Octave：向/从文本文件写入/读取 f(x,y) 出错

Question

I have problem with reading and drawing function from text file. Code to make and write function:

clc, clear;
plik=fopen('dane.txt', 'w')
x= -5:.1:5;
y= flip(rot90((-5:0.1:5)));
z =sin(x).*y;
fprintf(plik,'x          y         z\n');
for i=1:101
  for j=1:101
    fprintf(plik,'%6.3f   \t%6.3f   \t%6.3f \n', [x(i); y(j); z(i,j)]);
  endfor
 endfor
mesh (x,y,z)
fclose(plik);

And code to read and draw function f(x,y).

clear,clc;
plik = fopen('dane.txt','r');
a = dlmread(plik, '\t', 1, 0);

x=a(:,1);
y=a(:,2);
z=a(:,3);

plot3(x, y, z);
grid on
fclose(plik);

The problem is with drawing plot. It draws desired function BUT it also draws something that looks like f(x,y)=x*y plot. What can be an issue here?

Answer 1

The plot3 command takes X, Y, and Z column-vectors, and then treats each row as an (x,y,z) coordinate to plot. As it moves from one row to the next, it draws a line connecting one point to the next .

The way your data is arranged in the file, most points are rather close to each other, so drawing a line from one point to the next is mostly harmless, except that you have a big jump from when you go from point (x_i, y_N) to (x_{i+1}, y_0). The line that is drawn in this case is quite visible, since it connects the right end of the graph to the left one.

If you'd like to get rid of that line, either plot markers only (eg plot3(x,y,z,'o') , or reshape your data into a square matrix; in the presence of a matrix instead of a single vector, plot3 treats each column of the matrix as a separate line to draw, therefore this would enable the 'big' line to go away. Note that if you do this however, you'll have to specify a colour, otherwise each line will be drawn in an arbitrarily new colour.

Also, note that the output will still not be identical to that of 'mesh'; instead your surface will be created as a series of "parallel lines". If you want to recreate the mesh, just pass the reshaped matrices into mesh as before.

In other words compare

r = @(x) reshape( x, [101,101] )
plot( r(x), r(y), r(z) );

to:

mesh( r(x), r(y), r(z) );