Why find_if not working in my program?

Question

I have a simple program that calls std::find_if, I think I've pass the first two argument as iterator and the third as a predict, however the code still won't compile, any ideas?

#include <string>
#include <cctype>
#include <algorithm>

bool notspace(char ch);
bool space(char ch);

int main()  {
    typedef std::string::const_iterator iter;
    iter i;
    std::string s = "ab c";
    i = std::find_if(i, s.end(),space);
    return 0;
}

bool space(char ch)  {
    return std::isspace(ch);
}

Error message:

q-isspace.cpp: In function ‘int main()’:
q-isspace.cpp:12:38: error: no matching function for call to ‘find_if(iter&, std::__cxx11::basic_string<char>::iterator, bool (&)(char))’
     i = std::find_if(i, s.end(),space);
                                      ^
In file included from /usr/include/c++/5/algorithm:62:0,
                 from q-isspace.cpp:3:
/usr/include/c++/5/bits/stl_algo.h:3806:5: note: candidate: template<class _IIter, class _Predicate> _IIter std::find_if(_IIter, _IIter, _Predicate)
     find_if(_InputIterator __first, _InputIterator __last,
     ^
/usr/include/c++/5/bits/stl_algo.h:3806:5: note:   template argument deduction/substitution failed:
q-isspace.cpp:12:38: note:   deduced conflicting types for parameter ‘_IIter’ (‘__gnu_cxx::__normal_iterator<const char*, std::__cxx11::basic_string<char> >’ and ‘__gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >’)
     i = std::find_if(i, s.end(),space);

Answer 1

You pass i which is of type std::string::const_iterator(and also uninitialized) as first parameter to std::find_if. Then you pass s.end() which returns a std::string::iterator. The two iterators are of different types, while std::find_if expects them to be of the same type.

The correct rule of thumb, is to pair the calls to begin() and end()

#include <string>
#include <cctype>
#include <algorithm>

bool notspace(char ch);
bool space(char ch);

int main()  {
    typedef std::string::const_iterator iter;
    iter i,j;
    std::string s = "ab c";
    i = std::find_if(s.begin(), s.end(),notspace);
    j = std::find_if(s.begin(), s.end(),space);
    return 0;
}

bool space(char ch)  {
    return std::isspace(ch);
}

bool notspace(char ch)   {
    return !std::isspace(ch);
}

Answer 2

int main()  {
    typedef std::string::const_iterator iter;
    iter i,j;
    std::string s = "ab c";
    i = std::find_if(i, s.end(),notspace);
    j = std::find_if(i, s.end(),space);
    return 0;
}

You're not initalising your i variable, so it is not pointing to anything. This means that [i, s.end()) does not form a valid range, so your call to find_if() will not work correctly.

Try this instead:

i = std::find_if(s.begin(), s.end(), notspace);