Hash data type

The last Perl data type that will be considered is the hash. Hashes are similar to arrays in many respects. They hold multiple values, they can be indexed, and they can be looped over, one element at a time. What distinguishes hashes from arrays is that the elements of a hash are unordered, and the indexes are not numbers but strings. A few examples will clarify this ... 

The threat of inappropriate remote control can be addressed by requiring a validated electronic key to be transmitted with the command (Harris et al 1999). In the case of the first and second of the command types discussed in Section 15.1.3, this can be done by first calculating a hash or checksum values for the command message. A checksum or hash is a value calculated from the transmitted data, which is sensitive to any modification of the original data. Many such algorithms exist in the public domain (Schneier, 2000), e.g. CRC, RMD160. 

Now, suppose that Jessica Mosher Pollard, bom on October 19, 1970, needs to be assigned a number. The alternative month code has already been used, so a collision will occur unless the hashing function is somehow altered. Without such an alteration, her driver s license number will be either POLLAJM304PR or POLLAJM3047R. Either way, two people will be assigned the same number and a collision will occur. It is because the function is not one-to-one that problems result. When Jessica s number is typed into a computer, either Joseph s or John s records may be retrieved. Consequently, having no collisions (a one-to-one function) is very desirable. This function should be onto, as well. Otherwise, extra numbers will be floating around in the system without any drivers associated with them. This could cause data entry and retrieval problems. 

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