Python - Count number of rows/columns value appears in

Question

Say I have a 2-dimensional array of strings like so:

A = [['a', 'b', 'b'],
     ['c', 'c', 'a'],
     ['d', 'c', 'a']]

and I want to find out how many rows a given element appears in, so that I get the output:

In [1]: get_number_rows('a')
Out[1]: 3

In [2]: get_number_rows('b')
Out[2]: 1

In [1]: get_number_rows('c')
Out[1]: 2

In [2]: get_number_rows('d')
Out[2]: 1

Note that I don't want the total number of occurrences of 'a', but the number of rows it appears in.

I have tried looping over the rows and simply counting, but I'm dealing with a very large dataset (1000s x 1000s), so it's very slow. Any faster solutions would be appreciated.

Taylor Nelms · Accepted Answer · 2019-02-27 16:16:24Z

1

For rows, try something like

len([x for x in A if 'a' in x])

This list comprehension makes a list of all lists x in A satisfying the condition that 'a' in x. You then take the length of that list to get the total number of them.

answered Feb 27, 2019 at 16:16

Taylor Nelms

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1 Comment

Hannah Over a year ago

Thanks for this, I still struggle a bit with list comprehension!

Alan Kavanagh · Accepted Answer · 2019-02-27 16:14:19Z

1

You can use the following get_number_rows() method to sum if the number of arrays that that contain the character:

A = [['a', 'b', 'b'],
     ['c', 'c', 'a'],
     ['d', 'c', 'a']]

def get_number_rows(char):
    return len([x for x in A if char in x])

get_number_rows('a')
>> 3

get_number_rows('b')
>> 1

get_number_rows('c')
>> 2

get_number_rows('d')
>> 1

answered Feb 27, 2019 at 16:14

Alan Kavanagh

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Comments

Birdman · Accepted Answer · 2019-02-27 16:19:53Z

if __name__ == "__main__":
    A = [['a', 'b', 'b'],
         ['c', 'c', 'a'],
         ['d', 'c', 'a']]

    def get_number_rows(A, desired_element):
        """
        Pass in two dimensional array, A as first parameter
        Pass in desired char element, desired_element as 2nd parameter.
        Function will return number of occurrences of desired_element in A.
        """

        element_count = 0  # Int to keep track of occurrences
        for group in A:  # For nested array in A
            if desired_element in group:  # If our desired element is in the sub array
                element_count += 1  # Increment our counter
        return element_count  # After completion, return the counter

    print(get_number_rows(A, 'a'))

    print(get_number_rows(A, 'b'))

    print(get_number_rows(A, 'c'))

    print(get_number_rows(A, 'd'))

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