In this final example dealing with randomization, you'll collect some statistics on DNA in order to answer the question: on average, what percentage of bases are the same between two random DNA sequences? Although some simple mathematics can answer the question for you, the point of the program is to show that you now have the necessary programming ability to ask and answer questions about your DNA sequences. (If you were using real DNA, say a collection of some particular gene as it appears in several organisms in slightly different forms, the answer would be somewhat more interesting. You may want to try that later.)

So let's generate a set of random DNA, all the same length, then ask the following question about the set. What's the average percentage of positions that are the same between pairs of DNA sequences in this set?

As usual, let's try to sketch an idea of the program in pseudocode:

Generate a set of random DNA sequences, all the same length For each pair of DNA sequences How many positions in the two sequences are identical as a fraction? } Report the mean of the preceding calculations as a percentage

Clearly, to write this code, you can reuse at least some of the work you've already done. You certainly know how to generate a set of random DNA sequences. Also, although you don't have a subroutine that compares, position by position, the bases in two sequences, you know how to look at the positions in DNA strings. So that subroutine shouldn't be hard to write. ...

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