What is the difference between v(i):v(j) and v(i:j)?

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SelDen
SelDen 2021-10-28
评论： Star Strider 2021-10-28
I was trying to refer the consective elements in an array within a loop. When I used v(i):v(j) MATLAB stopped giving an array output after sometime. The error got fixed when I used v(i:j). What is the reason of this?

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Star Strider
Star Strider 2021-10-28
Without knowing what the elements of ‘v’ are, it is sifficult to say exactlly.
The ‘v(i):v(j)’ code creates a vector from ‘v(i)’ to ‘v(j)’ with a ‘step’ of 1. If the second is less than the first, no vector will be created. If the second is greater than the first while being less than 1 greater, it will return only the first value.
The ‘v(i:j)’ code creeates a vector of ‘v’ values between the ‘i’ and ‘j’ indices.
The two are entirely different, and will return entirely different results.
.
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SelDen
SelDen 2021-10-28
Thanks very much. I thought that the colon would be defined in v for some reason. In the code first elements of the array were already consecutive so the results were correct eventhough the code was wrong.

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Steven Lord
Steven Lord 2021-10-28
编辑：Steven Lord 2021-10-28
Ran in:
v = (1:10).^2
v = 1×10
1 4 9 16 25 36 49 64 81 100
If you ask for v(4:7) you're asking for the fourth, fifth, sixth, and seventh elements of v.
x1 = v(4:7) % [16 25 36 49]
x1 = 1×4
16 25 36 49
If you ask for v(4):v(7) you're asking for the vector 16:49 which is much longer.
x2 = v(4):v(7)
x2 = 1×34
16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
Now imagine that v was not in ascending order.
v2 = flip(v)
v2 = 1×10
100 81 64 49 36 25 16 9 4 1
v2(4:7) is still the fourth through seventh elements of v2, which in this case is x1 flipped.
x3 = v2(4:7) % flip(x1)
x3 = 1×4
49 36 25 16
But v2(4):v2(7) is now 49:16. You can't get from 49 to 16 in steps of +1 so that result is empty.
x4 = v2(4):v2(7)
x4 = 1×0 empty double row vector
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SelDen
SelDen 2021-10-28
Thank you very much. It was very helpful. The array I was working with was [ 1 2 3 4 5 4 3 2 1]. It went downhill when v(i=4) = 4 and v(i=4)=4 and printed out an element as there was nothing between and after that since the numbers were in descending order the result became empty like you said.

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