find previous value in column before specific set value and delete all others

Hello,

i have a vector 31x2 (see attachment). I need the previous values of column1 and value of column1 when value of column2 hit values greater then 120. See example result below

182 0

195 13

197 2

207 10

210 3

219 9

222 3

356 134

367 11

371 4

380 9

383 3

388 5

393 5

396 3

529 133

542 13

545 3

554 9

.... ....

Goal (clamp values just for understanding)

222 (3)

356 (134)

396 (3)

529 (133)

Hope you understand it and can help me out. Thank You!

Answer 1

[Rik]

You can use find to create a vector of indices.

data=[182	    0
195	    13
197	    2
207	    10
210	    3
219	    9
222	    3
356	    134
367	    11
371	    4
380	    9
383	    3
388	    5
393	    5
396	    3
529	    133
542	    13
545	    3
554	    9];
ind=find(data(:,2)>120);
ind=ind+[-1 0];%use implicit expansion (use bsxfun pre-R2016b)
ind=ind.';%transpose to make the indexing work as expected
disp(ind)

     7    15
     8    16

selected=data(ind,:)

selected = 4×2

222 3 356 134 396 3 529 133

disp(selected)

   222     3
   356   134
   396     3
   529   133

Answer 2

[DGM]

Something like this maybe:

load('example.mat')

idx = find(example(:,2)>120);
idx = sort([idx; idx-1]);
valuesiwant = example(idx,1)

gives

valuesiwant =
         222
         356
         396
         529
         569
         702
         739
         874
         911
        1047

Alternatively, you can use logical indexing. This is slightly faster and can probably still be improved:

mask = example(:,2)>120;
mask = mask | [mask(2:end); false];
valuesiwant = example(mask,1)