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Plotting two columns from matrix.

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JLV on 1 Jan 2020
Commented: JLV on 4 Jan 2020
I am trying to plot Column 4 against Column 10 as a line plot, but only when Columns 8 and 9 are between 0 +/- 0.0005. (Columns 8, 9, 10 are x, y, and z locations).
I would have done this with two if loops which would print the row if the conditions were met. This could have then been plotted. I feel as there are over 400,000 entries this is inefficient (file attached represents a smaller quantity due to size constraints). Prealloaction is not possible as I don't know the size of the final array. What would the best way to go about this?


Walter Roberson
Walter Roberson on 1 Jan 2020
In the cases where the conditions are not met, what do you want to do? Are you wanting to do a scatter plot? Are you wanting to plot a disconnected line segment for each consecutive area where the conditions are met? Are you wanting to create a single line plot as if the entries that do not met the condition were not present at all ?
JLV on 1 Jan 2020
I do not want to use the data at all where the conditions mentioned above is not met.
A scatter plot with a line of best fit will be plotted (Column4 vs Column10).

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Accepted Answer

per isakson
per isakson on 1 Jan 2020
Edited: per isakson on 1 Jan 2020
See Indexing with Logical Values half way down the page
A starter
M = dlmread('matrix.txt');
is_show = abs(M(:,8))<5e-4 & abs(M(:,9))<5e-4;
plot( M(is_show,10), M(is_show,4), '.' )


JLV on 2 Jan 2020
Very useful, reminds me logic plays an important role in coding rather than if and for loops. Thanks.
A follow up here
What would be the best way to plot Column4 if:
is_show = abs(M(:,10))<5e-4
I am trying to plot M(is_show,4) against its respective x and y coordinate locations M(is_show,8) and M(is_show,9), similar to a filled 2D contour plot. I have tried scatter too with no avail. Any suggestions on plot type will be useful.
Walter Roberson
Walter Roberson on 2 Jan 2020
x = M(is_show,8);
y = M(is_show,9);
z = M(is_show,4);
Finterp = scatteredInterpolant(x, y, z);
xmin = min(M(:,8));
xmax = max(M(:,8));
ymin = min(M(:,9));
ymax = max(M(:,9));
N = 500;
xrange = linspace(xmin, xmax, N);
yrange = linspace(ymin, ymax, N);
[XGrid, YGrid] = meshgrid(xrange, yrange);
ZGrid = Finterp(XGrid, YGrid);
contourf(XGrid, YGrid, ZGrid);
xlabel('M 8');
ylabel('M 9');
zlabel('M 4');
JLV on 4 Jan 2020
I will take a look into this and the code deeper when Im back in the office.

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