How to index on a loop for first occurrence?
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Hi,
I have a set of data which I would like to loop through for each row.
I would like for every row, the first occurrence of a one to cause all other subsequent values in that row to be replaced by a zero.
Does anybody have any advice on how I could go about doing this?
Thanks
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답변 (2개)
Andrei Bobrov
2015년 6월 1일
편집: Andrei Bobrov
2015년 6월 1일
a = [1 0 1 0 0 0 1
1 0 0 0 0 0 0
0 0 0 0 1 0 0
0 0 0 0 0 0 0
0 0 0 1 1 0 0
0 0 0 0 0 0 0
0 0 0 1 0 0 0
0 1 0 0 0 0 0
0 0 0 0 1 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 1 0 0
0 0 0 1 0 0 1
0 0 0 1 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 1
1 0 0 0 0 0 0
0 0 1 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
1 0 0 0 0 1 0
0 0 0 0 0 0 0
0 0 0 0 0 0 1
0 0 0 0 0 0 0
0 0 1 0 0 1 0
0 0 1 0 0 0 0
0 0 0 0 0 1 0
0 0 0 0 1 0 0]
out = cumsum(a==1,2)==1 & a==1;
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Walter Roberson
2015년 9월 9일
편집: Walter Roberson
2015년 9월 9일
The ==1 examines each element independently. It does not process row by row.
The & will result in 1 if both values are true, and 0 otherwise. It is a binary operator. The cumsum(a==1,2)==1 part is going to create a logical matrix the same size as "a" and the a==1 part is going to create a logical matrix the same size as "a", and then the & is going to compare corresponding elements one at a time, returning true only if both are set. Another way of writing the code would be
L1 = cumsum(a==1,2)==1;
L2 = a==1;
out = false(size(L1));
for K = 1 : size(L1,2)
for J = 1 : size(L1,1)
out(J,K) = L1(J,K) & L2(J,K);
end
end
In the case where "a" contains only 0 and 1, then the code could be shorter:
out = cumsum(a,2)==1 & a;
or
out = (cumsum(a,2)==1) .* a;
If "a" can contain values other than 0 and 1, then none of the posted versions in this sub-thread are correct. I would need to think more about a good solution for that case.
Adam
2015년 6월 1일
idx = find( x == 1, 1 );
x( (idx + 1):end ) = 0;
will do this for an array x. I will leave it up to you to make the trivial change to do it in a loop.
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Adam
2015년 6월 1일
편집: Adam
2015년 6월 1일
The final 1 after the comma is the 2nd argument to the find function which tells it to just find the first occurrence rather than all occurrences (or some other number than 1 if explicitly defined).
If you don't add that argument you will get all indices on the row corresponding to ones.
For example:
a = [0 0 1 1 0 0 1]
a =
0 0 1 1 0 0 1
>> find( a == 1 )
ans =
3 4 7
>> find( a == 1, 1 )
ans =
3
When I have something like this I don't fully understand I find it invaluable to just create a quick example on the command line so I can see for myself what is going on. It is one of the big advantages of Matalb over, for example, C++ where you need to do a lot more work to just test to see what simple syntax alternatives give you.
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