More Concise Version of Max/Min Without the Block

More concise version of max/min without the block

['abcd', 'def'].max_by &:length

Finding the element of a Ruby array with the maximum value for a particular attribute

array.max_by do |element|
  element.field
end

Or:

array.max_by(&:field)

ruby comparing groups of objects on a specific property

To expand the answer here:

class Person
  attr_accessor :height

  def initialize(height)
    self.height = height
  end
end

people = [ Person.new(10), Person.new(20), Person.new(30) ]

tallest_person = people.max_by &:height

MIN and MAX in C

Where are MIN and MAX defined in C, if at all?

They aren't.

What is the best way to implement these, as generically and type safe as possible (compiler extensions/builtins for mainstream compilers preferred).

As functions. I wouldn't use macros like #define MIN(X, Y) (((X) < (Y)) ? (X) : (Y)), especially if you plan to deploy your code. Either write your own, use something like standard fmax or fmin, or fix the macro using GCC's typeof (you get typesafety bonus too) in a GCC statement expression:

 #define max(a,b) \
   ({ __typeof__ (a) _a = (a); \
       __typeof__ (b) _b = (b); \
     _a > _b ? _a : _b; })

Everyone says "oh I know about double evaluation, it's no problem" and a few months down the road, you'll be debugging the silliest problems for hours on end.

Note the use of __typeof__ instead of typeof:

If you are writing a header file that
must work when included in ISO C
programs, write __typeof__ instead of
typeof.

Rails: Finding max of array that may contain nil

I'd go for:

shipping_costs.values.map(&:to_i).max

nil.to_i is 0.

Implementing variadic min / max functions

live example

This does perfect forwarding on arguments. It relies on RVO for return values, as it returns a value type regardless of the input types, because common_type does that.

I implemented common_type deduction, allowing mixed types to be passed in, and the "expected" result type output.

We support the min of 1 element, because it makes the code slicker.

#include <utility>
#include <type_traits>

template<typename T>
T vmin(T&&t)
{
  return std::forward<T>(t);
}

template<typename T0, typename T1, typename... Ts>
typename std::common_type<
  T0, T1, Ts...
>::type vmin(T0&& val1, T1&& val2, Ts&&... vs)
{
  if (val2 < val1)
    return vmin(val2, std::forward<Ts>(vs)...);
  else
    return vmin(val1, std::forward<Ts>(vs)...);
}

int main()
{
  std::cout << vmin(3, 2, 0.9, 2, 5) << std::endl;

  std::cout << vmin(3., 1.2, 1.3, 2., 5.2) << std::endl;

  return 0;
}

Now, while the above is a perfectly acceptable solution, it isn't ideal.

The expression ((a<b)?a:b) = 7 is legal C++, but vmin( a, b ) = 7 is not, because std::common_type decays is arguments blindly (caused by what I consider an over-reaction to it returning rvalue references when fed two value-types in an older implementation of std::common_type).

Simply using decltype( true?a:b ) is tempting, but it both results in the rvalue reference problem, and does not support common_type specializations (as an example, std::chrono). So we both want to use common_type and do not want to use it.

Secondly, writing a min function that doesn't support unrelated pointers and does not let the user change the comparison function seems wrong.

So what follows is a more complex version of the above. live example:

#include <iostream>
#include <utility>
#include <type_traits>

namespace my_min {

  // a common_type that when fed lvalue references all of the same type, returns an lvalue reference all of the same type
  // however, it is smart enough to also understand common_type specializations.  This works around a quirk
  // in the standard, where (true?x:y) is an lvalue reference, while common_type< X, Y >::type is not.
  template<typename... Ts>
  struct my_common_type;

  template<typename T>
  struct my_common_type<T>{typedef T type;};

