How to create an empty array to be filled?

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Hi everyone,
I have this:
Ma=[];
d=yq1-yq1;
A=pi*(d.*d)/4;
mdot=(A.*Ma*P0*sqrt(k/(R*T0)))/(1+(k-1)*Ma^2/2)^((k+1)/(2*(k-1)));
(I highlighted important part)
A is a 1x100 array and I need Ma is to be 1x100 vector too. But I got this error:
Error using .*
Matrix dimensions must agree.
How can I create an empty 1x100 Ma array which will be calculated with the equation above?
All helps are appriciated!

Accepted Answer

Steven Lord
Steven Lord on 18 May 2020
What value or values do you want to be stored in Ma before that line of code executes?
Take a look at the list of functions in the "Create and Combine Arrays" section and the "Creating, Concatenating, and Expanding Matrices" Topic on this documentation page for some functions that may be useful in defining your Ma vector.
  2 Comments
Steven Lord
Steven Lord on 18 May 2020
Call fzero once per element of the variables that you know that are not scalars (1-by-1) to solve mdot-(A.*Ma*P0*sqrt(k/(R*T0)))/(1+(k-1)*Ma^2/2)^((k+1)/(2*(k-1))) = 0 for Ma.
Alternately if you have Symbolic Math Toolbox you could solve the system(s) symbolically for Ma as a function of a symbolic variable A then use subs to substitute the array of values for A into the symbolic solution.

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More Answers (3)

TADA
TADA on 18 May 2020
Ma is a 0x0 empty vector:
Ma = []
Ma =
[]
You can't multiply your 1x100 vector A by that using element by element multiplatinum, you have to set something of the same size as A into Ma

Kamilu Sanusi
Kamilu Sanusi on 27 Aug 2022
Hello everyone. Please can someone explain the essense of creating empty matrix A, B, YG, & E in the following program:
% Winkel(zeit)=0-atan(arbeitspunkt.Vy(3)/arbeitspunkt.Vx(3))*180/pi;
PP(:,zeit)=arbeitspunkt.P;
QQ(:,zeit)=arbeitspunkt.Q;
UXT(zeit,:)=arbeitspunkt.Vx;
UYT(zeit,:)=arbeitspunkt.Vy;
IXT(zeit,:)=arbeitspunkt.Ix;
IYT(zeit,:)=arbeitspunkt.Iy;
A=[];
B=[];
YG=[];
E=[];
for i=1:size(DatGen,1)
[Gen, delta, Eq]=Generator(DatGen(i,:), arbeitspunkt);
% Matrix der Systemgelcihung von Generator i
% Ai(i)={Gen.Axy};
% Bi(i)={Gen.Bxy};
% YGi(i)={Gen.YGxy};
% Ei(i)={Gen.Exy};
% Matrix der Systemgelcihung von allen Generatoren
A=blkdiag(A, Gen.Axy);
B=blkdiag(B, Gen.Bxy);
YG=blkdiag(YG, Gen.YGxy);
E=blkdiag(E, Gen.Exy);

Alisha
Alisha on 23 Sep 2022
Edited: Steven Lord on 23 Sep 2022
[SL: removed spam link]
It is not possible to create a blank array and then allow it to grow dynamically each time a user types a number into the command line. Instead, you ought to read the integers and add them to an Array. An ArrayList can grow dynamically and does not require an initial size.
  1 Comment
Steven Lord
Steven Lord on 23 Sep 2022
It is possible to create an empty array and fill it by growing it dynamically. That's not a very efficient technique, though. Prefer to preallocate the array and fill it in so it doesn't have to grow with each new element you add to it.

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