Flattening multidimensional linked lists, part 1  Parsing
Further ideas on parsers and flattening data structures
Recently, I have been writing a lot about flattening multidimensional arrays. The next series of posts will be along the same lines, but this time we will be discussing multidimensional linked lists.
Ok then, so what is a multidimensional linked list? Basically, it's a set of linked lists connected together, such that you can depict them into a multilayer structure. Something like this:
1  2  3  4
 
5  6 7

represents a pointer to the next element (as usual in linked lists), and 
represents a connection to another "dimension". There are 3 lists in that example: the list 1  2  3  4
, the list 5  6
, and finally, the list 7
. This can be represented in a C structure in the following way:
struct mdl_node {
int val;
struct mdl_node *child;
struct mdl_node *next;
};
Where child
is the representation for 
, and next
has the usual meaning in linked lists. Note that both pointers are unidirectional; this means that 2
only has references to 3
and 5
, and 5
only has a reference to 6
.
And now, the question:
Given a multidimensional linked list structure, where each node can have a pointer to a child and a pointer to the next node, write a function, flatten(), that receives a pointer to the head node, and flattens the list, making it possible to print every element on the list starting on the head and just following "next" pointers.
I won't be coding this now  today, we're going to be developing a format to read multidimensional linked lists from input and build them according to struct mdl_node
, which is itself a pretty nice exercise, and perhaps as complex as the question. Part 2 will cover the implementation of flatten()
. So, can you think of a suitable textual representation to describe a multidimensional linked list? It should be easy to parse, and it should also be nice for a human reader  we don't want to get all confused when inputing data. I challenge the reader to think about it before moving on  this is not fun if you jump right into the solution!
The first step is to devise a representation for a regular linked list. Choosing a format to read a linked list is easy. We can go with:
x>y>z>...
x
, y
, and z
represent numbers. For example, if we wanted to write the list 1  2  3  4
, our input would be.
1>2>3>4
The grammar that describes this is simple:
list > node
 node '>' list
By the way, this article, once again, shows the grammar using a yacclike notation. Terminals are surrounded with single quotes.
That was quick, and to the point  we don't need anything more complex than that. Now, how can this be extended to handle multidimensional linked lists? To the unexperienced, this may seem hard at first because we as humans are used to picturing this using a multilayer structure that expands into multiple lines. Parsing something like that would be extremely difficult and is utterly unnecessary, it is easier to read everything in one line.
As it turns out, it is not that hard to invent some representation. In fact, we just need to insert another list after a node, in such a way that we can say if that list is a child of the current node, or if it's just what next
points to. I chose to represent a child list within a pair of parentheses. So, now, when a node has a child, there's an opening parenthesis, the child structure is represented, there's a closing parenthesis, and finally, if the node has next
elements, the usual representation is used.
An example should clear any doubts remaining. Let's look at our earlier list:
1  2  3  4
 
5  6 7
1  2  3  4
can be represented as:
1>2>3>4
2
's child is 5>6
, we can say so by writing:
1>2(5>6)>3>4
4
's child is 7
, so our final representation is:
1>2(5>6)>3>4(7)
Note that child lists can have children themselves, the notation recursively applies to each one of them. Let's look at another example:
1  2  3  4  5  6  7  8
  
