如何不使用内置函数或切片在python中查找字符串的子字符串

Question

I'm trying to work on a basic python program that is to find a substring of a string in python but the challenge was I can't use built functions or slicing.

MAINSTRING = raw_input('Enter a string : ')

print 'You entered %s' %MAINSTRING

isSubString = raw_input('Enter the substring : ')

print 'You entered %s' %isSubString

if isSubString in MAINSTRING:
    print isSubString + " is a substring of " + MAINSTRING

It works but I can't use in syntax which is the frustrating part. I also know to use the slicing method in python but my challenge was to break it to the basics.

Sorry for being so vague, but I just got a hint.

The complete code consist of two 'for' loops, one for the string and one for the substring

Answer 1

To avoid anything other than for loops and a function you could do it like this although I have no idea why. Also you will have to use in which is pointed out in the comments. If a letter in the mainstring matches the first letter in the substring then add it to the temp string t , then go to the next letters in each and see if they match. If the temp string equals the substring then it exists and it will return.

def substring(ss, s):
    x = 0
    t = ""
    for i in range(len(s)):
        if s[i] == ss[x]:
            t += ss[x]
            if t == ss:
                return s
            x+=1
        else:
            x = 0
            t = ""