2.2. Developing Basic, Recursive List-processing Functions¶
2.2.1. Recursive List Processing Example: sum( list )¶
In the previous section, we introduced three predicates that are part
of the fp module, namely isEq, isZero, and isNull. We will now
introduce some additional predicates and arithmetic functions that we
will then use to write recursive list-processing functions, including
sum, isMember, removeFirst, and subst.
First, to check whether something is a list or not, you must use the
isList function:
> fp.isList( [ ] )
true
> fp.isList( [1,2,3] )
true
> fp.isList( 1 )
false
Second, the two helper functions add and sub perform
the addition and subtraction, respectively, of their two integer
arguments:
> fp.add(2,3)
5
> fp.sub(2,3)
-1
Third, the two predicates isLT and isGT test whether
their first argument is less than or greater than their second argument,
respectively:
> fp.isGT(2,3)
false
> fp.isLT(2,3)
true
We’re now ready to write a recursive sum function that takes in an
integer list and returns the sum of all of the values in the input
list.
> var fp = require('./fp')
> sum( [ 1, 2, 3 ] )
6
> sum( [ ] )
0
> sum( [ 1, -2, 3, -4] )
-2
When we design a recursive algorithm on lists, we must keep in mind the recursive BNF definition of integer lists:
Following the two paths for a <list_of_ints> in this grammar, one for the empty list and one for a non-empty list, leads us to structure a sum function as shown below.
Think about how to complete this function.
Then try the following review problem, which uses similar recursive list-processing logic. Note that this problem is randomized. You must solve it correctly three times in a row to earn the point associated with it.
2.2.2. Recursive List Processing Example: isMember( num, list )¶
Next consider a function isMember that takes in an integer n and an integer list ns and returns true if and only if its first argument is a member of the second argument:
> var fp = require('./fp')
> isMember( 2, [ 1, 2, 3 ] )
true
> isMember( 4, [ 1, 2, 3 ] )
false
> isMember( 2, [ 1, [ 2, 3 ] ] )
false
Note that the second argument in the last call above is not an integer list. Keep in mind the recursive definition of integer lists:
Following this recursive definition, we design a recursive algorithm for isMember using the template provided in the first slide below.
Using a recursive pattern similar to the one for isMember, think about how to design a similar list-processing function removeFirst that takes in an integer n and an integer list l and returns a list identical to l but with the first occurrence of n removed:
> var fp = require('./fp')
> removeFirst(3,[1,2,3])
[ 1, 2 ]
> removeFirst(4,[1,2,3])
[ 1, 2, 3 ]
> removeFirst(2,[1,2,3,2])
[ 1, 3, 2 ]
Once you have the correct logic for removeFirst, consider the following review problem, which asks you to slightly modify removeFirst.
2.2.3. Recursive List Processing Example: subst( new, old, list )¶
As a final example in this section, consider a function that takes in two integers \(n\) (for ’new’) and \(o\) (for ’old’) and an integer list \(l\) and returns a list identical to \(l\) except that all occurrences of \(o\) in \(l\) have been replaced by \(n\):
> var fp = require('./fp')
> subst(10,1,[1,2,3,2,1])
[ 10, 2, 3, 2, 10 ]
> subst(50,5,[1,2,3])
[ 1, 2, 3 ]
> subst(10,1,[[1,2],3])
[ [ 1, 2 ], 3 ]
Note that we stretched the semantics of the subst function a bit since the third argument in the last call above is not an integer list. Again the template for the subst function follows the pattern established by the BNF grammar for a <list_of_ints>.
Now that we have established the correct logic for this function, consider the final review problem for this section, which asks you to slightly modify the subst function.
