What is the simplest way to extract lengths of NaN sequences from a vector?

What is the simplest way to extract lengths of NaN sequences from a vector? Plus the indices of their locations!

Answer 1

[John D'Errico]

Given sequences in a vector of NaNs, you find the location of the start, then find the end. the length is given by the difference. So how do you do it? SIMPLE. Use string search tools.

S = 1:100;
S(rand(size(S)) > .5) = NaN;

So roughly 50% NaNs. I'd bet there are some sequences in there.

Nanstart = strfind([0,isnan(S)],[0 1]);
Nanend = strfind([isnan(S),0],[1 0]);

The length of each sequence is just 1 more than the difference.

Nanlength = Nanend - Nanstart + 1;

S
S =
  Columns 1 through 31
   NaN     2   NaN     4     5   NaN     7     8   NaN   NaN   NaN   NaN   NaN   NaN    15    16   NaN   NaN    19   NaN   NaN    22    23   NaN    25   NaN    27    28    29    30    31
  Columns 32 through 62
    32   NaN   NaN   NaN   NaN    37   NaN   NaN   NaN    41    42    43   NaN    45   NaN    47    48    49   NaN   NaN   NaN   NaN    54   NaN   NaN    57    58    59    60    61   NaN
  Columns 63 through 93
    63   NaN    65   NaN    67   NaN    69    70   NaN    72    73   NaN    75   NaN    77   NaN   NaN   NaN    81    82   NaN    84    85    86   NaN    88    89   NaN    91   NaN   NaN
  Columns 94 through 100
    94   NaN   NaN   NaN    98   NaN   NaN

Nanstart
Nanstart =
     1     3     6     9    17    20    24    26    33    38    44    46    50    55    62    64    66    68    71    74    76    78    83    87    90    92    95    99

Nanlength
Nanlength =
     1     1     1     6     2     2     1     1     4     3     1     1     4     2     1     1     1     1     1     1     1     3     1     1     1     2     3     2

[Mr M.]

no tricky effects on the ends?

[Bruno Luong]

No

Answer 2

[Bruno Luong]

% a is the input vector
b = isnan(a);
counts = sum(b);
positions = find(b);

[Stephen23]

+1 isnan is correct.

[Mr M.]

No, I want to count sequences! not single elements!

[Bruno Luong]

Sorry, for sequence:

b = isnan(a(:)');
d = diff([0 b 0]);
i1 = find(d==1);
i9 = find(d==-1)-1;
if ~isempty(lgt)
    fprintf('nan from (%d,%d)\n', [i1;i9]);
end