Results for
No
50%
Yes, but I am not interested
8%
Yes, but it is too expensive
20%
Yes, I would like to know more
18%
Yes, I am cert. MATLAB Associate
2%
Yes, I am cert. MATLAB Professional
3%
4779 votes
Always!
29%
It depends
14%
Never!
21%
I didn't know that was possible
36%
1810 votes
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")
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")
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))
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")
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))
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.
quick / easy
21%
themed / in a group
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challenge (e.g., banned functions)
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puzzle / game
16%
educational
28%
other (comment below)
3%
117 votes
isstring
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ischar
7%
iscellstr
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isletter
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isspace
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ispunctuation
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2455 votes
Don't use / What are Projects?
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1–10
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11–20
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21–30
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31–50
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51+ (comment below)
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4070 votes
2
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3
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4
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6
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8
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Other (comment below)
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6419 votes
numel(v)
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length(v)
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width(v)
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nnz(v)
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size(v, 1)
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sum(v > 0)
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2537 votes
ismissing( { [ ] } )
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ismissing( NaN )
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ismissing( NaT )
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ismissing( missing )
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ismissing( categorical(missing) )
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ismissing( { '' } ) % 2 apostrophes
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896 votes
isempty( [ ] )
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isempty( { } )
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isempty( '' ) % 2 single quotes
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isempty( "" ) % 2 double quotes
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c = categorical( [ ] ); isempty(c)
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s = struct("a", [ ] ); isempty(s.a)
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1324 votes
sort(v)
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unique(v)
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union(v, [ ])
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intersect(v, v)
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setdiff(v, [ ])
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All return sorted output
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1193 votes
s = ['M','A','T','L','A','B']
9%
char([77,65,84,76,65,66])
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"MAT" + "LAB"
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upper(char('matlab' - '0' + 48))
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fliplr("BALTAM")
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rot90(rot90('BALTAM'))
30%
2929 votes
eye(3) - diag(ones(1,3))
11%
0 ./ ones(3)
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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 votes
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
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2
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3
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4
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5
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6+
6%
1643 votes
<= 6 GB
10%
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 votes
Yes, the available tools are great
12%
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No, but I would like to
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7%
What is test-driven development?
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1955 votes
Always
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Sometimes
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Never
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What is Simulink Project?
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2443 votes
Always
8%
Sometimes
9%
In the past, but not now
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Never
23%
What is MATLAB Project?
58%
4533 votes
A bunch of quick, simple problems
16%
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15%
Brain-teasers
16%
Random miscellany
3%
Something else
2%
What are Problem Groups?
49%
1777 votes
