Trying to create a vector in steps of 0.5

I'm trying to get the vector to go up in steps of 0.5.

 v_numerical(0.5)= g.*0.5;

 for i =  1:0.5:10

    v_numerical(i) = v_numerical(i-0.5)+(g-((c.*v_numerical(i-0.5))./m).*(0.5));
 end

However whenever i try running it I get the error:

Attempted to access v_numerical(0.5); index must be a positive integer or logical.

Any idea how I can solve this problem? Many thanks in advance!

Answer 1

[Stephen23]

MATLAB indices start at one, and must be whole numbers (i.e. no fraction allowed). With your code you are trying to index with 0.5 and lots of other fractions, which will always be an error, like this:

>> A(0.5) = 3 % fraction index is an error
??? Attempted to access (0.5); index must be a positive integer or logical.
>> A(1) = 3  % whole number index is okay
A =
     3

You can easily create a vector with 0.5 spacing:

V = 1:0.5:10;

but when I tried to fix your code it made very little sense to me what you are trying to achieve. Can you please explain what you are trying to do with this code: what should the input and output be?

[eoin247]

I'm trying to plot a graph of velocity against time in steps of 0.5 and compare the analytic and numerical approaches. Here is the whole code:

 close all; clear; 

 %defining known variables
 g = 9.81; c = 12.5; m = 70; 

 %defining timestep
 tstep = 0.5 ;

 %defining analytic time vector
 t = (0:tstep:10);

 %get analytic velocity
 v_analytic =(g.*m./c).*(1-(exp((-c.*t)./m)));
 %first numerical velocity point
 v_numerical(0.5)= g.*0.5;

 %setting up plot and plotting analytic velocity 
figure(1); hold on; grid on; box on; 
plot(t,v_analytic,'b*-')
title('parachutist');
xlabel('time (s)');
ylabel('velocity (m/s)');

 %numerical calculations using a 'for' loop

 for i =  1:0.5:10

    v_numerical(i) = v_numerical(i-0.5)+(g-((c.*v_numerical(i-0.5))./m).*(0.5));
end 

 %numerical results and plot
plot(t,v_numerical,'ro-')
legend('analytical','numerical')

[eoin247]

Actualy i think I just realised that your original answer solved my problem. I needed to define the vector in Ste[s of 0.5 before hand which i had not done.

Thank you for the answer!

Answer 2

[Nemanja]

g = linspace(1:10:20) %goes from 1 to 10 and has 20 elements because 10/20 is 0.5 so it will go up from 1 to 10 with 0.5 spacing%

[Stephen23]

"goes from 1 to 10 and has 20 elements because 10/20 is 0.5 so it will go up from 1 to 10 with 0.5 spacing"

No, you are making this error:

https://en.wikipedia.org/wiki/Off-by-one_error#Fencepost_error

It is easy to check that your statement is incorrect: did you try running the code yourself before posting here?

g = linspace(1,10,20) % fixed incorrect syntax

g = 1×20

1.0000 1.4737 1.9474 2.4211 2.8947 3.3684 3.8421 4.3158 4.7895 5.2632 5.7368 6.2105 6.6842 7.1579 7.6316 8.1053 8.5789 9.0526 9.5263 10.0000

Tip: to split the range 1..10 into steps of 0.5 then you need 19 elements. Lets try it now:

g = linspace(1,10,19)

g = 1×19

1.0000 1.5000 2.0000 2.5000 3.0000 3.5000 4.0000 4.5000 5.0000 5.5000 6.0000 6.5000 7.0000 7.5000 8.0000 8.5000 9.0000 9.5000 10.0000