I keep getting undefined function when preallocating

Hi,

I am trying to create a for loop wherein the temperature that is outputted is then subbed back in, I have preallocated but i get the error undefined function '(index)' when it comes to the line: S1QAmb(index)=zeros(size(index)) %surface 1, ambient load. Since the size of the array will change with the size of the index, I did not think the (index) needed to be defined. Also, if any other mistakes are spotted, please do let me know. Thank you.

My script is below:

U=1

S1=1.7*1.2 %Surface area of surface 1

S2= 1.7*1.2 %Surface 2 area

S3= 1.7*1.2 %surface 3 area

S4= 1.7*1.2 %surface 4 area

S5= 1.7*1.2 %surface 5 area

S6= 1.7*1.2 %surface 6 area

S7= 1.7*1.2 %surface 7 area

S8= 1.7*1.2 %surface 8 area

S9= 1.7*1.2 %surface 9 area

S10= 1.7*1.2 %surface 10 area

Tout=5

ma=1

ca=2

DTM=1

deltat=1

%Prealloacate

S1QAmb(index)=zeros(size(index)) %surface 1, ambient load

S2QAmb(index)=zeros(size(index)) %surface 2, ambient load

S3QAmb(index)=zeros(size(index)) %surface 3, ambient load

S4QAmb(index)=zeros(size(index)) %surface 4, ambient load

S5QAmb(index)=zeros(size(index)) %surface 5, ambient load

S6QAmb(index)=zeros(size(index)) %surface 6, ambient load

S7QAmb(index)=zeros(size(index)) %surface 7, ambient load

S8QAmb(index)=zeros(size(index)) %surface 8, ambient load

S9QAmb(index)=zeros(size(index)) %surface 9, ambient load

S10QAmb(index)=zeros(size(index)) %surface 10, ambient load

QAmb_Total(index)=zeros(size(index)) %Total Ambient Load

deltaTin(index)=zeros(size(index))

Tin(index)=zeros(size(index))

for index=(1:30)

if (index)==1

Tin(index)=20

else

Tin(index)=Tin(index-1)+deltaTin(index)

end

%Ambient Load

S1QAmb(index)=S1*U*(Tout-+Tin(index)) %surface 1, ambient load

S2QAmb(index)=S2*U*(Tout-+Tin(index)) %surface 2, ambient load

S3QAmb(index)=S3*U*(Tout-+Tin(index)) %surface 3, ambient load

S4QAmb(index)=S4*U*(Tout-+Tin(index)) %surface 4, ambient load

S5QAmb(index)=S5*U*(Tout-+Tin(index)) %surface 5, ambient load

S6QAmb(index)=S6*U*(Tout-+Tin(index)) %surface 6, ambient load

S7QAmb(index)=S7*U*(Tout-+Tin(index)) %surface 7, ambient load

S8QAmb(index)=S8*U*(Tout-+Tin(index)) %surface 8, ambient load

S9QAmb(index)=S9*U*(Tout-+Tin(index)) %surface 9, ambient load

S10QAmb(index)=S10*U*(Tout-+Tin(index)) %surface 10, ambient load

%Total Ambient Load in Array Form

QAmb_Total(index)=S1QAmb(index)+S2QAmb(index)+S3QAmb(index)+S4QAmb(index)+S5QAmb(index)+S6Amb(index)+S7QAmb(index)+S8QAmb(index)+S9QAmb(index)+S10QAmb(index)

%changein air cabin temperature where deltat is timestep

deltaTin(index)=(QAmb_Total(index)/((ma*ca)+DTM))*(deltat)

%New car cabin temperature

Tin(index)=Tin(index-1)+deltaTin(index)

end

Answer 1

[Thorsten]

index = 1:30;
% Preallocate
S1QAmb = zeros(size(index)); % index has to be defined before it can be used
for i = index
    S1QAmb(i) = S1*U*(Tout+Tin(i));     % you wrote -+; that's invalid syntax
    % and so on
end

You should also consider to use an array for S

S = 1.7*1.2*ones(1, 10);

and compute all S values in one line in the for loop

SQAmb(i,:) = S*U*(Tout+Tin(i)); 

with SQAmb preallocated as a matrix

SQAmb = zeros(numel(index), numel(S));

[Jamil Dudhwala]

Thank you,

Could i use 'i' instead of 'index'? the only reason I have been using index is because matlab uses i as a notation form for an imaginary number.

As for the S values, they're actually all supposed to be different, but i kept them the same as I was just testing to see if the code worked, would I still be better off doing what you said even if the values were different?

[Thorsten]

I used index to store all indices such that you can pre-allocate using size(index). And than you need another variable that runs through all indices. I used i. You can always get the imaginary unit as 1i or 30i or whatever imaginary value you need.

Yes, it should work also for different values. That one of Matlab's greated strength that you can vectorize your code.

[Steven Lord]

Thorsten's suggestion to avoid tucking an index into the variable name is a good one, and is the generally recommended best practice on Answers.

As an aside, this isn't technically correct.

% you wrote -+; that's invalid syntax

Writing something like "1-+2" is valid. It is perhaps confusing to read, but it is valid.

The unary plus operator has higher precedence (level 4 unless raising to a power is involved, in which case it's level 3) than the subtraction operator (level 6), so this is equivalent to "1-(+2)" which is 1-2 or -1. That's the same reason "1--1" returns 2 -- unary plus and unary minus are at the same precedence level.

[Jamil Dudhwala]

Sorry to bother you guys, but it seems I keep getting the error "subscript must either be real positive integers or logicals".

This must be because I have started of from i = 1, hence i - 1 = 0 and thats where the problem is. I thought that my ' if ' loop woould have sold that issue, but it does not seem to be working.

Thank you,

My revised script is below:

U=1

S = 1.7*1.2*ones(1, 10);

Tout = 5

ma = 1

ca = 2

DTM = 1

deltat = 1

index = 1:30;

%Prealloacate

SQAmb = zeros(numel(index), numel(S));

QAmb_Total = zeros(size(index));

deltaTin = zeros(size(index));

Tin = zeros(size(index));

%for loop

for i = index

if (i)==1

Tin(i) = 20

else

Tin(i) = Tin(i-1)+deltaTin(i)

end

SQAmb(i,:) = S*U*(Tout-Tin(i));

%Total Ambient Load in Array Form

QAmb_Total(i) = sum(SQAmb(i,:))

%change in air cabin temperature where deltat is timestep

deltaTin(i) = (QAmb_Total(i)/((ma*ca)+DTM))*(deltat)

%New car cabin temperature

Tin(i) = Tin(i-1)+deltaTin(i)

end

[Thorsten]

The if clause is fine. The problem occurs in the last but one line. You need to handle the case i == 1 also for this line.