Find row indices at which a specific string appears in a cell array

Question

I'm trying to find a list of row indices at which a specific string appears in a cell array. For example, for the following cell array:

input = {[546]   'Blah blah blah'
         [783]   'Magic string'
         [1341]  'Blah, blah, other stuff'
         [1455]  'Magic string'
         [1544]  'Another irrelevant string'
         [1700]  'Yet another pointless string'
         [1890]  'Magic string'}

...If I wanted to find the string 'Magic string', and return the row indices of these, I would hope to end up with a vector:

output =

        2   4   7

... Because the 'Magic string' exists on the 2nd, 4th and 7th rows of the input.

I have tried doing the following:

output = strfind({input{:,2}},'Magic string');
output = cell2mat(output);
output = find(output);

However, this fails at the second line, because when the cell2mat() operation is run, it removes all the blanks, rather than returning NaN for these cells. I can't run an isnan() operation on a cell array, so how could I acheive this?

Answer 1

For input

input = {[546]   'Blah blah blah'
         [783]   'Magic string'
         [1341]  'Blah, blah, other stuff'
         [1455]  'Magic string'
         [1544]  'Another irrelevant string'
         [1700]  'Yet another pointless string'
         [1890]  'Magic string'}

just use

output = find(strcmp(input(:,2), 'Magic string'));

Or, if strings can appear in any column,

output = find(any(strcmp(input, 'Magic string'), 2));

The key is that strcmp can be applied to cell arrays containing not only strings. For cells that contain anything other than a string it simply returns 0.