How to access elements of 1-D array through some other 1-D array?
How to access elements of 1-D array through some other 1-D array?
I am very new to Julia, even new to programming. Therefore, please excuse me for simple doubts.
consider the below Matlab Example:
A=[10; 20; 30; 40; 50];
B=[1; 3; 5];
The result of A(B)=0 in the matlab shall be [0.0 20 0.0 40 0.0].
A(B)=0
[0.0 20 0.0 40 0.0]
How do I achieve the same in Julia for 1-D array??
I have a variable A and B:
A
B
julia> A
5×1 ArrayInt64,2:
10
20
30
40
50
julia> B
2-element ArrayInt64,1:
1
3
5
when I execute this A[[B]]
A[[B]]
ERROR: ArgumentError: invalid index: ArrayInt64,1[[1, 2]]
ERROR: ArgumentError: invalid index: ArrayInt64,1[[1, 2]]
HOWEVER, this statement provides this result:
julia> A[[1, 3 ,5]]
3-element ArrayInt64,1:
5
3
1
Please guide me. I know that Julia has the flat array, but how to access them through any other flat array.
A[[1, 3, 5]]
A[[[1, 3, 5]]]
Hello Crstnbr, Thank you for writing to me. I wish to get the result of Matlab's statement: A(B)=0, in Julia. If I execute this statement: A[B]=0, in Julia, the result is 0; whereas in matlab the result is [0.0 20 0.0 40 0.0].
– NIT_GUP
Sep 8 '18 at 21:35
2 Answers
2
You have an extra pair of brackets.
A[B]
A[ [1; 3; 5] ]
A[ [1, 3, 5] ]
A[ [1 3 5] ]
A[ 1:2:5 ]
all work as desired. You can index an array with any valid index or any collection of indices.
However, A[[B]] tries to index A at the location [1;3;5], which is an error.
A[[B]]
A
[1;3;5]
You can get your desired result by overwriting the elements of A at indices given by B with zeros as follows:
A
B
julia> A=[10; 20; 30; 40; 50];
julia> B=[1; 3; 5];
julia> A[B] .= 0;
julia> A
5-element ArrayInt64,1:
0
20
0
40
0
Here, the dot assignment .= indicates to change the elements of A in-place.
.=
Okay. That is a very basic thing I didn't know. If I can ask you, is there any good reading source where I can start learning these nuances? I appreciate your help in this. Thanks a lot.
– NIT_GUP
Sep 8 '18 at 21:45
In general, the Julia documentation is fairly easy to read. Have a look here and here for example. I also recommend this blog post
– crstnbr
Sep 8 '18 at 22:14
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Hi Nit_GUP, it is not really clear to me what you actually want do do. Regarding your error, it is just the difference between
A[[1, 3, 5]](works fine) andA[[[1, 3, 5]]](errors because a vector of vectors is not a proper vector of indices).– crstnbr
Sep 8 '18 at 20:28