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Highlights

goc3
최근 활동: 2024년 10월 21일

Do you have the tab key insert spaces instead of tabs?

Always!

29%

It depends

14%

Never!

21%

I didn't know that was possible

36%

Congratulations to William!

I was browsing the MathWorks website and decided to check the Cody leaderboard. To my surprise, William has now solved 5,000 problems. At the moment, there are 5,227 problems on Cody, so William has solved over 95%. The next competitor is over 500 problems behind. His score is also clearly the highest, approaching 60,000.

Please take a moment to congratulate @William.

Tips & Tricks

goc3
최근 활동: 2024년 9월 8일

+1 for Backwards Loops!

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.

Cody

goc3
최근 활동: 2024년 6월 7일

Trick: Easy Digit Manipulation

There are a host of problems on Cody that require manipulation of the digits of a number. Examples include summing the digits of a number, separating the number into its powers, and adding very large numbers together.

If you haven't come across this trick yet, you might want to write it down (or save it electronically):

digits = num2str(4207) - '0'

That code results in the following:

digits =
     4     2     0     7

Now, summing the digits of the number is easy:

sum(digits)
ans =
    13

Highlights

goc3
최근 활동: 2024년 9월 30일

My favorite type of Cody problem is:

quick / easy

21%

themed / in a group

20%

challenge (e.g., banned functions)

13%

puzzle / game

16%

educational

28%

other (comment below)

Which of the following is not a built-in string-checking function?

isstring

11%

ischar

iscellstr

13%

isletter

21%

isspace

ispunctuation

37%

How many code files are typically in your MATLAB Projects (scripts, functions, classes, tests)?

Don't use / What are Projects?

26%

1–10

31%

11–20

15%

21–30

31–50

51+ (comment below)

12%

How many spaces per tab do you prefer?

17%

12%

59%

Other (comment below)

Which of the following does not return the correct length (5) of v = [6,2,9,7,3]?

numel(v)

length(v)

13%

width(v)

14%

nnz(v)

size(v, 1)

27%

sum(v > 0)

31%

Which of the following is not really a missing value? (Or, which option is false?)

ismissing( { [ ] } )

26%

ismissing( NaN )

18%

ismissing( NaT )

11%

ismissing( missing )

21%

ismissing( categorical(missing) )

ismissing( { '' } ) % 2 apostrophes

16%

Which of the following produces a false value?

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%

Which does NOT return sorted output for a vector of unique values (e.g., v = [3,7,2,5,1])?

sort(v)

unique(v)

16%

union(v, [ ])

17%

intersect(v, v)

14%

setdiff(v, [ ])

12%

All return sorted output

33%

Which of the following will not result in MATLAB as the output? (It may be a string or char. array.)

s = ['M','A','T','L','A','B']

char([77,65,84,76,65,66])

"MAT" + "LAB"

21%

upper(char('matlab' - '0' + 48))

17%

fliplr("BALTAM")

17%

rot90(rot90('BALTAM'))

30%

Which of the following will not produce a 3x3 array of zeros in MATLAB?

eye(3) - diag(ones(1,3))

11%

0 ./ ones(3)

cos(repmat(pi/2, [3,3]))

16%

zeros(3)

20%

A(3, 3) = 0

32%

mtimes([1;1;0], [0,0,0])

12%

I use MATLAB on x setups. (work, home; laptop, tablet; local installation, online; etc.)

33%

34%

18%

On the main computer you use for MATLAB, how much RAM is installed?

<= 6 GB

10%

7–12 GB

26%

13–22 GB

34%

23–46 GB

19%

47–90 GB

>= 91 GB

Do you use test-driven development in MATLAB?

Yes, the available tools are great

12%

Yes, the available tools need help

No, but I would like to

14%

No, it is not important to me

What is test-driven development?

61%

For your Simulink work, do you use Projects (.prj files)?

Always

12%

Sometimes

11%

In the past, but not now

Never

20%

What is Simulink Project?

53%

For your MATLAB work, do you use Projects (.prj files)?

Always

Sometimes

In the past, but not now

Never

23%

What is MATLAB Project?

58%

What is your favorite type of Cody Problem Group?

A bunch of quick, simple problems

16%

A structured, educational set

15%

Brain-teasers

16%

Random miscellany

Something else

What are Problem Groups?

49%