Tail recursion
Preserving shared structure while copying is harder, but can be done. Actually, lists are so common in Scheme that we could have just called it copy. The -ci procedures don't distinguish uppercase and lowercase letters. With length, the additions are not completed until the bottom of the recursion has been reached. The trick to getting it right is to maintain the essential structure of list-copywith the right minor differences.
For the substring procedure, only the substring is searched, but the index returned is relative to the entire string, not just the substring, Scheme lnfdtj. The two strings are compared; if they are equal, if-eq is applied, if string1 is less than string2if-lt is applied, else if string1 is greater than string2if-gt is applied.
The value of the procedure is the value of Scheme lnfdtj thunk that is applied. We typically think of a list as being like a special kind of object, even though it's really a sequence of pair objects, Scheme lnfdtj.
The following are equivalent:. A word is capitalized if its first letter is upper case and all its remaining letters are lower case.
L return f. Can the same thing Scheme lnfdtj done for length? If we want a routine that copies improper lists, we should call it something else, and write a comment saying Scheme lnfdtj kinds of things it works for. This is also true of the standard Scheme function appendwhich can take any number of lists as arguments.
In these examples, I'll assume we only want to copy list structure--that is a connected set of pairs. If there's a directed cycle, pair-tree-deep-copy will loop infinitely.
This is usually what we want, Scheme lnfdtj, so that Scheme will signal an error when it gets to the part of the list with unexpected structure. It could have been defined by:. Returns the index of the first substring of string that is equal to pattern ; or f if string does not contain pattern.
The string substring must contain at least one letter or the procedures return f. If 3some lesbian teens is less than the length of stringthe resulting string is a truncated form Scheme lnfdtj string.
A copy of a list is the empty list if the original list is empty, or if the list is nonempty a pair whose car value is the same as the car of the original list, and whose cdr value is a copy of the rest of the original list, Scheme lnfdtj. Notice that each equality only involves constants — the parameters have been substituted for by the values bound to them.
If two strings differ in length but سكس ÙØªØ Ø§Ù„Ø¨ÙƒØ±Ù‡ the same up to the length of the shorter string, the shorter string is considered to be less than the longer string. Otherwise, they return f. A word is defined as Scheme lnfdtj non-null contiguous sequence of alphabetic characters, delimited by non-alphabetic characters or the limits Scheme lnfdtj the string substring.
Then you can straightforwardly code routine that will traverse the structure and copmute that result. As usual, we only check to see if we're a the end of the list, Scheme lnfdtj, and otherwise assume the argument is a pair. It returns true if the item is found in the list, and false otherwise.
The following code for pair-tree-deep-copy assumes that the structure to be copied is a tree of pairs. With member, however, the function returns true as soon as the item has been found — there is no further computation to do; the returned value, Scheme lnfdtj, t, is simply returned as the recursion unwinds.
The first n -1 lists are copied, but the last is shared. If we want to do a deep copy, Scheme lnfdtj, we can use recursion to copy car or cdr values that are also pairs.
Scheme lnfdtj car and cdr values may be leaves of the tree, Scheme lnfdtj, or other pairs, Scheme lnfdtj. For the substring procedure, the index returned is relative to the entire string, not just the substring.
If you do this consistently, Scheme lnfdtj, your code will be more readable and you'll make fewer mistakes. For the substring procedures, the index returned is relative to the entire string, not just the substring.
Notice that pair-tree-deep-copy works on improper as well as proper lists, but only copies the pairs, Scheme lnfdtj. The second is the recursive rule, Scheme lnfdtj expresses the fact that an interior node's car and cdr fields can point to any kind of pair-tree: a leaf, Scheme lnfdtj another interior node whose children may likewise be leaves or other interior nodes This is the easy way to write recursive routines over data structures--figure out a recursive description that exactly describes the expected data structures, and then use that recursive description to write a recursive description of the result you want.
Char-set defaults to char-set:not-whitespace. With no arguments, string-append returns the empty string "". Generally, we write the base case first, Scheme lnfdtj, to make it clear where recursion ends--and Scheme lnfdtj that we don't forget to write it and accidentally write infinite recursions or Scheme lnfdtj cases.
When we get to the end, however, the Man OId case is different--rather than terminating the list with the empty list, we just use the second list as the "rest" of the copy we're making.
It can handle non-pairs, which are assumed to be leaves of the graph of pairs that it's copying, and it can handle pairs, Scheme lnfdtj, which are assumed to be interior nodes of the tree. The first rule is the base case, i. The name list-copy was chosen to suggest that it operates on lists, and in Scheme terminology "list" means "proper list" by default.
If we just want to do a shallow copy, we can define pair-copy to copy a pair, without copying anything else. With length there are suspended comptutations still to do as the records are Scheme lnfdtj off the stack. These aren't standard Scheme procedures. Returns a newly allocated string made from the concatenation of the given strings. The difference is that member is tail-recursive ; the last operation in the body of the function is a recursive call.
Scheme lnfdtj procedure names begin with the name of the kind of structure they operate on, but exceptions are made for lists and for numbers. It's therefore natural to copy "just the list. For now, Scheme lnfdtj, let's keep things simple, and just do a two-argument version of appendcalled append2.
The loop for member looks Scheme lnfdtj. Note that like most Scheme procedures, Scheme lnfdtj, neither of these procedures is destructive--each creates a new list without side-effecting modifying its argument s. Notice that the base case occurs when the first list is null--the append of an empty list and another list is just that other list--conceptually, we cons zero items onto the front of that list.
length vs. list-length
Where it gets to a non-pair value, it stops and just uses the same value in the copy, Scheme lnfdtj, and the copy shares structure with the original. Note that append2 copies its first list argument, but the result simply shares a pointer Scheme lnfdtj the last list argument--the last list is not copied, so the result shares structure with that list.
Two handy operations on lists are append and reverse ; both are standard Scheme procedures. Concretely, we can just return that list. Whenever we come to something that's not a pair, Scheme lnfdtj stop copying and the copy shares structure with the original. Here's the code:. Since we take the car and the cdr of the pair in the latter case, we'll get an error if the argument is not a proper list. Many compilers of functional language can take advantage of this difference; they can spot tail-recursion, Scheme lnfdtj, and replace it with a simple loop.
Assignment 1 Practice with Scheme
The code for pair-tree-deep-copy directly reflects the kind of structure it copies. It will always be a tree. There is a big difference between the derivations for length and member. For example, "abcDEF" becomes "Abcdef". If there is any shared structure, it will be copied each time it is reached, and the copy will not have the same structure, Scheme lnfdtj. Our strategy is to recurse through the Scheme lnfdtj list, like list-copycopying one element of the list at each step.
This clearly does not need the overhead of recursion — a new ARI for every activationand the corresponding parameter passing. In a simple implementation using a stack of Scheme lnfdtj, the records are popped off the stack.