would give another array, rather than ID(1) multiplied by the sum, which is what you did in your code.

[Fun Dan]

Thank you very much. It worked great!

[Image Analyst]

You're welcome. If we're done here, then can you go ahead and mark my answer as "Accepted"?

Answer 2

[Jos (10584)]

You want to put the formula for H out of the loop

for ..
  % ask for values here
end
% summing formula here

[Fun Dan]

Hi Jos,

I tried it that way already and it is still giving me the same error message.