python – What is the complexity of the algorithm and how is it calculated?

Question:

What is the complexity of the algorithm for finding a substring in a string, and how is it calculated? Please explain. (the algorithm is crude, but simplified as an example and for understanding)

text='bla-bla-this-bla'
subtext='this'
for i,element in enumerate(text):
    if element == subtext[0]:
        if subtext == text[i:i+len(subtext)]:
            print(i)

Answer:

Let:

n = len(text)
m = len(subtext)

It is obvious that the cycle for make worst case n iterations, in each iteration in the worst case we do slice text[i:i+len(subtext)] , which is performed in O (m) , and then the comparison for the same O (m) , as a result we get O (nm).