Whats happening this code. Anagram as a substring in a string

Question

I found this question posted on here, but couldn't comment or ask a question, so I am creating a new question.

The original post stated the following:

t = "abd"
s = "abdc"

s trivially contains t. However, when you sort them, you get the strings abd and abcd, and the in comparison fails. The sorting gets other letters in the way.

Instead, you need to step through s in chunks the size of t.

t_len = len(t)
s_len = len(s)
t_sort = sorted(t)
for start in range(s_len - t_len + 1):
   chunk = s[start:start+t_len]
   if t_sort == sorted(chunk):
   # SUCCESS!!

In the for loop why are they taking S-len then subtracting t_len? Why are they adding 1 at the end?

Answer 1

alvits and d_void already explained the value of start; I won't repeat that.

I strongly recommend that you learn some basic trace debugging. Insert some useful print statements to follow the execution. For instance:

Code:

t = "goal"
s = "catalogue"

t_len = len(t)
s_len = len(s)
t_sort = sorted(t)
print "lengths & sorted", t_len, s_len, t_sort

for start in range(s_len - t_len + 1):
   chunk = s[start:start+t_len]
   print "LOOP start=", start, "\tchunk=", chunk, sorted(chunk)
   if t_sort == sorted(chunk):
      print "success"

Output:

lengths & sorted 4 9 ['a', 'g', 'l', 'o']
LOOP start= 0   chunk= cata ['a', 'a', 'c', 't']
LOOP start= 1   chunk= atal ['a', 'a', 'l', 't']
LOOP start= 2   chunk= talo ['a', 'l', 'o', 't']
LOOP start= 3   chunk= alog ['a', 'g', 'l', 'o']
success
LOOP start= 4   chunk= logu ['g', 'l', 'o', 'u']
LOOP start= 5   chunk= ogue ['e', 'g', 'o', 'u']

Does that help illustrate what's happening in the loop?