how to compare 2 columns of a 2d array at a time with columns of another array in python

问题

I have two string arrays each with three columns.I want to compare first two columns of both 2-d arrays(having 3 cols and 4000 rows). if they match then i need those matching values.But my code is not working.Here is a sample.

array1=["1stcolumn...", "2ndColumn...", "3rdColumn..."]
array2=[1stcolumn 2ndColumn 3rdColumn]
if (array1[0]==array2[0] and array1[1]==array2[1]):
       array3.append('matches: {!r}'.format(array1))
print(array3)

回答1:

EDIT

if array1[:2] == array2[:2]: compares all items from the index 0 to 2(2 is not included), and comes up with the same result as if array1[0] == array2[0] and array1[1] == array2[1]:. Also, it is simpler.(Thanks to Wyatt for comment)

If your arrays are 2-dimensional:

def compare_columns(array1, array2):
    if len(array1) != len(array2):
        return False # If row numbers are not same, return false

    for row_number in range(len(array1)):
        if array1[row_number][:2] != array2[row_number][:2]:
            return False # If the content is not equal, return false

    return True # All of the content is equal, the return true

# For example, these are 2-dimensional arrays
array1 = [["1.1", "1.2", "Lord of the Day of Judgment!"],
          ["2.1", "2.2", "Lord of the Day of Judgment!"]]
array2 = [["1.1", "1.2", "مَالِكِ يَوْمِ الدِّينِ"],
          ["2.1", "2.2", "مَالِكِ يَوْمِ الدِّينِ"]]

array3 = []       
if compare_columns(array1, array2):
       array3.append('matches: {!r}'.format(array1))
print(array3)

Output:

["matches: [['1.1', '1.2', 'Lord of the Day of Judgment!'], ['2.1', '2.2', 'Lord of the Day of Judgment!']]"]

BEFORE EDIT:

If your array is one dimensionel, you don't need to say column, it is just item. Then your job is easy like you have done above. Just, you have a few syntax errors. Use this code:

array1 = ["1stcolumn", "2ndColumn", "1-3rdColumn"]
array2 = ["1stcolumn", "2ndColumn", "2-3rdColumn"]
array3 = []
if array1[0] == array2[0] and array1[1] == array2[1]:
       array3.append('matches: {!r}'.format(array1))
print(array3)

Output:

["matches: ['1stcolumn', '2ndColumn', '1-3rdColumn']"]

So, if you have any other problem, let us know.

回答2:

There are a number of errors in this code snippet which will prevent it from even running without errors.

Python lists are declared with commas between elements. For example, a declaration of a list of strings could be:

array1 = ["this", "is", "a", "list"]

Examples of using lists in Python (3) can be found here.
The Python logical 'and' operator is not &. It is and. See this question.
In Python, as in most languages, variables must be declared before they can be referenced. In your code, array3 is never declared. You can always declare an empty list like this:

array3 = []

回答3:

As always we need a sample data set

In [1]: from random import randint

In [2]: a = [[randint(0, 1) for _ in range(3)] for __ in range(10)]

In [3]: b = [[randint(0, 1) for _ in range(3)] for __ in range(10)]

Have a look at it

In [4]: for aa, bb in zip(a, b): print(aa, bb)
[1, 1, 0] [0, 1, 0]
[0, 0, 0] [1, 0, 0]
[1, 1, 0] [1, 1, 0]
[1, 1, 0] [0, 1, 0]
[0, 0, 0] [1, 0, 0]
[0, 0, 0] [1, 0, 1]
[1, 1, 1] [1, 1, 1]
[0, 1, 0] [1, 0, 0]
[1, 0, 1] [1, 0, 1]
[1, 1, 1] [1, 0, 0]

It seems that there are a few candidates... let's see if we can sort out the sub-lists of the first list where the first 2 elements are equal to the corresponding elements of the corresponding sub-list in the second list.

A possible solution involves a list comprehension, using zip to pair corresponding sub-lists and filtering according to our criterium:

In [5]: c = [aa for (aa, bb) in zip(a, b) if aa[:2]==bb[:2]]

where the comparison is done on two slices of the sub-lists, avoiding the use of the logical operator and.

Comparing c with the dump of a and b (see input cell #4)

In [6]: c
Out[6]: [[1, 1, 0], [1, 1, 1], [1, 0, 1]]

In [7]:

it seems to me that the list comprehension procedure here proposed is correct.

来源：https://stackoverflow.com/questions/46867709/how-to-compare-2-columns-of-a-2d-array-at-a-time-with-columns-of-another-array-i

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python

arrays

list

indexing