Cannot get min_element to work in C++

Question

I'm writing a small recursive algorithm. Here is the code:

#include <iostream> #include <vector> #include <algorithm> using namespace std; vector<int> coins; int checkchange(int left) { vector<int> choices (coins.size()); if (left == 0) return 0; else { int min; for (int i=0;i<coins.size();i++) { choices.at(i) = (1 + checkchange(left - coins.at(i))); } return min_element(choices.front(),choices.back()); } } int main() { int N; cin >> N; for (int i=0;i<N;i++) { int c,m,temp,change; cin >> c >> m; for (int j=0;j<c;j++) { cin >> temp; coins.push_back(temp); } for (int j=0;j<m;j++) { cin >> temp; change = checkchange(temp); cout << change; } } return 0; }

I get the following error:

In file included from
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/algorithm:62,
from burningcoins.cpp:3: /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/stl_algo.h:
In function ‘_FIter std::min_element(_FIter, _FIter) [with _FIter =
int]’: burningcoins.cpp:19: instantiated from here
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/stl_algo.h:5998:
error: invalid type argument of ‘unary *’
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/stl_algo.h:5998:
error: invalid type argument of ‘unary *’

I've tried compiling with both g++ and gcc, both give me the same error. What am I doing wrong?

Edit:

New Code:

int checkchange(int left) { vector<int> choices (coins.size()); if (left == 0) return 0; else { for (int i=0;i<coins.size();i++) { choices[i] = (1 + checkchange(left - coins.at(i))); } return *min_element(choices.begin(), choices.end()); } }

New error message:

/tmp/ccV3VLsK.o: In function main': burningcoins.cpp:(.text+0x16a): undefined reference tostd::cin'
burningcoins.cpp:(.text+0x16f): undefined reference to std::basic_istream<char, std::char_traits<char> >::operator>>(int&)' burningcoins.cpp:(.text+0x187): undefined reference tostd::cin'
burningcoins.cpp:(.text+0x18c): undefined reference to std::basic_istream<char, std::char_traits<char> >::operator>>(int&)' burningcoins.cpp:(.text+0x19b): undefined reference tostd::basic_istream >::operator>>(int&)'
burningcoins.cpp:(.text+0x1b0): undefined reference to std::cin' burningcoins.cpp:(.text+0x1b5): undefined reference tostd::basic_istream >::operator>>(int&)'
burningcoins.cpp:(.text+0x1ec): undefined reference to std::cin' burningcoins.cpp:(.text+0x1f1): undefined reference tostd::basic_istream >::operator>>(int&)'
burningcoins.cpp:(.text+0x208): undefined reference to std::cout' burningcoins.cpp:(.text+0x20d): undefined reference tostd::basic_ostream >::operator<<(int)'
/tmp/ccV3VLsK.o: In function __static_initialization_and_destruction_0(int, int)': burningcoins.cpp:(.text+0x261): undefined reference tostd::ios_base::Init::Init()'
burningcoins.cpp:(.text+0x266): undefined reference to std::ios_base::Init::~Init()' /tmp/ccV3VLsK.o: In functionstd::vector >::_M_range_check(unsigned long) const':
burningcoins.cpp:(.text._ZNKSt6vectorIiSaIiEE14_M_range_checkEm[std::vector >::_M_range_check(unsigned long) const]+0x2d): undefined reference to std::__throw_out_of_range(char const*)' /tmp/ccV3VLsK.o: In functionstd::vector >::_M_insert_aux(__gnu_cxx::__normal_iterator > >, int const&)':
burningcoins.cpp:(.text._ZNSt6vectorIiSaIiEE13_M_insert_auxEN9__gnu_cxx17__normal_iteratorIPiS1_EERKi[std::vector >::_M_insert_aux(__gnu_cxx::__normal_iterator > >, int const&)]+0x259): undefined reference to __cxa_begin_catch' burningcoins.cpp:(.text._ZNSt6vectorIiSaIiEE13_M_insert_auxEN9__gnu_cxx17__normal_iteratorIPiS1_EERKi[std::vector<int, std::allocator<int> >::_M_insert_aux(__gnu_cxx::__normal_iterator<int*, std::vector<int, std::allocator<int> > >, int const&)]+0x2be): undefined reference to__cxa_rethrow'
burningcoins.cpp:(.text._ZNSt6vectorIiSaIiEE13_M_insert_auxEN9__gnu_cxx17__normal_iteratorIPiS1_EERKi[std::vector >::_M_insert_aux(__gnu_cxx::__normal_iterator > >, int const&)]+0x2c8): undefined reference to __cxa_end_catch' /tmp/ccV3VLsK.o: In functionstd::vector >::_M_check_len(unsigned long, char const*) const':
burningcoins.cpp:(.text._ZNKSt6vectorIiSaIiEE12_M_check_lenEmPKc[std::vector >::_M_check_len(unsigned long, char const*) const]+0x4c): undefined reference to std::__throw_length_error(char const*)' /tmp/ccV3VLsK.o: In function__gnu_cxx::new_allocator::deallocate(int*, unsigned long)':
burningcoins.cpp:(.text._ZN9__gnu_cxx13new_allocatorIiE10deallocateEPim[__gnu_cxx::new_allocator::deallocate(int*, unsigned long)]+0x1c): undefined reference to operator delete(void*)' /tmp/ccV3VLsK.o: In function__gnu_cxx::new_allocator::allocate(unsigned long, void const*)':
burningcoins.cpp:(.text._ZN9__gnu_cxx13new_allocatorIiE8allocateEmPKv[__gnu_cxx::new_allocator::allocate(unsigned long, void const*)]+0x35): undefined reference to std::__throw_bad_alloc()' burningcoins.cpp:(.text._ZN9__gnu_cxx13new_allocatorIiE8allocateEmPKv[__gnu_cxx::new_allocator<int>::allocate(unsigned long, void const*)]+0x45): undefined reference tooperator new(unsigned long)'
/tmp/ccV3VLsK.o:(.eh_frame+0x12): undefined reference to __gxx_personality_v0' /tmp/ccV3VLsK.o:(.eh_frame+0x4f): undefined reference to__gxx_personality_v0'
collect2: ld returned 1 exit status

