Why does the line plot depend on the generator type in a for loop ?

Question

Alexandre Kazantsev 2023-3-8

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1925020-why-does-the-line-plot-depend-on-the-generator-type-in-a-for-loop

评论： Voss 2023-3-15

采纳的回答： Voss

I have :

a set of points in two dimensions given by their coordinates x_stations and y_stations;
a set of couples of points given by the indices of point 1 (couples_st1) and point 2 (couples_st2) for each couple;
a subset i_subset of those couples

I would like to plot i_subset graphically as lines linking point 1 and point 2 for each couple in the subset.

The problem : depending how I am looping on i_subset :

for i = i_subset

i_couple = i;

% some code...

% generate plot

% ouptut the plotted vectors

end

or

for i = 1:length(i_subset)

i_couple = i_subset(i);

% some code ...

% generate plot

% ouptut the plotted vectors

end

I get the same output of plotted vectors, but not the same plots. Why ?

A minimal example is enclosed with input data. Just run "main_minimal_example.m".

main_minimal_example

max difference in x outputs : 0 max difference in y outputs : 0

Thank you !

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Answer 1

Voss 2023-3-8

1
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1925020-why-does-the-line-plot-depend-on-the-generator-type-in-a-for-loop#answer_1188555

The i_subset passed in to make_line_plot.m is a column vector, and that is assigned to generator if mode == 1. Then generator is used in the for loop

for i_gen = generator

A for loop iterates over the columns of what you give it, so this loop iterates one time, with i_gen being generator.

When mode is 0, then generator is 1:length(i_subset), which is a row vector, and the for loop operates as intended (I assume).

To get the mode == 1 case to work like the mode == 0 case, one way would be to make the for loop iterate over elements of generator, as in:

for i_gen = generator(:).'

[ From the documentation for for, when you have for index = valArray:

valArray — Create a column vector, index, from subsequent columns of array valArray on each iteration. For example, on the first iteration, index = valArray(:,1). The loop executes a maximum of n times, where n is the number of columns of valArray ]