问题
bool pred(int k, int l, int num1, int num2)
{
return (num1 < num2);
}
int main()
{
vector <int> nums;
for (int i=50; i > 0; --i)
{
nums.push_back(i);
}
std::sort (nums.begin(), nums.end(), boost::bind(&pred, 5, 45));
}
I am a boost newbie. I was learning to use boost::bind and I wanted to sort a vector of integers and get rid of all those elements in the vector that are greater than 45 and less than 5. Had a hard time doing it. Would be great if anyone could help me do it?
The reason I am facing problem is because I am trying to get rid of a vector element while iterating through the vector to sort it. I know it would be much easier if i sort it first and then remove elements from it. But I want to do it this way. Any help is appreciated.
回答1:
You can't do that from sort
.
Remove the elements before or after sort
.
bool outOfRange(int low, int high, int num) {
return low > num || num > high;
}
...
nums.erase(
std::remove_if(nums.begin(), nums.end(),
boost::bind(&outOfRange, 5, 45, _1)),
nums.end()
);
Though you really don't need boost::bind
at all. Heck, we can make it a bit more generic too:
template<typename T, class cmp = std::less<T> >
struct outOfRange : std::unary_function<T, bool> {
outOfRange(const T &low, const T &high) : low(low), high(high) {}
bool operator()(const T &val) { return cmp()(val, low) || cmp()(high, val); }
const T &low, &high;
}
...
nums.erase(
std::erase_if(nums.begin(), nums.end(), outOfRange<int>(5, 45)),
nums.end()
);
回答2:
There many ways to do it. The easiest is to first remove all unwanted elements and then sort:
bool outsideRange(int num, int min, int max)
{
return (num < min) || (num > max);
}
nums.erase(std::remove_if(nums.begin(),nums.end(),boost::bind(&outsideRange,_1,5,45) ) ); // See comments about remove-erase idiom.
std::sort(nums.begin(),nums.end());
Note that when using boost bind you need to include the placeholder (_1), that tells it which argument is the one being iterated over.
If you prefer to to it in one step, you could conditionally copy all your ints to a multiset, which sort the items for you:
bool outideRange(int num, int min, int max)
{
return (num < min) || (num > max);
}
std::multiset numsInSet;
std::remove_copy_if(
nums.begin(),
nums.end(),
std::inserter(numsInSet,numsInSet.begin()),
boost::bind(&outideRange,_1,5,45) );
回答3:
Your idea is not quite possible as std::sort can only affect the ordering of your vector and cannot modify the values themself.
The closet thing I can think of that would approximate what you want is to have all the valid values (those that are >= 5 and <= 45) come before invalid values but have both the valid and invalid value sorted.
bool pred(int min, int max, int num1, int num2)
{
bool num1_valid = (num1 >= min) && (num1 <= max);
bool num2_valid = (num2 >= min) && (num2 <= max);
if (num1_valid == num2_valid)
{
return num1 < num2;
}
else
{
return num1_valid;
}
}
来源:https://stackoverflow.com/questions/1677211/sort-using-boostbind