Find at least 4 consecutive values less than 1 in an array

I have an array in which I need to find four or more consecutive values that are less than one. For example: M = [1 0 .3 .5 .2 .1 6 7 .3 .5 10 1 .8 .9 .7 .2 .1 .3]

The program should return the indexes 3-6 and 13-18.

How can I go about solving this?

[José-Luis]

That would be 2-6 and 13-18

Answer 1

[Roger Stafford]

Here's another one-liner if you accept initial indices as an answer.

 f = find(all(hankel(M(1:4),M(4:end))<1,2));

[Jos (10584)]

+1

Answer 2

[Azzi Abdelmalek]

Edit

M= [1 0 .3 .5 .2 .1 6 7 .3 .5 10 1 .8 .9 .7 .2 .1 .3]
if diff(size(M))<0
 M=M'
end
a=M<1;
b=strrep(num2str(a),' ','');
[ii,jj]=regexp(b,'1+','start','end')
d=jj-ii>=4
out=[ii;jj];
out=out(:,d)

[Neha]

I got an error when I tried this:

Error using strrep Input strings must have one row.

Error in accelfilter (line 636) b=strrep(num2str(a),' ','');

[Azzi Abdelmalek]

If you copy and paste the code, there is no error, unless you are using M that is not a 1xn array. What is the size of M

size(M)

[Azzi Abdelmalek]

If M is a column array, just add at the beginning of your code

M=M'

Answer 3

[Azzi Abdelmalek]

Edit

M= [1 0 .3 .5 .2 .1 6 7 .3 .5 10 1 .8 .9 .7 .2 .1 .3];
if diff(size(M))<0
 M=M'
end
a=[0 M<1 0];
ii=strfind(a,[0 1]);
jj=strfind(a,[1 0])-1;
d=jj-ii>=4;
out=[ii;jj];
out=out(:,d)

[Neha]

suppose that the array M length could change? How can I make the code still work for a matrix of size (1,n)?

[Azzi Abdelmalek]

It doesn't matter if the length changes. Maybe you are asking if the size changes, from 1xn to nx1. In this case we can edit our code. Look at edited answesrs

[Neha]

Okay ,yes that works. Thank you very much.

Answer 4

[José-Luis]

M = [1 0 .3 .5 .2 .1 6 7 .3 .5 10 1 .8 .9 .7 .2 .1 .3];
numVal = 4;
your_vals = conv(double(M<1),ones(1,numVal)/numVal,'valid') == 1;
start = find(diff([0 your_vals]) == 1)
finish = fliplr(numel(M) + 1 - find(diff([0 fliplr(your_vals)]) == 1))

Answer 5

[Andrei Bobrov]

t = M<1&M>0;
t1 = [true;diff(t(:))~=0];
idx=accumarray(cumsum(t1),(1:numel(M))',[],@(x){x});
ii = idx(t(t1));
out = ii(cellfun(@numel,ii)>=4);

Answer 6

[Jos (10584)]

Here is a relatively simple one-liner:

M = [1 0 .3 .5 .2 .1 6 7 .3 .5 10 1 .8 .9 .7 .2 .1 .3] % example data
[startIDX, endIDX] = regexp(char((M(:).'< 1)+'0'), '1111+')