Assuming you mean the Universal Product Code, and assuming you mean computing the numbers into binary, I've linked a page that gets into a lot of detail about it.
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Compute means to figure out the question and get an answer.
I don't understand what you mean by "missing"? Did it fall off the package or did someone steal it? All joking aside, some product manufactures do not have a registered UPC code. Some products do not have a UPC code, because they are part of a bulk package or not meant for resale. If it's important for you to know a UPC code number, you can always look on the package for the name and address of the manufacture or distributor. If you were to contact them. I'm sure they would be happy to give you the info about the UPC number (if they have one).
look-up UPC code
A UPC barcode identifies an item by a series of black lines with numbers underneath. It allows for data on such items to be stored such as what the item is as well as price.
It seems you already have the result. What is it you want to compute, and based on what data?