Media Scripting in Scheme Summary: We examine techniques for
doing computation using each element of a list. As you may
recall from our initial discussions of algorithms, there are
four primary kinds of control that help us write algorithms: We
sequence operations, we choose between operations, we
encapsulate groups of operations into functions, and we repeat
operations. At this point, you've learned mechanisms for
sequencing operations (by listing one after another or by
nesting them), for choosing (with when), and for grouping
operations (as procedures). You've also seen some very specific
mechanisms for repeating image-transform! apply a procedure to
each pixel in an image, and its variants) build pixels by
applying a function to the column/row of a position. However,
you have not learned a general mechanism for repeating
operations. It is time to remedy that situation. We will start
by looking at how you iterate over the values in a list. What if
we want to iterate over other kinds of values? Very soon, y