# 0×1 empty double column vector

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Patrick Hartzsch on 12 Feb 2020
Commented: Patrick Hartzsch on 15 Feb 2020
Hi,
I have an array of 19x2 double (r_F), contains 19 integers in column 1 and 2. Now I want to do a vlookup of integers that are stored in the array auswahl; the size is 100x1. If the integer of auswahl were find in column 1 of r_F then write the integer of column 2 in the array R_X. The following code do that for me.
R_X = [];
for i = 1:size(auswahl, 1)
index = find(r_F(:, 1) == auswahl(i));
content = r_F(index, 2);
disp(i + ": " + index + " " + content)
R_X = [R_X; content];
end
Strangely enough contains the result on index 23 and 74 0x1 empty double column vector?! Has anyone an idea why this is happen?
Kind regards,
Patrick

#### 1 Comment

Guillaume on 12 Feb 2020
Comment by Patrick Hartzsch originally posted as an answer moved here:
Supplement: I fill the vector auswahl with the following command:
auswahl = round(rand(100,1)*19,0);
It seems, that auswahl contains not only integers.

Guillaume on 12 Feb 2020
a) If you want to generate integers between 0 and 19 use:
auswahl = randi([0 19], 100, 1); %use randi to generate random integers
b) Despite what you may have been taught, loops are rarely needed in matlab and often complicates everything. To keep the elements in the second column of r_F for which the first column is found in auswahl:
R_X = r_F(ismember(r_F(:, 1), auswahl), 2); %keep elements of column 2 of r_F for which column 1 is a member of auswahl

Patrick Hartzsch on 14 Feb 2020
test.m
Guillaume on 14 Feb 2020
I'm afraid I can't see anything wrong with the results produced by your script. Both auswahl and the first column of r_F are guaranteed to contain integers due to the way you construct them.
edit: However note that auswahl contains integers from 0 to 19 included, while r_F contains integers from 1 to 19. Perhaps, you meant to have integers from 1 to 19 in auswahl all along, in which case:
• I did write "If you want to generate integers between 0 and 19 use"
• That's what your original round(rand(100,1)*19,0) did
If you want integers between 1 and 19 (included) then:
auswahl = randi(19, n, 1)
See the documentation of randi.
Patrick Hartzsch on 15 Feb 2020
Yes, that's was the problem. Your last code solve the problem!
Many thanks for your support and patience