How can I fill the remainder of the first column of an N x N matrix with 1's, once a certain co-CodePudding

I have an N x N matrix filled with 0's as follows:

str1 = "Patrick"
str2 = "Garrick"

matrix = [[0 for _ in range(len(str1))] for _ in range(len(str2))]

I am iterating over the first column of the matrix, and if a certain condition is met, I would like to fill in the rest of the first column with 1's, how can I do this? Here is my code so far:

for i in range(len(str2)):
    if str2[i] == str1[0]:   #this is the condition!
        matrix[i:][0] = 1
        break
    else:
        matrix[i][0] = 0

But the above gives me an error, since we cannot do matrix[i:][0] = 1, and I do understand why, but how else can I achieve the above?

CodePudding user response：

The pure python answer is that you need to iterate:

flag = 0
for i in range(len(str2)):
    if str2[i] == str1[0]:
        flag = 1
    matrix[i][0] = flag

You can make it simpler by finding the index up front since your matrix is already zeros:

try:
    i = str2.find(str1[9])
except:
    pass
else:
    for j in range(i, len(str2)):
        matrix[j][0] = 1

You can cheat for the first column (won't work for the rest):

try:
    i = str2.find(str1[9])
except:
    pass
else:
    matrix[i:] = [1]   [0 for _ in range(len(str1) - 1)]

If you're willing to use numpy, you can do this without looping explicitly, delegating instead to under-the-hood vectorized loops:

s1 = np.array(list(str1))
s2 = np.array(list(str2))
matrix = np.maximum.accumulate(s2[:, None] == s1, axis=0)

CodePudding user response：

I think this does what you want. Note I'm using different strings as input so the conditional will be be met (several times).

from pprint import pprint

str1 = "Patrick"
str2 = "GaPPicP"

matrix = [[0 for _ in range(len(str1))] for _ in range(len(str2))]

for i in range(len(str2)):
    if str2[i] == str1[0]:   # this is the condition!
        for j in range(len(str2)):
            matrix[j][i] = 1

pprint(matrix)

Output:

[[0, 0, 1, 1, 0, 0, 1],
 [0, 0, 1, 1, 0, 0, 1],
 [0, 0, 1, 1, 0, 0, 1],
 [0, 0, 1, 1, 0, 0, 1],
 [0, 0, 1, 1, 0, 0, 1],
 [0, 0, 1, 1, 0, 0, 1],
 [0, 0, 1, 1, 0, 0, 1]]

CodePudding user response：

You just need to loop through range(i, len(str2)) and assign 1 to matrix[j][0].

Demo:

str1 = "Patrick"
str2 = "Garrick"

matrix = [[0 for _ in range(len(str1))] for _ in range(len(str2))]
print(matrix)

for i in range(len(str2)):
    if str2[i] == str1[0]:  # this is the condition!
        for j in range(i, len(str2)):
            matrix[j][0] = 1
        break
    else:
        matrix[i][0] = 0

print(matrix)

Note: For the given strings, your condition will never become True. You can test it some other values e.g. with str1 = "aatrick".