Day 18 of 30 Ruby Coding Challenge  Finding the Missing Number Game
We're going to play a game: find the missing number in a given array. This first solution will be not that great but we'll get the job done!
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This is the blog post version of the Youtube video from the 30 Ruby Coding Challenges in 30 Days series
Let’s go straight to the point
Coding Challenge
Create a function that takes an array of numbers between 1 and 10 (excluding one number) and returns the missing number.
A few points:
 The array of numbers will be unsorted (not in order).
 Only one number will be missing.
Examples
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find_missing_num([1, 2, 3, 4, 6, 7, 8, 9, 10]) ➞ 5
find_missing_num([7, 2, 3, 6, 5, 9, 1, 4, 8]) ➞ 10
find_missing_num([10, 5, 1, 2, 4, 6, 8, 3, 9]) ➞ 7
Pretty simple, isn’t it?
Algorithm in Ruby
Let’s break the algorithm down to a few small steps:
Step 1
We need to go through a list of items (an array), going from 1 to 10.
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def find_missing_num(array)
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10].each do item
end
end
array = [1, 2, 3, 4, 6, 7, 8, 9, 10]
find_missing_num(array)
Ruby provides us a really better way to do this:
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def find_missing_num(array)
(1..10).each do item
end
end
array = [1, 2, 3, 4, 6, 7, 8, 9, 10]
find_missing_num(array)
Step 2
We’re going to compare each number (from 1 to 10) with the numbers in the given array.
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def find_missing_num(array)
(1..10).each do item
array.each do number
if item == number
break # breaking the execution
end
end
end
end
array = [1, 2, 3, 4, 6, 7, 8, 9, 10]
find_missing_num(array)
We break the execution once if find the desired number because we don’t need to continue looking for the number
Step 3
Because we want to return the number that wasn’t found in the array, I’m going to create a local variable to indicate that.
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def find_missing_num(array)
(1..10).each do item
found = false
array.each do number
if item == number
found = true
break
end
end
if found = false
return item
end
end
end
array = [1, 2, 3, 4, 6, 7, 8, 9, 10]
find_missing_num(array)
That’s it!
I know, that’s not beautiful but it works : )
Do you have a better/different version? Drop that in the comments and bring more healthy discussions :)
Thanks for the visit and in the next coding challenge we’re going to explore a more decent way to solve this puzzle, see you there!
Don’t forget to come by and say hi!
Published at DZone with permission of Alexandre Gama. See the original article here.
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