[STDCXX-1033] incorrect documentation of nth_element in User's Guide

• Type: Bug
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 4.2.1
• Fix Version/s: 4.2.2
• Component/s: Documentation
• Severity: Incorrect Behavior
• Assignee: Martin Sebor
• Reporter: Martin Sebor
• Created: 07/Apr/09 22:29
• Updated: 07/Apr/09 22:49
• Resolved: 07/Apr/09 22:49

Description

From an external source:

There appears to be an error in your documentation here:

http://stdcxx.apache.org/doc/stdlibug/14-3.html#idx353

This is your example:

The example program illustrates finding the fifth largest value in a vector of random numbers.

void nth_element_example()
// illustrates the use of the nth_element algorithm
// see alg7.cpp for complete source code
{
  // make a vector of random integers
  std::vector<int> aVec(10);
  std::generate(aVec.begin(), aVec.end(), randomValue);
 
  // now find the 5th largest
  std::vector<int>::iterator nth = aVec.begin() + 4;
  std::nth_element(aVec.begin(), nth, aVec.end());
 
  std::cout << "fifth largest is " << *nth << std::endl;
}

However nth_element uses < as the default predicate so your example will produce the 5th smallest element, not the 5th largest.

To get the fifth largest you should use std::greater<int>() as a predicate or use the fifth from the other end, i.e. aVec.end() - 5.