Check if one string is a prefix of another

Question

I have two strings which I\'d like to compare: String and String:. Is there a library function that would return true when passed these two strings

Answer 1

This is both efficient and convenient:

str.compare(0, pre.size(), pre) == 0

compare is fast because it uses the fast traits::compare method and doesn't have to copy any data.

Here, it will compare std::min(str.size(), pre.size()) characters but if the characters in the two ranges are equal it also checks the length of pre and returns a non-zero value if pre is longer than this.

See the documentation at cplusplus.com.

I've written a test program that uses this code to compare prefixes and strings given on the command line.

Answer 2

With string::compare, you should be able to write something like:

bool match = (0==s1.compare(0, min(s1.length(), s2.length()), s2,0,min(s1.length(),s2.length())));

Alternatively, in case we don't want to use the length() member function:

bool isPrefix(string const& s1, string const&s2)
{
    const char*p = s1.c_str();
    const char*q = s2.c_str();
    while (*p&&*q)
        if (*p++!=*q++)
            return false;
    return true;
}

Answer 3

bool IsPrefix(const std::string& prefix, const std::string& whole)
{
  return whole.size() >= prefix.size() && whole.compare(0, prefix.size(), prefix) == 0;
}

Answer 4

std::string(X).find(Y) is zero if and only if Y is a prefix of X

Answer 5

If you can reasonably ignore any multi-byte encodings (say, UTF-8) then you can use strncmp for this:

// Yields true if the string 's' starts with the string 't'.
bool startsWith( const std::string &s, const std::string &t )
{
    return strncmp( s.c_str(), t.c_str(), t.size() ) == 0;
}

If you insist on using a fancy C++ version, you can use the std::equal algorithm (with the added benefit that your function also works for other collections, not just strings):

// Yields true if the string 's' starts with the string 't'.
template <class T>
bool startsWith( const T &s, const T &t )
{
    return s.size() >= t.size() &&
           std::equal( t.begin(), t.end(), s.begin() );
}

Answer 6

What's wrong with the "find" and checking the result for position 0 ?

string a = "String";
string b = "String:";

if(b.find(a) == 0)
{
// Prefix

}
else
{
// No Prefix
}