How do I initialise all entries of a matrix with a specific value

Question

In Haskell  if I wanted to get a 10 element list which only contained the number 5  I could do something like this   take 10   repeat 5   Output    5 5 5 5 5 5 5 5 5 5    Is there anything like this in Matlab

User · Answer

The ones method is much faster than using repmat:

>> tic; for i = 1:1e6, x=5*ones(10,1); end; toc
Elapsed time is 3.426347 seconds.
>> tic; for i = 1:1e6, y=repmat(5,10,1); end; toc
Elapsed time is 20.603680 seconds.

And, in my opinion, makes for much more readable code.

User · Answer

See repmat in the documentation   B   repmat 5 1 10

User · Answer

Given a predefined m-by-n matrix size and the target value val  in your example   m   1  n   10  val   5    there are currently 7 different approaches that come to my mind     1  Using the repmat function  0 094066 seconds   A   repmat val m n    2  Indexing on the undefined matrix with assignment  0 091561 seconds   A 1 m 1 n    val   3  Indexing on the target value using the ones function  0 151357 seconds   A   val ones m n     4  Default initialization with full assignment  0 104292 seconds   A   zeros m n   A      val   5  Using the ones function with multiplication  0 069601 seconds   A   ones m n    val   6  Using the zeros function with addition  0 057883 seconds   A   zeros m n    val   7  Using the repelem function  0 168396 seconds   A   repelem val m n      After the description of each approach  between parentheses  its corresponding benchmark performed under Matlab 2017a and with 100000 iterations  The winner is the 6th approach  and this doesn t surprise me   The explaination is simple  allocation generally produces zero-filled slots of memory    hence no other operations are performed except the addition of val to every member of the matrix  and on the top of that  input arguments sanitization is very short   The same cannot be said for the 5th approach  which is the second fastest one because  despite the input arguments sanitization process being basically the same  on memory side three operations are being performed instead of two    the initial allocation the transformation of every element into 1 the multiplication by val

User · Answer

As mentioned in other answers you can use    gt  gt  tic  x 5 ones 10 1   toc Elapsed time is 0 000415 seconds    An even faster method is    gt  gt  tic   x 5  x x ones 10 1    toc Elapsed time is 0 000257 seconds

User · Answer

It is easy to assign repeated values to an array   x 1 10    5    If you want to generate the array of elements inline in a statement try something like this   ones 1 10    5   or with repmat  repmat 5  1  10

[arrays] How do I initialise all entries of a matrix with a specific value?

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