  template<typename T0, typename T1, typename... Ts>
  struct my_common_type<T0, T1, Ts...> {
    typedef typename std::common_type<T0, T1>::type std_type;
    // if the types are the same, don't change them, unlike what common_type does:
    typedef typename std::conditional< std::is_same< T0, T1 >::value,
      T0,
    std_type >::type working_type;
    // Careful!  We do NOT want to return an rvalue reference.  Just return T:
    typedef typename std::conditional<
      std::is_rvalue_reference< working_type >::value,
      typename std::decay< working_type >::type,
      working_type
    >::type common_type_for_first_two;
    // TODO: what about Base& and Derived&?  Returning a Base& might be the right thing to do.
    // on the other hand, that encourages silent slicing.  So maybe not.
    typedef typename my_common_type< common_type_for_first_two, Ts... >::type type;
  };
  template<typename... Ts>
  using my_common_type_t = typename my_common_type<Ts...>::type;
  // not that this returns a value type if t is an rvalue:
  template<typename Picker, typename T>
  T pick(Picker&& /*unused*/, T&&t)
  {
    return std::forward<T>(t);
  }
  // slight optimization would be to make Picker be forward-called at the actual 2-arg case, but I don't care:
  template<typename Picker, typename T0, typename T1, typename... Ts>
  my_common_type_t< T0, T1, Ts...> pick(Picker&& picker, T0&& val1, T1&& val2, Ts&&... vs)
  {
    // if picker doesn't prefer 2 over 1, use 1 -- stability!
    if (picker(val2, val1))
      return pick(std::forward<Picker>(pick), val2, std::forward<Ts>(vs)...);
    else
      return pick(std::forward<Picker>(pick), val1, std::forward<Ts>(vs)...);
  }

  // possibly replace with less<void> in C++1y?
  struct lesser {
    template<typename LHS, typename RHS>
    bool operator()( LHS&& lhs, RHS&& rhs ) const {
      return std::less< typename std::decay<my_common_type_t<LHS, RHS>>::type >()(
          std::forward<LHS>(lhs), std::forward<RHS>(rhs)
      );
    }
  };
  // simply forward to the picked_min function with a smart less than functor
  // note that we support unrelated pointers!
  template<typename... Ts>
  auto min( Ts&&... ts )->decltype( pick( lesser(), std::declval<Ts>()... ) )
  {
    return pick( lesser(), std::forward<Ts>(ts)... );
  }
}

int main()
{
  int x = 7;
  int y = 3;
  int z = -1;
  my_min::min(x, y, z) = 2;
  std::cout << x << "," << y << "," << z << "\n";
  std::cout << my_min::min(3, 2, 0.9, 2, 5) << std::endl;
  std::cout << my_min::min(3., 1.2, 1.3, 2., 5.2) << std::endl;
  return 0;
}

The downside to the above implementation is that most classes do not support operator=(T const&)&&=delete -- ie, they do not block rvalues from being assigned to, which can lead to surprises if one of the types in the min does not . Fundamental types do.

Which is a side note: start deleting your rvalue reference operator=s people.

Minimum value in each row and each column of a matrix - Python

Use numpy.

>>> import numpy as np
>>> matrix =[[12,34,28,16],
...         [13,32,36,12],
...         [15,32,32,14],
...         [11,33,36,10]]
>>> np.min(matrix, axis=1) # computes minimum in each row 
array([12, 12, 14, 10])
>>> np.min(matrix, axis=0) # computes minimum in each column
array([11, 32, 28, 10])

How to use awk to find the largest and smallest numbers?

The problem is the lines if (max < $2) max=$2 cmax=$1 and if (min > $2) min=$2 cmin=$1 the should be if (max < $2){ max=$2; cmax=$1 } and if (min > $2){ min=$2; cmin=$1} respectively. Another error is with the line print cmin min \n cmax max which should at least be print cmin min "\n" cmax max. Errors are easierly spotted when you format the code better:

BEGIN{
    first=1
}
{
    if (first) {  # We should move this to it's own block and test against NR
        max = min = $2
        first = 0
        next
    }    
    if (max < $2) {  # confusing using max to store min
        max=$2 
        cmax=$1 
    }
    if (min > $2) { # and min to store max your operators are the wrong way round
       min=$2 
       cmin=$1
    } 
}
END { 
    print cmin min "\n" cmax max  
}

Your script should now work but still has a few issue, compare with the version below:

NR==1{                   # If we are on the first line in the file
    min=max=$2           # Set initial min/max values
    next                 # Skip to next line
}
{
    if ($2 > max) {      # If the value in field 2 is greater than current max
        max=$2           # Store the new values
        cmax=$1 
    } 
    if ($2 < min) {      # If the value in field 2 is less than current min
        min=$2           # Store the new values
        cmin=$1
    } 
}
END {
    # Use printf to properly format the output
    printf "%s %d\n%s %d\n",cmin,min,cmax,max
}

A side note: if you presort the file on the second field you can be more concise:

sort -nk2 file | awk 'NR==1{print}END{print}' 

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