STL正确使用find_if（）打印出奇数

Question

How can I possibly make use of find_if algorithm from STL to find and print out odd numbers from vector.

Let me just give you an example of what I'm on about:

#include <iostream> 
#include <algorithm> 
#include <vector> 

using namespace std; 


bool isOdd(int x) 
{ 
   return x%2==0; 
} 

int main(void)
{ 
   int tab[]={1,2,3,4,5,6,7,8,9,10}; 
   vector<int> myVec(tab, tab + sizeof(tab)/sizeof(tab[0])); 
   vector<int>::iterator it; 

   //printing out all numbers 

  cout << "Vector contains the following numbers: " << endl; 

  for(it = myVec.begin(), it != myVec.end(), ++it)
  { 
      cout << *it << ' '; 
  } 

 // an unsuccesful attempt to print out odd numbers while using find_if and while loop 

 vector<int>::iterator bound = find_if(myVec.begin(), myVec.end(), isOdd);  

 while(bound != myVec.end())
 {
    cout << *bound << ' '; 
 } 


 }

What is wrong with while loop, I guess it's the core problem of my code. I'm assigning whatever find_if function will return to iterator, and than I simply can't figure out how to cherry pick odd values from vector ;(

Answer 1

The problem is that you are not advancing the iterator in your loop:

while(bound != myVec.end())
{
    cout << *bound << ' '; 
    bound = find_if(bound+1, myVec.end(), isOdd);
}

In C++11 you can use std::next(bound) instead of bound+1 .

Also, your isOdd returns true when the number is even. It should be

bool isOdd(int x) 
{ 
   return x%2 != 0; 
} 

Demo.

Answer 2

Just adding that for this use I'd just use std::copy_if :

std::copy_if(myVec.begin(), myVec.end(), 
             std::ostream_iterator<int>(std::cout, " "), isOdd);

Similarly, the first for loop in your code (and those should be semicolons, not commas) can be replaced with std::copy :

std::copy(myVec.begin(), myVec.end(), std::ostream_iterator<int>(std::cout, " "));

Demo .