Why do I get "Array indices must be positive integers or logical values"?

I am trying to graph Thrust vs Mach number. I took posted all the parts that are related to the error message

clc
clear
close all

Ta = 504;
y = 1.4;
Ra = 1716;

for M = .1:.1:5
    u(M) = M.* sqrt(y.*Ra.*Ta);
    disp(u)
end

The error I get is "Array indices must be positive integers or logical values. Error in test (line 10) u(M) = M.* sqrt(y.*Ra.*Ta);" Can someone shine some light on what I am doing wrong?

Answer 1

[Cris LaPierre]

Your values of M are not valid indices for u. Your indices must be integer values >=1.

A=1:5;
% This works because the index is a positive integer
A(2)

ans = 2

% This does not work because the index is a decimal number
A(0.2)

Array indices must be positive integers or logical values.

The most robust solution would be to create a vector M, and then have your for loop loop through each element of M, with the loop counter being used assign the result to u.

M = .1:.1:5;
for v = 1:length(M)
    u(v) = M(v).* sqrt(y.*Ra.*Ta);
    disp(u)
end

Even better would be to get rid of the loop altogether.

M = .1:.1:5;
u = M.* sqrt(y.*Ra.*Ta)

[Mims Hillis]

Thank you I should have mentioned that the posted code was apart of a larger loop

Answer 2

[Torsten]

Ta = 504;
y = 1.4;
Ra = 1716;
M = .1:.1:5;
for i = 1:numel(M)
    u(i) = M(i)*sqrt(y*Ra*Ta);
end
plot(M,u)

or simply

Ta = 504;
y = 1.4;
Ra = 1716;
M = .1:.1:5;
u = M*sqrt(y*Ra*Ta);
plot(M,u)