Efficient way to build a matrix

Question

David Velasco 2021-5-19

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https://ww2.mathworks.cn/matlabcentral/answers/833738-efficient-way-to-build-a-matrix

评论： David Velasco 2021-5-19

I am looking for a more elegant and efficient way to build a matrix that I need for some subsequent computations and plotting. I start with a matrix called IndexSunrise, which is 1:7000 and holds sequential values but spaced at irregular intervals, e.g. IndexSunrise = [12 45 93 141 194 ... 7000]. I want create a new matrix that includes these plus the 7 sequential values that preceed these, e.g. IndexSunriseFinal = [5 6 7 8 9 10 11 12 37 38 39 40 41 42 43 45 86 87 88 89 90 91 92 93 ... 7000].

Presently I have it like the below

Index1 = IndexSunrise;
Index2 = IndexSunrise-1;
Index3 = IndexSunrise-2;
Index4 = IndexSunrise-3;
Index5 = IndexSunrise-4;
Index6 = IndexSunrise-5;
Index7 = IndexSunrise-6;
IndexSunriseFinal = [Index1 Index2 Index3 Index4 Index5 Index6 Index7]; 

While this works, it's ugly and inneficient and makes debuging harder (i.e. if I want to look at the 8 or 9 preceeding values instead, I have to rebuild the above).

I'm sure there has to be a better way to do this without using something worse like eval.

Suggestion?

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Answer 1

Atsushi Ueno 2021-5-19

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在 MATLAB Online 中打开

> I want create a new matrix that includes these plus the 7 sequential values that preceed these, e.g. IndexSunriseFinal = [5 6 7 8 9 10 11 12 37 38 39 40 41 42 43 45 86 87 88 89 90 91 92 93 ... 7000].

IndexSunrise = [12 45 93 141 194 7000];
n = 7;
IndexSunriseFinal = repelem(IndexSunrise, n+1) - repmat((n:-1:0), size(IndexSunrise));

>> repelem(IndexSunrise, n+1)

ans = 12 12 12 12 12 12 12 12 45 45 45 45 45 45 45 45 93 93 93 93 93 93 93 93 141 141 141 141 141 141 141 141 194 194 194 194 194 194 194 194 7000 7000 7000 7000 7000 7000 7000 7000

>> repmat((n:-1:0), size(IndexSunrise))

ans = 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0

>> repelem(IndexSunrise, n+1) - repmat((n:-1:0), size(IndexSunrise))

ans = 5 6 7 8 9 10 11 12 38 39 40 41 42 43 44 45 86 87 88 89 90 91 92 93 134 135 136 137 138 139 140 141 187 188 189 190 191 192 193 194 6993 6994 6995 6996 6997 6998 6999 7000

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David Velasco 2021-5-19

Beautiful! Thank you!

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Answer 2

Steven Lord 2021-5-19

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Take advantage of implicit expansion.

IndexSunrise = [12 45 93 141 194 7000]
IndexSunrise = 1×6
          12          45          93         141         194        7000
offsets = (-6:0).'
offsets = 7×1
    -6
    -5
    -4
    -3
    -2
    -1
     0
values = IndexSunrise + offsets
values = 7×6
           6          39          87         135         188        6994
           7          40          88         136         189        6995
           8          41          89         137         190        6996
           9          42          90         138         191        6997
          10          43          91         139         192        6998
          11          44          92         140         193        6999
          12          45          93         141         194        7000