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Always!
29%
It depends
14%
Never!
21%
I didn't know that was possible
36%
1810 票
goc3
goc3
Last activity 2024 年 9 月 8 日

Base case:
Suppose you need to do a computation many times. We are going to assume that this computation cannot be vectorized. The simplest case is to use a for loop:
number_of_elements = 1e6;
test_fcn = @(x) sqrt(x) / x;
tic
for i = 1:number_of_elements
x(i) = test_fcn(i);
end
t_forward = toc;
disp(t_forward + " seconds")
0.10925 seconds
Preallocation:
This can easily be sped up by preallocating the variable that houses results:
tic
x = zeros(number_of_elements, 1);
for i = 1:number_of_elements
x(i) = test_fcn(i);
end
t_forward_prealloc = toc;
disp(t_forward_prealloc + " seconds")
0.035106 seconds
In this example, preallocation speeds up the loop by a factor of about three to four (running in R2024a). Comment below if you get dramatically different results.
disp(sprintf("%.1f", t_forward / t_forward_prealloc))
3.1
Run it in reverse:
Is there a way to skip the explicit preallocation and still be fast? Indeed, there is.
clear x
tic
for i = number_of_elements:-1:1
x(i) = test_fcn(i);
end
t_backward = toc;
disp(t_backward + " seconds")
0.032392 seconds
By running the loop backwards, the preallocation is implicitly performed during the first iteration and the loop runs in about the same time (within statistical noise):
disp(sprintf("%.2f", t_forward_prealloc / t_backward))
1.08
Do you get similar results when running this code? Let us know your thoughts in the comments below.
Beneficial side effect:
Have you ever had to use a for loop to delete elements from a vector? If so, keeping track of index offsets can be tricky, as deleting any element shifts all those that come after. By running the for loop in reverse, you don't need to worry about index offsets while deleting elements.
goc3
goc3
Last activity 2024 年 9 月 30 日

quick / easy
21%
themed / in a group
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challenge (e.g., banned functions)
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puzzle / game
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educational
28%
other (comment below)
3%
117 票
isstring
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ischar
7%
iscellstr
13%
isletter
21%
isspace
9%
ispunctuation
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2455 票
Don't use / What are Projects?
26%
1–10
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11–20
15%
21–30
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31–50
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51+ (comment below)
12%
4070 票
goc3
goc3
Last activity 2024 年 10 月 7 日

2
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3
12%
4
59%
6
4%
8
5%
Other (comment below)
3%
6419 票
numel(v)
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length(v)
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width(v)
14%
nnz(v)
8%
size(v, 1)
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sum(v > 0)
31%
2537 票
ismissing( { [ ] } )
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ismissing( NaN )
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ismissing( NaT )
11%
ismissing( missing )
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ismissing( categorical(missing) )
9%
ismissing( { '' } ) % 2 apostrophes
16%
896 票
isempty( [ ] )
10%
isempty( { } )
13%
isempty( '' ) % 2 single quotes
13%
isempty( "" ) % 2 double quotes
24%
c = categorical( [ ] ); isempty(c)
18%
s = struct("a", [ ] ); isempty(s.a)
22%
1324 票
sort(v)
8%
unique(v)
16%
union(v, [ ])
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intersect(v, v)
14%
setdiff(v, [ ])
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All return sorted output
33%
1193 票
s = ['M','A','T','L','A','B']
9%
char([77,65,84,76,65,66])
7%
"MAT" + "LAB"
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upper(char('matlab' - '0' + 48))
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fliplr("BALTAM")
17%
rot90(rot90('BALTAM'))
30%
2929 票
eye(3) - diag(ones(1,3))
11%
0 ./ ones(3)
9%
cos(repmat(pi/2, [3,3]))
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zeros(3)
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A(3, 3) = 0
32%
mtimes([1;1;0], [0,0,0])
12%
3009 票
goc3
goc3
Last activity 2023 年 11 月 6 日

Have you ever learned that something you were doing manually in MATLAB was already possible using a built-in feature? Have you ever written a function only to later realize (or be told) that a built-in function already did what you needed?
Two such moments come to mind for me.
1. Did you realize that you can set conditional breakpoints? Neither did I, until someone showed me that feature. To do that, open or create a file in the editor, right click on a line number for any line that contains code, and select Set Conditional Breakpoint... This will bring up a dialog wherein you can type any logical condition for which execution should be paused. Before I learned about this, I would manually insert if-statements during debugging. Then, after fixing each bug, I would have to delete those statements. This built-in feature is so much better.
2. Have you ever needed to plot horizontal or vertical lines in a plot? For the longest time, I would manually code such lines. Then, I learned about xline() and yline(). Not only is less code required, these lines automatically span the entire axes while zooming, panning, or adjusting axis limits!
Share your own Aha! moments below. This will help everyone learn about MATLAB functionality that may not be obvious or front and center.
(Note: While File Exchange contains many great contributions, the intent of this thread is to focus on built-in MATLAB functionality.)
1
33%
2
34%
3
18%
4
5%
5
3%
6+
6%
1643 票
<= 6 GB
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7–12 GB
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13–22 GB
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23–46 GB
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47–90 GB
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>= 91 GB
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15925 票
Yes, the available tools are great
12%
Yes, the available tools need help
6%
No, but I would like to
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No, it is not important to me
7%
What is test-driven development?
61%
1955 票
Always
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Sometimes
11%
In the past, but not now
3%
Never
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What is Simulink Project?
53%
2443 票
Always
8%
Sometimes
9%
In the past, but not now
2%
Never
23%
What is MATLAB Project?
58%
4533 票
A bunch of quick, simple problems
16%
A structured, educational set
15%
Brain-teasers
16%
Random miscellany
3%
Something else
2%
What are Problem Groups?
49%
1777 票
goc3
goc3
Last activity 2020 年 4 月 6 日

Cody has a wealth of problem groups that allow users of various skill levels to improve programming skills vis-à-vis MATLAB in an engaging way.

I would like to highlight the Draw Letters group, composed of problems created by Majid Farzaneh.

If you haven't yet visited Cody or solved that problem group, I would recommend that you head over there now. What are you waiting for?

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