9 28  29  30 31  32  33

10  18  19  20  21  22  23  42  43  44  45  46
   
11  12  13  14 24  25  26 47 48
  
15  16 17 27
 
39 34  35  36  37
 
40  41 38
Holy crap, what now? As I said, the notation conveniently applies itself recursively. Here's the textual representation of the above example:
1(9(10(11>12(15(39(40>41))>16)>13>14(17(34(38)>35>36>37)))>18>19>20>21>22(24>25(27)>26)>23>42>43>44>45(47)>46(48)))>2>3(28>29>30)>4>5>6>7(31>32>33)>8
Let's code the parser for this thing. Before jumping into code, we should define the grammar, so as not to lose ourselves in the way. The grammar is:
mdlist > node
 node '>' mdlist
node > id
 id '(' mdlist ')'
id > integer (..., 2, 1, 0, 1, 2, ...)
The starting rule is mdlist
. You should convince yourself that this grammar is correct and fully describes the notation chosen for multidimensional lists. Now, all that's left is, as Jon Bentley says, A Small Matter of Programming.
Parsers are generally built on top of tokens that are read, recognized, and returned by a token function. We will call this function next_token()
. For this exercise, tokens can be one of (
, )
, >
, and integers. For the purposes of our small program, we can assume that there is a globally known variable with enough information holding the result of the last invocation of next_token()
. This variable must be capable of telling us what's the type of the token (parenthesis, number, etc), and its value. We can do so with a simple structure:
struct mdl_token {
enum token_type {
NUMBER,
OPEN_P = '(',
CLOSE_P = ')',
PTR = '>',
END
} type;
int n; /* meaningless if type != NUMBER */
} token;
next_token()
will read from stdin
and place the new token read in token
. We can then use token.type
to learn about what was just read, and it if was a number, we can read its value in token.n
. END
is just a special token type that denotes the end of input.
With this in mind, we can write our next_token()
function:
void next_token(void) {
int c;
c = getchar();
if (c == '(')
token.type = OPEN_P;
else if (c == ')')
token.type = CLOSE_P;
else if (c == '>')
token.type = PTR;
else if (isdigit(c)  c == '') {
ungetc(c, stdin);
scanf("%d", &token.n);
token.type = NUMBER;
}
else
token.type = END;
}
The call ungetc(c, stdin)
is necessary because by the time next_token()
realizes it read a digit from input, it must push this digit back to make it available to scanf()
. The explicit test c == ''
is there to support negative numbers. It is assumed that any invalid token signalizes end of input.
The parser routines are built on top of next_token()
. The idea is to bind every I/O operation on next_token()
, so that the rest of the parser can think about tokens rather than single characters coming from input.
Now comes the fun part: implementing the grammar. Recall that our grammar for multidimensional linked lists is:
mdlist > node
 node '>' mdlist
node > id
 id '(' mdlist ')'
id > integer (..., 2, 1, 0, 1, 2, ...)
Basically, each nonterminal is turned into a function that is smart enough to recognize which rule is selected to expand. md_list()
will implement the logic for the expansions in mdlist
, and md_node()
will implement the logic for node
. There is no need to implement an equivalent function for id
because that's part of next_token()
's job.
For example, md_list()
, which is the entry point, will call md_node()
to read the leading node. Note that this node can be an arbitrarily complex child list, but after md_node()
returns, we have processed node
from mdlist
in the grammar. Now, md_list()
must look at the next token to decide which rule to expand  both of the options start with node
, so we can only tell them apart when we look at the next token after processing node
. The choice is simple: in the absence of further tokens, the first rule is chosen and md_list()
returns; if the next token is >
, md_list()
recursively calls itself to process the rest of the list, and the cycle begins again. Eventually, md_list()
will find an empty token sequence after processing node
, reaching the base case.
The implementation for md_node()
is similar, and it follows the grammar appearance: unsurprisingly enough, it starts by reading an integer, and then it decides whether to call md_list()
(which in turn will call md_node()
again) based on the next token  it will restart the cycle if the next token is an opening parenthesis. However, notice, if you will, that inside md_node()
there is further processing to be made after md_list()
returns: we have a closing parenthesis to read and consume (otherwise, that would be seen by the md_list()
instance that invoked the current md_node()
 it is very easy to screw things up in code like this).
So basically, this is what we end up with:
struct mdl_node {
int val;
struct mdl_node *child;
struct mdl_node *next;
};
struct mdl_node *md_node(void);
struct mdl_node *md_list(void) {
struct mdl_node *n = md_node();
if (token.type == PTR) {
next_token();
n>next = md_list();
}
return n;
}
struct mdl_node *md_node(void) {
struct mdl_node *n = malloc(sizeof(*n));
n>val = token.n;
n>child = n>next = NULL;
next_token();
if (token.type == OPEN_P) {
next_token();
n>child = md_list();
if (token.type != CLOSE_P)
fprintf(stderr, "Missing close parentheses\n");
else
next_token();
}
return n;
}
As usual in this kind of stuff, we need a lookahead token, so that the parser knows which rule to expand next. This is assured by using the convention that both md_list()
and md_node()
always read one more token than they need, so that it is easy to look at the next token and take appropriate action. It is a good exercise to try to implement your own version of the parser based on the grammar I presented here, and to compare it with my code. It may look easy, or it may look scary and confusing. I must confess, had I not wrote down the grammar before implementing the parser, I would definitely get it wrong, and readers of the code would stare at it wondering how and why it magically works.
Stay tuned for the next article, where we will be covering the implementation for flatten()
.