Careful with the using namespace std;. You have a variable named min and a function named min. It's not quite a conflict, but it's getting close. — Qaz
– Qaz, Commented Oct 17, 2013 at 20:14
min_element works properly. You're just not using it properly. You might want to change your question title. — syam
– syam, Commented Oct 17, 2013 at 20:15
Yeah I changed it, using it like you proposed and its still not working. — spurra
– spurra, Commented Oct 17, 2013 at 20:31

David G · Accepted Answer · 2013-10-18 16:18:41Z

4

std::min_element takes a range. front and back return references to actual values. You should be using the begin and end methods to return an iterator to the corresponding positions in the vector:

min_element(choices.begin(), choices.end()); // ^^^^^ ^^^

If you find this tedious, you can create a function which wraps around the standard min_element:

template <class Container> auto min_element(Container c) -> decltype(std::min_element(c.begin(), c.end())) { return std::min_element(c.begin(), c.end()); }

And use it as:

min_element(choices);

edited Oct 18, 2013 at 16:18

answered Oct 17, 2013 at 20:13

David G

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10 Comments

spurra Over a year ago

I've changed it to what you said, and get following error: burningcoins.cpp: In function ‘int checkchange(int)’: burningcoins.cpp:19: error: cannot convert ‘__gnu_cxx::__normal_iterator<int*, std::vector<int, std::allocator<int> > >’ to ‘int’ in return I imagine that is because it returns a pointer. But dereferencing it with the star operator gives me even more errors?

David G Over a year ago

@BananaCode Is that the only change you made?

spurra Over a year ago

Yes, it is the only change.

David G Over a year ago

@BananaCode What are some of the errors you get when you dereference. Dereferencing is the right thing to do in this case.

spurra Over a year ago

burningcoins.cpp: In function ‘int checkchange(int)’: burningcoins.cpp:20: error: expected primary-expression before ‘;’ token. This is the relevant part of the code: return min_element(choices.begin(), choices.end())*;

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