Return the longest common Substring Function c ++

Question

string longest_common_Substring(string a, string b) {

string best = ""; 
string subString = ""; 
//vector < vector<int>> s[a.length]; 

int max = a.length() + b.length(); 


for (int i = 0; i <= a.length(); i++)
{
    for (int k = 0; k <= b.length() -i - 1; k++)
    {
        subString = a.substr(i, k);
        if (b.find(subString)== true && subString.length() > best.length())
        {
            best = subString;
        }
    }

}

return best; }

My sample input is longest_common_Substring("the rain", " in spain") and the output is "in". The correct output is "ain"

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Lastly, please search for "longest common substring c++". It will give you a lot of hits, including many here on Stack Overflow. — Some programmer dude, Jun 19 '18 at 07:36
Possible duplicate of [How to find Longest Common Substring using C++](https://stackoverflow.com/questions/10248728/how-to-find-longest-common-substring-using-c) — Fantastic Mr Fox, Jun 19 '18 at 07:39
@FantasticMrFox In this case OP needs to debug and fix their code. I don't think "just remove it and copy another implementation" is an answer. — user202729, Jul 14 '18 at 06:04

Thomas Sablik · Accepted Answer · 2018-06-19T08:26:22.937

0

You should change your if condition. string::find returns the position of the found string or string::npos if no string was found. string::find does not work with true and false. true is converted to 1. Therefor b.find(subString)== true is only true if subString is found at position 1 in b. You can verify this behavior with

#include <iostream>
#include <string>

int main() {
    std::string a = "Text";

    if (a.find("Text") == true) std::cout << 1 << std::endl;
    if (a.find("ext") == true) std::cout << 2 << std::endl;
    if (a.find("xt") == true) std::cout << 3 << std::endl;
    if (a.find("t") == true) std::cout << 4 << std::endl;
    return 0;
}

Output is 2

Try

b.find(subString) != b.npos

to fix your problem.

edited Jun 19 '18 at 08:26

answered Jun 19 '18 at 07:54

Thomas Sablik

16,127
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I added a code to verify my statement. If my explanation is wrong, please correct me. – Thomas Sablik Jun 19 '18 at 08:22
`Only 1 is converted to true` - Correction, anything except 0 is true. – Fantastic Mr Fox Jun 19 '18 at 08:23
Why is the result of my code example `2`? And not also `3` and `4`? Why is the result of the code in the question `in` and not `ain`? – Thomas Sablik Jun 19 '18 at 08:24
1

Because you have it backwards. In this specific conversion, true is converted into 1. – Fantastic Mr Fox Jun 19 '18 at 08:25
Thanks, that explains it. – Thomas Sablik Jun 19 '18 at 08:25
Thank you this fixed my issue. – user2839454 Jun 20 '18 at 05:53

Return the longest common Substring Function c ++

1 Answers1