How to plot (x-1)/(y+2)^1.8?

Question

I tried to plot the equation (x-1)/(y+2)^1.8 in octave using surf() method. But the graph is something else.

Here's my code:

p = linspace(1,50, 100);
t = linspace(1,48,100);
ans = zeros(length(p), length(t));
ans = compute_z(p, t, ans);
figure;
surf(p, t, ans');

trying to compute z = (x-1)/(y+2)^1.8 using helper function compute_z

function [ans] = compute_z(ans, p, t)
        for i = 1:length(p)
            for j = 1:length(t)
               ans(i,j) = (p(i) - 1) / (t(j)+2)^1.8;
            end
        end

I was trying to generate this graph.

`ans = zeros(length(p), length(t));` is redundant. If anything, you should allocate ans inside of compute_z. — Marcus Müller, Oct 19 '15 at 08:33
@MarcusMüller Okay.. but no change in graph.. how to i plot correct graph. — CodeNinja101, Oct 19 '15 at 08:37

score 4 · Accepted Answer · answered Oct 19 '15 at 09:31

4

There is no need for your compute_z-method as you can you meshgrid and vectorisation.

p = linspace(1,50, 100);
t = linspace(1,48,100);

[P, T] = meshgrid(p,t); Z = (P-1) ./ (T+2).^1.8;

figure;
surf(P, T, Z);

(Tested in Matlab, but should work in Octave as well)

answered Oct 19 '15 at 09:31

harre

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Looks like you beat me to the answer :) – Dev-iL Oct 19 '15 at 09:39

score 0 · Answer 2 · answered Oct 19 '15 at 09:04

Your problem is you define compute_z inputs as ans, p and t in that order. However, you call the function with p, t and ans in that order. Do you see the problem?

ans isn't an input to the function, just an output, so no need to list it as an input. Furthermore, don't call your variable ans, it's the default variable name MATLAB uses when no output variable name is specified, so is likely to be overwritten.

Here's my suggestion:

p = linspace(1,50, 100);
t = linspace(1,48,100);
z = zeros(length(p), length(t));
z = compute_z(p, t);
figure;
surf(p, t, z');

with compute_z defined as follows:

function z = compute_z(p, t)
   for i = 1:length(p)
       for j = 1:length(t)
           z(i,j) = (p(i) - 1) / (t(j)+2)^1.8;
       end
   end

How to plot (x-​1)/​(y+​2)^​1.8?

2 Answers2

How to plot (x-1)/(y+2)^1.8?