Solution for Drill: Researching Array Methods

method1_array#transpose.md

Solution for Challenge: Drill: Researching Array Methods. Started 2014-02-12T03:13:11+00:00

METHOD 1: Array#transpose

Array#transpose transposes an array matrix--that is, it flips the rows to columns and columns to rows.

Our boggle board challenge presents a good example for showcasing this method.

dice_grid =               [['b', 'r', 'a', 'e'],
                           ['i', 'o', 'd', 't'],
                           ['e', 'c', 'l', 'r'],
                           ['t', 'a', 'k', 'e']]  
             
dice_grid.transpose #=>
#                          [["b", "i", "e", "t"],
#                           ["r", "o", "c", "a"],
#                           ["a", "d", "l", "k"],
#                           ["e", "t", "r", "e"]]

Most solutions that I saw (including my own)...

def get_col(col)
  dice_grid.map {|row|  row[col]}
end

get_col(0).join('') #=> biet

Another way....

def get_col(col)
  dice_grid.transpose[col]
end

get_col(0).join('')  #=> biet

There are a couple important assumptions/requirements for using this method successfully.

• The input array must a 2D array (an array of arrays). Every element of the array must be an array or else a TypeError is raised.

arr1 = ["a", "b", "c"].transpose #=> TypeError: no implicit conversion of String into Array
arr2 = [1,2,[3,4]].transpose #=> TypeError: no implicit conversion of Fixnum into Array

• The inner arrays must be of the same length or it will raise an IndexError. The inner arrays themselves may hold different combinations of data types (e.g., arrays, hashes, numbers, strings, etc.), but each inner array must contain the same number of elements.

# an array which holds 3 array elements, each of which is a 2-element array;
# one of which contains a 2-element array as one of its elements
arr3 =    [[  1,     2],
           ["a",   "b"],
           [  3, [4,5]]]

# success! 
arr3.transpose #=> 
#  [[1, "a",      3], 
#   [2, "b", [4, 5]]]

# an array which holds 3 array elements;
# of which two are 2-element arrays and one is a 3-element array
arr4 = [[1, 2], ["a", "b"], [3, 4, 5]]

# fail!
arr4.transpose #=> IndexError: element size differs (3 should be 2)

method2_array#count.md

METHOD 2: Array#count

Input: an array object
Output: an integer

By itself, Array#count is pretty straightfoward; it takes an array as input and returns the number of elements in the array as an integer.

array = ['a', 'b', 'b', 'd', 'b']

array.count #=> 5

Array's #count may take an argument or a block.

When an object is passed as an argument, the method returns the integer number of elements for which comparison by ==, as inherited or otherwise defined by the object's class, returns true.

array.count('b') #=> 3

When passed a block, Array#count returns the integer number of elements for which evaluating the block on the input array returns true.

array.count {|x| x >= 'b'} #=> 4