Which rows of a 2-column array contain elements of 1-column array.

When I look for the elements of 1-column array, a, inside a 2-column array, b, I would like to know which rows of b contain the elements of a.

By using ismember:

[~,i2] = ismember(a,b)

the index i2 indicates the position of the elements of a inside b, as b was a 1-column array, instead of a 2-column array. How can I know in which rows of b I can find the elements of a ?

% here my example:
a = [   6208
        2920
        6630
         725
        7069
         434
        4334
        8343
        2935
        2920
        6976
        3202
        2920
        1409
        4842
        3545
        8079
        3908
        5663
        5333
        3495
        6142
        6866
        6361
        5402
        5731
        5743
        2331
        8083
        6866
        8158
        7537
        5198
        8003
        3514
        6752
        4618
        2577
        1637
        7026
        6842
        1409
         921
        7208
        6866
        7778
        8600
        3571
        4439
        8003
        4288
        7244
        8656
        6519
        7262
        4619
        8478
        5504
        1227
        1778
        8872
        1447
        5027
        6614
        7243
        2639];

b =  [  5446        7487
        2321        2930
        5446        7026
        5333        5783
        4446        5333
         523        1227
        6231        7026
         822        6349
         822        8478
        4439        4921
        5611        8343
        2331        8810
        5333        6231
         523        6614
        4446        7208
        8083        8343
        6231        8478
        6691        8656
        4446        5783
        8083        8656
        6543        7208
        3363        5446
         741        1301
        6614        6842
        2331        5783
        1095        6842
        2321        8478
        4439        5333
        5611        6349
         741        5783
         822        2321
        1227        6349
        2930        7026];

[~,i2] = ismember(a,b)

% desired output (I would like to know the rows of "b" that contain the elements of "a")
b(i2(i2~=0),:)
Index in position 1 exceeds array bounds. Index must not exceed 33.

% eventually, I would like to compare "a(find(i2))" with "b(i2(i2~=0),:)"

Answer 1

[Voss]

ismember returns linear indices as its second output. You can use ind2sub to convert to row + column indices.

a = [   6208
        2920
        6630
         725
        7069
         434
        4334
        8343
        2935
        2920
        6976
        3202
        2920
        1409
        4842
        3545
        8079
        3908
        5663
        5333
        3495
        6142
        6866
        6361
        5402
        5731
        5743
        2331
        8083
        6866
        8158
        7537
        5198
        8003
        3514
        6752
        4618
        2577
        1637
        7026
        6842
        1409
         921
        7208
        6866
        7778
        8600
        3571
        4439
        8003
        4288
        7244
        8656
        6519
        7262
        4619
        8478
        5504
        1227
        1778
        8872
        1447
        5027
        6614
        7243
        2639];

b =  [  5446        7487
        2321        2930
        5446        7026
        5333        5783
        4446        5333
         523        1227
        6231        7026
         822        6349
         822        8478
        4439        4921
        5611        8343
        2331        8810
        5333        6231
         523        6614
        4446        7208
        8083        8343
        6231        8478
        6691        8656
        4446        5783
        8083        8656
        6543        7208
        3363        5446
         741        1301
        6614        6842
        2331        5783
        1095        6842
        2321        8478
        4439        5333
        5611        6349
         741        5783
         822        2321
        1227        6349
        2930        7026];

Notice that b has 33 rows, so for instance the element at linear index 44 is at row 11, column 2 (i.e., all 33 in the first column plus 11 more in the second column).

[~,i2] = ismember(a,b);

% optional: remove all the zeros from i2 (doesn't affect the final result, b(r(r~=0),:))
i2 = i2(i2 > 0) 

i2 = 12×1

44 4 12 16 36 57 48 10 51 42

[r,c] = ind2sub(size(b),i2)

r = 12×1

11 4 12 16 3 24 15 10 18 9

c = 12×1

2 1 1 1 2 2 2 1 2 2

b(r(r~=0),:)

ans = 12×2

5611 8343 5333 5783 2331 8810 8083 8343 5446 7026 6614 6842 4446 7208 4439 4921 6691 8656 822 8478

[Sim]

Thanks a lot @Voss!! The trick was to use ind2sub :-)

[Voss]

You're welcome!