Find value based on adjacent value condition and insert into new variable

I have a variable that contains four columns. The first column has increment integers from 0 to 50. The fourth column has some random numbers.

Every number in the fourth column is associated with a number from the first column.

For example, 6.9119 is associated with 0 and -10.6901 to 1.

Now I have another variable where the first column has some integers.

These numbers are also between 0 and 50, but numbers might be repetitive in some rows.

I am looking to fill the second column of this new array with the corresponding value from the fourth column of the first array.

For example, everywhere that there is 0 in the first column, the value 6.9119 should be inserted into the second column.

Your help would be welcome.

Answer 1

[Akira Agata]

ismember function will be helpful for this task, like:

% Sample matrix
A = [(0:50)', rand(51,1)];

% Second matrix with index only
B = repelem((0:50)', 2);

% Apply ismember
[~, pt] = ismember(B, A(:,1));

% Add corresponding values to B
B = [B, A(pt, 2)];

% Show the result
disp(B)

         0    0.4116
         0    0.4116
    1.0000    0.9358
    1.0000    0.9358
    2.0000    0.2611
    2.0000    0.2611
    3.0000    0.9179
    3.0000    0.9179
    4.0000    0.1945
    4.0000    0.1945
    5.0000    0.8413
    5.0000    0.8413
    6.0000    0.1172
    6.0000    0.1172
    7.0000    0.8611
    7.0000    0.8611
    8.0000    0.9853
    8.0000    0.9853
    9.0000    0.9129
    9.0000    0.9129
   10.0000    0.5301
   10.0000    0.5301
   11.0000    0.1679
   11.0000    0.1679
   12.0000    0.4269
   12.0000    0.4269
   13.0000    0.3499
   13.0000    0.3499
   14.0000    0.8446
   14.0000    0.8446
   15.0000    0.2920
   15.0000    0.2920
   16.0000    0.6363
   16.0000    0.6363
   17.0000    0.1607
   17.0000    0.1607
   18.0000    0.6755
   18.0000    0.6755
   19.0000    0.5971
   19.0000    0.5971
   20.0000    0.6314
   20.0000    0.6314
   21.0000    0.8578
   21.0000    0.8578
   22.0000    0.4748
   22.0000    0.4748
   23.0000    0.8513
   23.0000    0.8513
   24.0000    0.0575
   24.0000    0.0575
   25.0000    0.1833
   25.0000    0.1833
   26.0000    0.4709
   26.0000    0.4709
   27.0000    0.8045
   27.0000    0.8045
   28.0000    0.4485
   28.0000    0.4485
   29.0000    0.6027
   29.0000    0.6027
   30.0000    0.1016
   30.0000    0.1016
   31.0000    0.5743
   31.0000    0.5743
   32.0000    0.5373
   32.0000    0.5373
   33.0000    0.4140
   33.0000    0.4140
   34.0000    0.2825
   34.0000    0.2825
   35.0000    0.7808
   35.0000    0.7808
   36.0000    0.2294
   36.0000    0.2294
   37.0000    0.9534
   37.0000    0.9534
   38.0000    0.3508
   38.0000    0.3508
   39.0000    0.0983
   39.0000    0.0983
   40.0000    0.7216
   40.0000    0.7216
   41.0000    0.8293
   41.0000    0.8293
   42.0000    0.5657
   42.0000    0.5657
   43.0000    0.0617
   43.0000    0.0617
   44.0000    0.2747
   44.0000    0.2747
   45.0000    0.3750
   45.0000    0.3750
   46.0000    0.0057
   46.0000    0.0057
   47.0000    0.3309
   47.0000    0.3309
   48.0000    0.3858
   48.0000    0.3858
   49.0000    0.6357
   49.0000    0.6357
   50.0000    0.1620
   50.0000    0.1620

[Fernando]

Thank you for your quick and detailed response.