Recursively find a string within a string in C++ - c++

Question

(C++)
Given myString, I want to check if myString contains substring. Here's what I have so far, but it only returns true if the string begins with the substring.
bool find(string myString, string substring)
{
if(mystring.length() < substring.length())
{
return false;
}
if(mystring == substring)
{
return true;
}
for(int i = 0; i < substring.length() - 1 ; ++i)
{
if(mystring.at(i) == substring.at(i))
{
continue;
}
else
{
string string2 = mystring.substr(1, mystring.length() - 1);
return find(string2, substring);
}
return true;
}
return false;
}
What is wrong with this function?

Answer 1

Check this function, it based on your code, with removal of extra code and fix of the errors.
I also changed the signature to get const reference to improve the efficiency.
bool find(const string& myString, const string& substring)
{
if(myString.length() < substring.length()){
return false;
}
else if(myString.substr(0,substring.size()) == substring){
return true;
}
else if (myString.length() > substring.length()){
return find(myString.substr(1), substring);
}
else{
return false;
}
}

Answer 2

First of all the function can be written simpler. For example
bool find( const std::string &myString, const std::string &subString )
{
return
( myString.substr( 0, subString.size() ) == subString ) ||
( subString.size() < myString.size() && find( myString.substr( 1 ), subString ) );
}
Here is a demonstrative program
#include <iostream>
#include <iomanip>
#include <string>
bool find( const std::string &myString, const std::string &subString )
{
return
( myString.substr( 0, subString.size() ) == subString ) ||
( subString.size() < myString.size() && find( myString.substr( 1 ), subString ) );
}
int main()
{
std::cout << std::boolalpha << find( "Hello World", "World" ) << std::endl;
std::cout << std::boolalpha << find( "Hello C", "C++" ) << std::endl;
}
The program output is
true
false
As for your function then it will return true only in the case when the both string have the same length and are equal each other
if(myString == substring){
return true;
}
And in case when myString.length() > substring.length() the function returns nothing
else if (myString.length() > substring.length()){
int start = 1;
int end = (int) myString.length() - 1;
string string2 = myString.substr(start, end);
find(string2, substring);
}
I think you mean
return find(string2, substring);
in this code snippet.
EDIT: I see that you changed the code of the function in your post. But in any case this code snippet
for(int i = 0; i < substring.length() - 1 ; ++i)
{
if(mystring.at(i) == substring.at(i))
{
continue;
}
else
{
string string2 = mystring.substr(1, mystring.length() - 1);
return find(string2, substring);
}
return true;
}
makes no sense.

Answer 3

You're missing a return before the recursive call to find. As it stands it falls through to the return false at the end.
Also, if (mystring == substring) should be checking if mystring starts with substring, not exact equality.

Answer 4

First, this is expensive because of the memory copies in substr.
Second, you havent checked for substring length > 0.
Third, the "else if" check for mystring.length > 0 is redundant if you have done the other checks (including substring length > 0).
Now to your core logic. In the recursion your start is never moving, so you are tied to the beginning. What you need to do is start with position 1, and increment start at every recursion, and also extract using substr the substring from "start" to "start + substring.length". That way you start from the beginning, keep moving forward, and check the correct length. You could also start from the end (as you have) and move back, what you would have to do there is: find sart position (end position minus length of substring), and check that the start position is not less than zero before calling the function recursively.

Answer 5

You're just removing the leftmost characters of myString and then comparing the rest to your substring. Obviously, this is not going to work in a general case, when your substring is somewhere in the middle of myString.
On each iteration try comparing not the whole myString, but rather the first substring.size() characters of it. This should fix your issue.

Answer 6

Here's what I have so far, but it only returns true if the string
begins with the substring.
It also fails for find("foo", "f").
To see why, add some test output to the function:
bool find(string myString, string substring)
{
std::cout << myString << ", " << substring << "\n";
// ...
}
It will print:
foo, f
oo, f
o, f
You see why this cannot work? You just keep removing the first character, until only the last character is compared with the substring to be found.
But it fails even for find("foo", "o"):
foo, o
oo, o
o, o
That's because of this line:
find(string2, substring);
You don't return the result of the recursive call.
All things considered, I think you just have the wrong algorithm here. It simply cannot work the way you have written the code.
A few other observations:
int start = 1;
int end = (int) myString.length() - 1;
That's not good style. For historical reasons, a std::string's size is unsigned, and you are using a C-style cast where static_cast should be preferred. You should just use std::string::size_type here, because it's just an internal piece of implementation code and you gain nothing from casting to int.
string string2 = myString.substr(start, end);
The second argument of substr defines the length of the substring, not the index of the last character. end sounds like you use the value as the index of the last character. Have a look at http://en.cppreference.com/w/cpp/string/basic_string/substr.