Efficient way to build a matrix

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?

Answer 1

[Atsushi Ueno]

> 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

[David Velasco]

Beautiful! Thank you!

Answer 2

[Steven Lord]

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

reshape the values array if desired.

[David Velasco]

Interesting. This works as well, and it's actually faster (by a tiny bit) than the first one:

Using repelem & and repmap: Elapsed time is 0.000327 seconds.

Using implicit expansion: Elapsed time is 0.000283 seconds.