Matlab for loop error with '>='

Dear Matlab gurus,

Having trouble plotting several graphs using a for loop. Error comes upto '>='. Tried with '>' only, and it too won't work.

"Error: File: ******.m Line: 14 Column: 7 Unexpected MATLAB operator."

I'm not too good at computing. Help would be much appreciated.

Thanks,

Kelvin

 % Drag force coefficient
  c = 1.3
  % Surface area of skydiver - m^2
  A = 1.2
  % Altitude - m
  h = 40000
  % Atmospheric pressure - Pa
  p = 101325
  % Specific gas constant for dry air - J/kg K
  R = 287.058
  % Density - mol
  D = P/R.*T

for h >= 0 
    % Temperature - K
    T = -1.033.*10.^-16.*h.^4 + 3.344.*10.^-12.*h.^3 + 2.521.*10.^7.*h.^2 - 0.009527.*h + 293;
    % Pressure
    P = p.*(1 - 2.256.*10.^-5.*h).^5.256;
    % Drag Force
    F = 1/2.*p.*v.^2.*c.*A;
    h = h - 1;
end
% Q1
figure(1);
plot(T, h)
title('Temperature as a fn of Altitude')
xlabel('Temperature (K)')
ylabel('Height (m)')

% Q2
figure(2);
plot(P, h)
title('Pressure as a fn of Altitude')
xlabel('Pressure (Pa)')
ylabel('Height (m)')
% Q3
figure(3);
plot (D, h)
title('Density as a fn of Altitude')
xlabel('Density (mol.)')
ylabel('Height (m)')

Answer 1

[Image Analyst]

It doesn't make sense. What is x??? You didn't define it. Even if you did, you would use "if" or "while" instead of "for". And if you used a while, x would have to change within the loop or else the loop would never end.

[Kelvin Kong]

Sorry, forgot to update x. x is meant to be h. Edited*.

Answer 2

[John D'Errico]

for x >= 0

is not valid MATLAB syntax. No matter what you think it means, you cannot make up new syntax and expect it to work. Computer languages don't work that way.

Perhaps you wanted a while loop. But x is undefined at that point.

I'd start by reading the help docs on for and while. Reading the manual is always a good idea.

doc for
doc while

[Kelvin Kong]

Gotcha!

I'll be back! In Arnold's voice hehe..

[Kelvin Kong]

What about this for loop? for (h > 0) && (h = 0)

Error with '>'.

[Image Analyst]

Uh, no. What value can be both equal to zero and greater than zero at the same time? Anyway, (h=0) is an assignment, and that will probably return "true" since the assignment can be made successfully. But a for loop is

for k = startValue : stepValue : endingValue

or

for k = setOfKValues

where setOfKValues is a set of values that k will take on in turn as the loop iterates.

I'm thinking you probably want

while h > 0

Answer 3

[Kelvin Kong]

So' I changed the 'for' into 'if', and the error went away. Updated my code, got the graphs windows to open up but no plots. What did I do wrong?

% Drag force coefficient
c = 1.3
% Surface area of skydiver - m^2
A = 1.2
% Altitude - m
h = 40000
% Atmospheric pressure - Pa
p = 101325
% Specific gas constant for dry air - J/kg K
R = 287.058
% Density - mol.
D = P/R.*T
% Mass of Space Diver - kg
m = 100
% Acceleration of gravity - m/s
g = 9.8
% Initial velocity - m/s
u = 0
% Drag 
DF = 0

for x = 0:40000:1
    if h >= 0 
    % Temperature - K
    T = -1.033.*10.^-16.*h.^4 + 3.344.*10.^-12.*h.^3 + 2.521.*10.^7.*h.^2 - 0.009527.*h + 293;
    % Pressure
    P = p.*(1 - 2.256.*10.^-5.*h).^5.256;
    % Acceleration Eq.
    a = (m.*g - DF)/m;
    % Velocity Eq.
    h0 = h
    s = h - h0
    v = sqrt(u.^2 + 2.*a.*s);
    % Drag Force
    DF = 1/2.*p.*v.^2.*c.*A;
    h = h - 1;
    end
end
% Q1
figure(1);
plot(T, h)
title('Temperature as a fn of Altitude')
xlabel('Temperature (K)')
ylabel('Height (m)')
grid on 

% Q2
figure(2);
plot(P, h)
title('Pressure as a fn of Altitude')
xlabel('Pressure (Pa)')
ylabel('Height (m)')
grid on

% Q3
figure(3);
plot (D, h)
title('Density as a fn of Altitude')
xlabel('Density (mol.)')
ylabel('Height (m)')
grid on

% Q4
figure(4);
plot (v, h)
title(' Velocity of Space Diver as Fn of Altitude')
xlabel('Velocity (m/s)')
ylabel('Height (m)')
grid on