spink_credit_check.rb

### Ryan Spink ###

###Asking for CC number:

print "What is your 16 digit credit card number?  >" 

####Storing the 16 digit CC number:

card_number = gets.chomp


###Storing the all the numbers in the CC as separate variables, multiplying every other by two according to the Luhn:

a = card_number[-1].to_i
b = card_number[-2].to_i*2
c = card_number[-3].to_i
d = card_number[-4].to_i*2
e = card_number[-5].to_i
f = card_number[-6].to_i*2
g = card_number[-7].to_i
h = card_number[-8].to_i*2
i = card_number[-9].to_i
j = card_number[-10].to_i*2
k = card_number[-11].to_i
l = card_number[-12].to_i*2
m = card_number[-13].to_i
n = card_number[-14].to_i*2
o = card_number[-15].to_i
p = card_number[-16].to_i*2

###If the odd [doubled] numbers are over 10, add their tens and ones place together:

if b.to_i >= 10
    b = (b.to_s[-1].to_i) + (b.to_s[-2].to_i)
end

if d.to_i >= 10
    d = (d.to_s[-1].to_i) + (d.to_s[-2].to_i)
end

if f.to_i >= 10
    f = (f.to_s[-1].to_i) + (f.to_s[-2].to_i)
end

if h.to_i >= 10
    h = (h.to_s[-1].to_i) + (h.to_s[-2].to_i)
end

if j.to_i >= 10
    j = (j.to_s[-1].to_i) + (j.to_s[-2].to_i)
end

if l.to_i >= 10
    l = (l.to_s[-1].to_i) + (l.to_s[-2].to_i)
end

if n.to_i >= 10
    n = (n.to_s[-1].to_i) + (n.to_s[-2].to_i)
end

if p.to_i >= 10
    p = (p.to_s[-1].to_i) + (p.to_s[-2].to_i)
end

### Add the original and adjusted variables together, as required by the Luhn:

firstcheck = a+b+c+d+e+f+g+h+i+j+k+l+m+n+o+p

###my check to see the final number output:

#puts firstcheck.to_i


###Output. If the ones place of the summed variables is 0, output valid. Else, Invalid. 

if (firstcheck.to_s[-1].to_i) === 0
    puts "Valid"
else
    puts "Invalid"
end

### It does work for the Amex numbers provided.