Find the first and last elements of consecutive values in a vector

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RZM 2018-3-27

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评论： Pawel Jastrzebski 2018-3-27

采纳的回答： Stephen23

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Let's say we have an arbitrary vector like this with ascending values:

a= [10 11 12 19 21 25 29 30 31 32 33 39 50];

and we want to find the first and last element of each consecutive (integer) values. The answer could be in this form:

First consecutive values: [10 11 12]
Second consecutive values: [29 30 31 32 33]
CI_f = [10 29]
CI_l = [12 33]

How could we do it in MATLAB without loop?

Is it also possible to put a condition for the length of the consecutive values to be between specific numbers of values, let's say between 3 and 8?

Thanks in advance for helping :)

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Stephen23 2018-3-27

编辑：Stephen23 2018-3-27

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The second sequence seems to have a mistake, given a it should be:

Second consecutive values: [29 30 31 32 33]
CI_f = [10 29]

RZM 2018-3-27

Yes, Thanks.

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Answer 1

Stephen23 2018-3-27

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编辑：Stephen23 2018-3-27

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It is easy to identify the first and last elements of consecutive sequences of values:

>> a = [10 11 12 19 21 25 29 30 31 32 33 39 50];
>> D = diff([0,diff(a)==1,0]);
>> first = a(D>0)
first =
   10   29
>> last = a(D<0)
last =
   12   33

"Is it also possible to put a condition for the length of the consecutive values to be between specific numbers of values, let's say between 3 and 8?"

Yes. Add find and experiment with the differences:

1 + find(D<0) - find(D>0)

Hint: compare against the require limits to generate an index.

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Answer 2

Pawel Jastrzebski 2018-3-27

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As to what you put as an answer, it can be coded this way:

CI_f = [CI{1}(1) CI{2}(1)]
CI_l = [CI{1}(end) CI{2}(end)]

And the output is going to be the same:

>> CI_f
CI_f =
      10    30
>> CI_l
CI_l =
      12    33

However I find it your question completely unclear.

You've got a vector 'a' consisting of 13 elements.
Values from that vector are used to create the cell arrays
1st cell array CI{1} holds 3 elements which are consecutive vales of 'a': indices 1 to 3
2nd cell array CI{2} holds 4 elements which are consecutive vales of 'a': indices 8 to 11
Where's the pattern here?
Are there going to be more cell arrays? I.e. CI{3} holding 5 elements of 'a': what indices??
What are you trying to achieve?
Why CI's are stored as cells? What's wrong with the row vectors?

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Pawel Jastrzebski 2018-3-27

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OK, from what I've seen, you've changed the consecutive values from cells to vectors.

Once you've got your consecutive vectors created , it easy to find the first and the last value in them - again:

a = [10 11 12 19 21 25 29 30 31 32 33 39 50];

CI_1 = [10 11 12];
CI_2 = [30 31 32 33];

% First and last value in each CI vector:

CI_f = [CI_1(1) CI_2(1)]
CI_l = [CI_1(end) CI_2(end)]

What is the rule that decides that CI_1 will have 3 elements, CI_2 will hold 4 elements etc.?
What is the rule that decides that CI_1 1st value = 1st value of a? And then the 1st value of CI_2 = 8th value of a? Are you choosing them randomly?

As to your sub-question:

Is it also possible to put a condition for the length of the consecutive values to be between specific numbers of values, let's say between 3 and 8?

Yes, it is possible.

Test the following code:

a = [10 11 12 19 21 25 29 30 31 32 33 39 50];

% STARTING POINT
% select a random staring point which is within the length of 'a'
StartPoint = randi(length(a))
% LENGTH
% Chose the ranodm lenth
Random_Length = randi([StartPoint length(a)])
CI_3 = a(StartPoint:Random_Length)

RZM 2018-3-27

编辑：RZM 2018-3-27

Thanks but this is still not what I meant. If we start reading "a" from left to right as we normally do, CI_1 is the first sequence of consecutive numbers. The next sequence of consecutive values in "a" is 29 to 33 so the CI_2 is [29 30 31 32 33]. There is no other rule in creation of "a" despite it has to have ascending integer values with growing indices and a sequence here refers to the consecutive ascending values with the difference of "one".

Pawel Jastrzebski 2018-3-27