Comparing content of two columns in MATLAB

I need to compare the content of two tables, more exactly two col开发者_高级运维umns (one column per table), in MATLAB to see for each element of the first column, if there is an equal element in the second column.

Should I use a for loop or is there an existing MATLAB function that does this?

If the order is important, you do element-wise comparison, after which you use all

%# create two arrays
A = [1,2;1,3;2,5;3,3];
B = [2,2;1,3;1,5;3,3];

%# compare the second column of A and B, and check if the comparison is `true` for all elements
all(A(:,2)==B(:,2))
ans = 
    1

If the order is unimportant and all elements are unique, use ismember

all(ismember(A(:,1),B(:,1))
ans = 
    1

If the order is unimportant, and there are repetitions, use sort

all(sort(A(:,1))==sort(B(:,2)))
ans = 
    0

did you know you could do this:

>> a = [1:5];
>> b = [5:-1:1];
>> a == b

ans =

     0     0     1     0     0

so you could compare 2 columns in matlab by using the == operator on the whole column. And you could use the result from that as a index specifier to get the equal values. Like this:

>> a(a == b)

ans =

     3

This mean, select all the elements out of a for which a == b is true.

For example you could also select all the elements out of a which are larger than 3:

>> a(a > 3)

ans =

     4     5

Using this knowledge I would say you could solve your problem.

For arithmetic values, both solutions mentioned will work. For strings or cell arrays of strings, use strcmp/strcmpi.

From the help file:

TF = strcmp(C1, C2) compares each element of C1 to the same element in C2, where C1 and C2 are equal-size cell arrays of strings. Input C1 or C2 can also be a character array with the right number of rows. The function returns TF, a logical array that is the same size as C1 and C2, and contains logical 1 (true) for those elements of C1 and C2 that are a match, and logical 0 (false) for those elements that are not.

An example (also from the help file):

Example 2

Create 3 cell arrays of strings:

A = {'MATLAB','SIMULINK';'Toolboxes','The MathWorks'};
B = {'Handle Graphics','Real Time Workshop';'Toolboxes','The MathWorks'};
C = {'handle graphics','Signal Processing';'  Toolboxes', 'The MATHWORKS'};

Compare cell arrays A and B with sensitivity to case:

strcmp(A, B)
ans =
     0     0
     1     1

Compare cell arrays B and C without sensitivity to case. Note that 'Toolboxes' doesn't match because of the leading space characters in C{2,1} that do not appear in B{2,1}:

strcmpi(B, C)
ans =
     1     0
     0     1

To get a single return value rather than an array of logical values, use the all function as explained by Jonas.

You can use for loop (code below) to compare the content of the column 1 and column % 2 in the same table:

clc
d=[     19        24      16      12      35      0    
        16        16      18      0       23      18    
        16        10      7       10      13      24      
        19        8       30      0       12      26      
        16        12      4       1       13      12      
        24        0       31      0       40      0      
        12        11      10      6       20      0      
        16        11      6       2       25      9      
        17        9       21      0       17      8      
        20        0       7       10      22      0      
        13        16      12      18      17      13    
        17        23      17      0       23      20      
        25        0       10      3       17      15      
        14        4       4       17      12      10      
        19        24      21      5       35      0      
        15        20      5       0       10      31      
        13        8       0       16      40      0      
        18        27      26      1       19      14      
        12        0       2       0       12      4      
        20        0       6       2       15      21      
        20        0       26      0       18      26      
        12        11      1       13      19      15      
        14        0       20      0       9       16      
        14        15      6       12      40      0      
        20        0       8       10      18      12      
        10        11      14      0       13      11      
         5        0       22      0       8       12 ];
 x1=d(:,1);
 y1=d(:,2);
 for  i=1:27
     if x1(i)>y1(i);
        z(i)=x1(i);
     else
        z(i)=y1(i);
     end
 end
 z'