I keep getting the error "subscript must either be real positive integers or logicals" even when I place an 'if' statement

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 would have solved that issue, but it does not seem to be working.

Thank you,

My 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

Answer 1

[Star Strider]

‘I thought that my ' if ' loop would have solved that issue, but it does not seem to be working.’

It is working in the if block, but not afterwards.

The line that throws the error is:

%New car cabin temperature         
Tin(i) = Tin(i-1)+deltaTin(i)

[Jamil Dudhwala]

I can't work out what the error is in that line, what is wrong there?

[Star Strider]

The same as was the problem in your original line. When ‘i’ is 1, the subscript is 0. You likely need another if block around it, or otherwise define:

Tin(1) = 20;

before the loop, and then start your loop at 2 instead of 1:

for i = 2:max(index)
    ...
end

That is likely the easiest way.

For example:

%for loop         
Tin(1) = 20
for i = 2:max(index)
    
    Tin(i) = Tin(i-1)+deltaTin(i)
    
    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

If you do this, you will need to account for the initial values being 0 in ‘SQAmb’, ‘QAmb_Total’, and ‘deltaTin’.

[Jamil Dudhwala]

Thank you, your solution worked athough I found another way which lets me start from i = 1.

[Star Strider]

As always, my pleasure.

Out of curiosity, what is your solution?

[Jamil Dudhwala]

I moved the equations for when i=1 before the else statement and the then when i>1 after the else statement. It was longer but seemed to work.

Revamped 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)); %Preallocation of Ambient Load matrix

QAmb_Total = zeros(size(index)); %Preallocation of Total Ambient Load matrix

deltaTin = zeros(size(index));

Tin = zeros(size(index));

for i=index

if (i)==1

SQAmb(i,:) = S*U*(Tout-20)

%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)=20+deltaTin(i)

else

SQAmb(i,:) = S*U*(Tout-Tin(i-1))

%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

end

[Star Strider]

Good solution!