boost::lambda expression doesn't compile

Question

I tried to write a function that calculates a hamming distance between two codewords using the boost lambda library. I have the following code:

#include <iostream> #include <numeric> #include <boost/function.hpp> #include <boost/lambda/lambda.hpp> #include <boost/lambda/if.hpp> #include <boost/bind.hpp> #include <boost/array.hpp> template<typename Container> int hammingDistance(Container & a, Container & b) { return std::inner_product( a.begin(), a.end(), b.begin(), (_1 + _2), boost::lambda::if_then_else_return(_1 != _2, 1, 0) ); } int main() { boost::array<int, 3> a = {1, 0, 1}, b = {0, 1, 1}; std::cout << hammingDistance(a, b) << std::endl; }

And the error I am getting is:

HammingDistance.cpp: In function ‘int hammingDistance(Container&, Container&)’: HammingDistance.cpp:15: error: no match for ‘operator+’ in ‘<unnamed>::_1 + <unnamed>::_2’ HammingDistance.cpp:17: error: no match for ‘operator!=’ in ‘<unnamed>::_1 != <unnamed>::_2’ /usr/include/c++/4.3/boost/function/function_base.hpp:757: note: candidates are: bool boost::operator!=(boost::detail::function::useless_clear_type*, const boost::function_base&) /usr/include/c++/4.3/boost/function/function_base.hpp:745: note: bool boost::operator!=(const boost::function_base&, boost::detail::function::useless_clear_type*)

This is the first time I am playing with boost lambda. Please tell me where I am going wrong. Thanks.

EDIT:

Thanks a lot guys! Here is the working code (just for reference):

#include <iostream> #include <numeric> #include <boost/function.hpp> #include <boost/lambda/lambda.hpp> #include <boost/lambda/if.hpp> #include <boost/lambda/bind.hpp> #include <boost/array.hpp> using boost::lambda::_1; using boost::lambda::_2; template<typename Container> int hammingDistance(Container & a, Container & b) { return std::inner_product( a.begin(), a.end(), b.begin(), 0, (_1 + _2), boost::lambda::if_then_else_return(_1 != _2, 1, 0) ); } int main() { boost::array<int, 3> a = {1, 0, 1}, b = {0, 1, 1}; std::cout << hammingDistance(a, b) << std::endl; }

You should call inner_product as std::inner_product(a.begin(), a.end(), b.begin(), 0, ...);. — kennytm
– kennytm, Commented Apr 1, 2010 at 9:14

shoosh · Accepted Answer · 2010-04-01 09:12:57Z

First problem: when using boost/lambda, include <boost/lambda/bind.hpp> instead of <boost/bind.hpp>

Second problem: you need a using namespace boost::lambda after the #includes

still doesn't compile though

Edit:
Third problem - you need 6 arguments for std::inner_product, you're missing an initialization argument. probably add 0 as the forth argument.

Alex Korban · Accepted Answer · 2010-04-01 09:11:08Z

I could be wrong but I think you should have using namespace boost::lambda; before your function as the placeholders (_1, _2 etc.) are in that namespace.

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