# how to plot a vector

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Elinor Ginzburg on 24 May 2020
Answered: Elinor Ginzburg on 24 May 2020
hello,
I'm trying to plot the points of a vector to see if it looks like I want it to, but I get a blank figure. can anyone please advice me on what's wrong with my code?
% units %
lambda = 1; L = 10; N = 500; a = 1; V0 = 5;
% Create lattice points %
dx = L/N;
x = zeros([1 N]);
for n = 1:N
x(n) = -L/2+(n-0.5)*dx;
end
% define potential %
Vho = zeros([1 N]);
for i = 1:N
Vsw = -V0*(heaviside(x(i)+a/2)-heaviside(x(i)-a/2));
end
figure
plot(Vsw)
Thank you very much for your help!

#### 1 Comment

Stephen Cobeldick on 24 May 2020
Given that heaviside belongs to the Symbolic Toolbox, you will probably need to consider it a bit more carefully.

Stephen Cobeldick on 24 May 2020
Edited: Stephen Cobeldick on 24 May 2020
"what's wrong with my code?"
• you preallocate a variable named Vho which you never use again.
• inside the loop you allocate to a variable named Vso , but you do not use any indexing, so on each loop iteration you simply overwrite Vso, thus leaving only the (scalar value) from the final iteration.
So even though you write that you are plotting a vector, if you actually take a look at Vso it is actually scalar (and plotting a scalar gives the appearance of an empty plot, although it isn't actually, just the default linespec does not include any point marker).
I fixed those mistakes (and replaced heaviside with a simple logical comparison):
Vsw = zeros(1,N);
for k = 1:N
Vsw(k) = -V0*((x(k)+a/2)>0 - (x(k)-a/2)>0);
end
plot(Vsw,'-*') Tommy on 24 May 2020
Vsw is a scalar, so nothing will show when you plot it. You are overwriting the value of Vsw in each loop of your for loop. Do you mean to instead store the values within Vho and plot that?
% define potential %
Vho = zeros([1 N]);
for i = 1:N
Vho(i) = -V0*(heaviside(x(i)+a/2)-heaviside(x(i)-a/2));
end
figure
plot(Vho)

Elinor Ginzburg on 24 May 2020
Thank you very much guys!
I mistyped and didn't index Vsw as you all mentioned.