{"id":2846,"date":"2021-10-24T00:14:50","date_gmt":"2021-10-24T08:14:50","guid":{"rendered":"https:\/\/wonghoi.humgar.com\/blog\/?p=2846"},"modified":"2025-12-01T19:52:07","modified_gmt":"2025-12-02T03:52:07","slug":"matlab-repeating-arrays-elementwise-array-replication-interleaved-repmat","status":"publish","type":"post","link":"https:\/\/wonghoi.humgar.com\/blog\/2021\/10\/24\/matlab-repeating-arrays-elementwise-array-replication-interleaved-repmat\/","title":{"rendered":"MATLAB repeating arrays (elementwise array replication, interleaved ‘repmat’)"},"content":{"rendered":"\n

Since MATLAB R2015b, there’s a new feature<\/a> called repelem(V, dim1, dim2, ...)<\/strong><\/code> which repeats each element by dimX times over dimension X. If N (dim1) is scalar, each V is uniformly repeated by N times. If N is a vector, it has to be the same length as V and each element of N says how many times the corresponding element in V is repeated.<\/p>\n\n\n\n

Here are some historical ways of doing it (as mentioned in MATLAB array manipulation tips<\/a>)<\/p>\n\n\n\n

The scalar case (repeat uniformly) can be emulated by a Kronecker product multiplying everything with 1 (self):<\/p>\n\n\n\n

kron(V, ones(N,1))<\/code><\/pre>\n\n\n\n
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Just replace all the elements b with 1 so we are left with elements of A repeating the way we wanted<\/figcaption><\/figure>\n\n\n\n
Kron method is conceptually smart but it has unnecessary arithmetic (multiply by 1). Nonetheless this method is reasonably fast until TMW finally developed a built-in function for it that outperforms all the tricks people have accumulated over decades.<\/p>\n\n\n\n
The vector case (each element is repeated a different number of times according to vector N) is basically decoding Run-Length Encoding (RLE), aka counts to placements, which you can download maturely written programs on MATLAB File Exchange (FEX). There are a bunch of cumsum<\/code>\/diff<\/code>\/accumarray<\/code>\/reshape <\/code>tricks but at the end of the day, they are RLE decoding in vectorized forms.<\/p>\n\n\n\n
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Since MATLAB R2015b, there’s a new feature called repelem(V, dim1, dim2, …) which repeats each element by dimX times over dimension X. If N (dim1) is scalar, each V is uniformly repeated by N times. If N is a vector, … Continue reading →<\/span><\/a><\/p>\n
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