Plot step function [x+1/2]

General Usage
1

Hi all,

I have been trying to plot [x+1/2] learning from the codes at my old thread:

But it returns not the plot that I want. I change 1 into 1.5 it still not working. Maybe it takes more than just that.

2

So you want the plot from your old post, but shifted 0.5 to the left? or am I interpreting that correctly?

You probably need to make sure you have the x-grid at correct locations to be able to draw that (i.e. at x.5 instead of x.0).

3

yes shifted 0.5 to the left, that is correct, because basically it is still using the rules of step function.

4

Yeah, but have you changed your x values so they are actually hitting the half steps instead of the integers? So x=-5.5:1:3.5 or something like that?

5

I use this:

using Plots; gr()

f(x) = floor(x)
x = -5.5:1:4.5
y = f.(x)
plot(x[1:2], [y[1], y[1]], label=L"f(x) = \lfloor x \rfloor", legend=:topleft,legend_foreground_color=:white, legend_font_color=:blue3, color=:blue3, showaxis=false, tickfontcolor=:white)
# axis and 
hline!((0,0), color=:black, label="")
vline!((0,0), color=:black, label="")
for i in x
    if i!= 0
        annotate!(i,-0.2, (i, 7))
    end
end
for i in y
    if i != 0
        annotate!(-.2, i, (i, 7))
    end
end

for i in 2:length(x)-1
    plot!(x[i:i+1], [y[i], y[i]], label="", color=:blue3, lw=2)
    scatter!(x[i], y[i], marker=:circle, markersize=8, label="")
end

got error:
LoadError: Cannot convert Float64 to series data for plotting
Stacktrace:

6

Seems like it is unhappy about getting floats, so maybe the line with scatter! could be changed to something like scatter!(x[i:i], y[i:i], ....) which would make them one element arrays. Does this work?

1 Like
7

It does not show error but shows no plot as well.

8

Yes scatter needs arrays, so the usual way to write this would be scatter([x], [y]).

For a plot to be shown it has to be the return value of the last evaluated expression. As loops in Julia return nothing, you need to either explicitly create a plot object and call it like this

p = plot()

for x ∈ 2:length(x)-1
    plot!(...)
    scatter!(...)
end
 
p

or add a call to current() to your code, which will just display the last plot object you’ve plotted to.

1 Like
9

This works finally, after few days / week

using Plots; gr()

f(x) = floor(x + 1/2)
x = -5.5:1:4.5
y = f.(x)
plot(x[1:2], [y[1], y[1]], label=L"f(x) = \lfloor x + 1/2 \rfloor", legend=:bottomleft,legend_foreground_color=:white, legend_font_color=:blue, color=:blue, showaxis=false, tickfontcolor=:white)
# axis and 
hline!((0,0), color=:black, label="")
vline!((0,0), color=:black, label="")
for i in x
	if i!= 0
		annotate!(i,-0.2, (i, 7))
	end
end
for i in y
	if i != 0
		annotate!(-.2, i, (i, 7))
	end
end
annotate!(-.2, -.2, ("0", 7))
for i in 2:length(x)-1
	plot!(x[i:i+1], [y[i], y[i]], label="", color=:blue, lw=2)	
end
scatter!(x[1:end-1], y[1:end-1], markersize=3, markercolor=:blue, label="")
scatter!(x[2:end], y[1:end-1], markersize=3, markercolor=:white, label="")