Question

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How to find all substrings of a string in PHP
Find all subsets of a list

How can I compute all the possible substrings of a string? For example given a string ABCDE. All its possible substrings will be

A,
B,
C,
D,
E,
AB,
BC,
CD,
DE,
ABC,
BCD,
CDE,
ABCD,
BCDE,
ABCDE

Thanks! A pseudocode will be highly appreciated. 😀

Answer 1

Just use two for-loops:

generate substrings(string):
    for start in [0,1,...,string.length-1]:
        for end in [start,...,string.length-1]:
            yield string[start...end]

You can also do it this way with two for-loops:

generate substrings(string):
    for substringLength in [1,2,...,string.length]:
        for start in range [0,1,...,string.length-substringLength]:
            yield string[start...(start+substringLength-1)]
    yield ""

You probably want to include the empty string "" in the sequence you return as well, as it is a substring of all strings.

You also need to consider if it is valid to yield a duplicate string multiple times (e.g. do you return “ABA” twice as a substring of “ABABA”?). If the answer is no, merely make a hashtable called alreadyYielded, and whenever you yield, abort if you’ve already yielded the string, otherwise add the value to the hashtable in case you see it again. For example:

seen = new HashTable()
...
        substring = string[...]
        if substring not in seen:
            seen.add(substring)
            yield substring
...