UPC-A is widely used in the United States and Canada to identify retail products at the time of checkout. A UPC-A symbol has a fixed length of 12 digits, with the last digit served as a checksum. The first digit is called “number system”, followed by the manufacturer code and product code. The Uniform Code Council has the authority assigning manufacturer code. The remaining digits, the product code, is assigned by the manufacturer. This arrangement ensures that every retail proudct has its unqiue identification number.

The UPC-A encoding scheme is relativley complicated. A digit has two different encodings depending on the position of the digit. A complete barcode string contains 15 characters, divided into 7 parts: ^[3]

The left guard and right guard have the same encoding patterns. They can be printed using asterisk, left and right parenthesis or brackets: *, ( and ), or [ and ]. The center guard can be printed using either a vertical bar symbol | or the hyphen -.

The patterns for the left halve digits are represented by 0-9 respectively; and the right halve digits are mapped to A-J:

For example, suppose we wish to encode the number 543000186706. The following three lines produce identical barcodes:

    *543000|BIGHAG*
    (543000-BIGHAG)
    [543000|BIGHAG] 

C Code

The following C code illustrates the check digit calculation and the barcode string generation.

static char barcode_string[20];
int upc_a_check_digit(char* data)
{
	int sum, check_digit;
	char* p;
	sum=0;
	sum = (data[0]+data[2]+data[4]+data[6]+data[8]+data[10])*3 +
          (data[1]+data[3]+data[5]+data[7]+data[9]+data[11]);
	check_digit = 10-(sum %10 );
	if (check_digit==10 ) check_digit=0;
	return check_digit;
}

char* upc_barcode_string(char* data)
{
	char* p;
	int i;
	int check_digit;

	memset(barcode_string, 0, sizeof(barcode_string));
	check_digit=upc_a_check_digit(data);
	barcode_string[0]='[';
	for(i=0; i<6; i++)
		barcode_string[i+1]=data[i];
    barcode_string[7]='|';
    for(i=6; i<10; i++)
	    barcode_string[i+2]=data[i]-'0'+'A';
    barcode_string[13]=check_digit +'A';
    barcode_string[14]=']';
    return barcode_string;
}

BASIC Code

Function upc_a_check_digit(data as String) As Integer
	Dim sum as Integer
	Dim i As Integer
	
	sum=0
	for i=1 to 11 Step 2
		sum = sum + Val(Mid(data,I,1))
	Next i
	sum = sum * 3
	For i=2 to 11 Step 2
		sum=sum+Val(Mid(data, I, 1))
	Next i
	sum = 10 - (10 Mod 10)
	if ( sum=10 ) Then sum=0 End IF
	upc_a_check_digit=sum;
End Function
Function upc_barcode_string(data as String) As String
	Dim check_digit As Integer
	Dim I As Integer
	Dim barcode_string As String
	Dim myChar as Integer
	
	barcode_string="["
	barcode_string = barcode_string & Left$(data, 6) & "|"
	for i=7 to 11
		myChar = Val(Mid(data,I,1))
		myChar = myChar + Asc("A")
		barcode_string=barcode_string & chr(myChar)
	Next i
	myChar = upc_a_check_digit(data)
	barcode_string = barcode_string & chr(myChar) & "]"
	upc_barcode_string=barcode_string
End Function