Get the first character from a string?

rubyonrails-talk
1

I've got a math function where I'm converting a base36 number to a base 10 integer and it works wonderfully, except when the first character is a zero. I could get this method to work if I could look at the first position in a string, and if it was a zero, I'd then add a zero to the first position of the resulting string. Here's the equation (and its compliment);

  def calc_ovalue(barcode)     barcode.to_i(36)   end

  def calc_other(barcode)     onum = calc_ovalue(barcode)     if onum.odd?       (onum + 1).to_s(36).upcase     else       (onum - 1).to_s(36).upcase     end   end

some examples of accepeting the first value and calculating the second two are; 0N5F 0N5G ovalue = 30003 0V9B 0V9C ovalue = 40511 6JYV 6JYW ovalue = 305815

Thank you, Kathleen

2

if you have a string @str, then @str[0] will give you the charactercode of the first char that's already enough to test against the charcode of a zero which is 48

if yo prefer to work with a string as result, use @str[0,1] this gives a string result (0 = from position, 1 = length) or @str[0..0] returns a string too, the range is startposition to endposition

3

Assuming that s is the string; s[0] is the first character. s[0].chr == "1" or s[0]==48 (see the irb session below)

a="1234"

=> "1234"

b="01234"

=> "01234"

a[0]

=> 49

a[0].chr

=> "1"

b[0]

=> 48

b[0].chr

=> "0"

Hope that helps, Matt

4

Matt, I'm seeing how to test the first character of the incoming string which is (barcode) (see function)   def calc_other(barcode)     onum = calc_ovalue(barcode)     if onum.odd?       (onum + 1).to_s(36).upcase     else       (onum - 1).to_s(36).upcase     end     if barcode[0] == 48       "0" + return     end   end

But then, I don't see how to attach the preceding zero if the incoming (barcode) has one. I know this looks stupid, but what I'm attempting is to add a zero to the first position of the result. I assume result = string? Thank you, Kathleen

5

will the result always be 4 chars? If so, could you do something like

return_val = my_val.rjust(4).gsub(" ",“0”)

6

Marlon, Yes. The problem is that if I submit a barcode like 0123 the other barcode is calculated as 124. If there is a character other than zero in the first position it works just fine. Where are you suggesting that I put the return_val line of code you show? Is that gsub example putting a zero at the end of the string? What / How would you amend the return value of the calc_other function. Isn't the result of any method the value inside the 'return' keyword? If so, why can't the 'return' value be modified before the function closes? I seem to be having a problem 'seeing the forest through the trees'. Kathleen

7

This is how it could be done:

def calc_ovalue(barcode) barcode.to_i(36) end

def calc_other(barcode) onum = calc_ovalue(barcode) if onum % 2 == 1 onum+=1 else onum-=1 end onum.to_s(36).rjust(4).gsub(" ",“0”).upcase end

8

How about this? All operations here should be (fairly) cheap.

def next_barcode(barcode)   length = barcode.length

  val = barcode.to_i(36)   if val.odd?     val += 1   else     val -= 1   end

  ret = val.to_s(36).upcase   ('0' * (length - ret.length)) + ret end

tests = [ 'AAAAAA', 'AAAAAZ', '012345', '00000Z', '00ZZZZ' ]

tests.each do |test|   puts "#{test} --> #{next_barcode(test)}" end