The UPC Bar Code - Reading and checking UPC barcodes

Handed out: 11/29/05
Due: by 10:00pm on 12/05/03

Here are the rules for UPC barcodes:
  1. Barcodes consist of black and white vertical lines of 4 thicknesses.
  2. The four thicknesses, from thinnest to thickest are 1, 2, 3, and 4.
  3. The barcode begins and ends with 3 thinnest vertical bars(black,white,black). Let's call this sequence "the margin" (technically, they are called respectively "left guard bar" and "right guard bar")
  4. In the middle of the barcode are 5 thinnest lines, in sequence, white, black, white, black, white. Let's call this sequence "the middle" (technically it is called "central guard bar").
  5. Between the beginning margin and the middle is the product manufacturer id. Between the middle and the end margin is the item id.
  6. The lines in the manufacturer id (and the same for the item id) are grouped to represent digits. There are in total 12 digits, 6 in the manufacturer id, 6 in the item id. The codes used to represent the digits in the manufacturer id are 
    	     Digit  Thickness	Colors
	     0		3211		WWWBBWB
	     1		2221		WWBBWWB
	     2		2122		WWBWWBB
	     3		1411		WBBBBWB
	     4		1132		BWBBBWW
	     5		1231		WBBWWWB
	     6		1114		WBWBBBB
	     7		1312		WBBBWBB
	     8		1213		WBBWBBB
	     9		3112		WWWBWBB
    You will have noticed that all digits have 4 lines and occupy the same amount of space: if you sum the thicknesses, you always get 7.
  7. The codes used to represent the digits in the item id are the same, but now the colors are reversed. So '0' becomes "BBBWWBW", '1' becomes "BBWWBBW", etcetera.
  8. The last digit of the item id is not really part of the item it, it is a check digit used to determine if there was a read error when scanning the barcode. Thus in the barcode we have: the start margin (BWB), the manufacturer id (6 digits), the middle (WBWBW), the item id (5 digits), the check digit, and the end margin (BWB). The check digit is computed as follows:
    Step 1: Add the digits at odd positions together (the first digit is at position 1)
    Step 2: Multiply this sum by 3
    Step 3: Add the digits at even positions together
    Step 4: Add results of steps 2 and 3.
    Step 5: Subtract this sum from the next multiple of 10: this is the check digit
    For example, if we are given the barcode 036000291452 the computation becomes: sum1 = (0+6+0+2+1+5)*3 = 42; sum2 = 3+0+0+9+4 = 16. sum1+sum2 = 58 Thus the check digit is 60-58 = 2 as in fact is the case (the 12th digit of the barcode is 2)

Additional information about UPC barcodes can be found at http://en.wikipedia.org/wiki/Universal_Product_Code and at http://electronics.howstuffworks.com/upc1.htm

Your program should in a loop prompt the user to enter a barcode in decimal representation (something like 036000291452 above). The program should in correspondence indicate if the check digit is correct. If not it should print out the correct check digit. Then if correct it should convert the decimal representation to a color representation - in the case of 036000291452 the color representation is
BWBWWWBBWBWBBBBWBWBWBBBBWWWBBWBWWWBBWBWWWBBWB
WBWBWBBWBBWWBBBWBWWBBWWBBWWBWWWBBBWWBBBWBBWBBWWBWB
print that color representation, then convert it back to a decimal representation. Of course you should get back the original decimal representation.

Email your program to the Teaching Assistant.