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SICP - Lists, Accumulate and Flatmap, part 2

Published: August 19, 2018
Last Updated: December 5, 2020

In the second post in the little series on lists, accumulate, and flatmap we’re going to take a closer look at accumulate, and some operations on lists that we run into during SICP.

The accumulate procedure is an example of what SICP drills into us in Chapter 1, which is composing higher order procedures which abstract functionality. In section 2.2.3, Sequences as Conventional Interfaces, the text describes a number of common operations which are performed on sequences. It includes such operations as enumeration, filtering, mapping, and accumulation. By organizing programs around these abstractions, it helps to increase the clarity of the code.

In this post we’re going to focus on one of these operations, accumulataion, and we’ll see how a number of other procedures can be written in terms of the accumulate procedure. The text offers more details and examples of incorporating the other generic procedures into programs, and there are a number of exercises for working with them.

The accumulate procedure, simply put, takes an operator (procedure), an initial value, and a sequence. It then applies the operator to the the sequence, recursively building up its return value, using the provided initial value as the base value to be applied. The operator shall be a procedure which takes two arguments, which will be elements of the sequence, or in the base case, an element of the sequence and the initial value.

By virtue of the dynamic typing in Scheme, this allows the accumulate procedure to be applied to differing types, and allows for different types of procedures to be used as the operator as well. For example, to sum up the numbers 1 through 10, we can use accumulate as follows:

(accumulate + 0 '(1 2 3 4 5 6 7 8 9 10))

In this case, we pass the ‘+’ operator as the procedure, and we use 0 as the initial value. If we wanted the product of the numbers from 1 through 10, we would have passed in 1 as the initial value.

It’s important to note, that with accumulate, the value is built from the right to the left. In effect, when the base case is reached, the last element of the sequence and the initial value will have the operator applied, with the element of the sequence being the left hand operand. Then the next to last element of the sequence has its value and the value of the previous computed, and so on. This results in the final value being built from right to left. This is also why the accumulate procedure is also known as fold-right. The opposite, fold-left, applies the initial value to the first value in the sequence, building from left to right. Examining the code at the end of this post in the gist shows this clearly.

This is most clear by examining the results of applying division:

(accumulate / 1 '(2))
; 2

(accumulate / 2 '(1))
; 1/2

The above would have the same result as calling fold-right. The fold-left procedure, shown below, produces the opposite:

(fold-left / 1 '(2))
; 1/2

(fold-left / 2 '(1))
; 2

Here you can see that the final value is built from the left to the right.

Accumulate is a versatile procedure which can be creatively used. SICP provides a number of problems to work on which involve applying the accumulate procedure in a number of ways. Some of the solutions to a few of the problems are shown in the gist below.

For example, it’s possible to build append, map, fold-right, and length all using the accumualte procedure. It’s also possible to reverse a list using fold-right and fold-left.

The code below shows how append is written without accumulate, and then shows how it can be constructed in terms of accumulate. I’ve also shown how map and length can be written in terms of accumulate. Also, the code from the book for fold-left is included.

If you are working through SICP you should take the time to work through these procedures and really understand how they work. Again, working with accumulate and lists and understanding how they are constructed are essential to getting through the text. Also, by this point, you should be somewhat more comfortable formulating solutions recursively. How accumulate works is easy recursion compared to what comes next with trees.

This code should work with DrRacket if you want to try it out by entering some test cases into the REPL.

In the next post in this series we will take a look at flatmap and how it is used in SICP.

© Copyright 2021, Tyler Rhodes