Dealing with arrays of text

30 August 13. [link] PDF version

Dealing with text in C is annoying for several reasons, primary among them the fact that changing a string often involves a memory reallocation. In 21st Century C, I talk about the lovely asprintf function; this was one of the most popular bits of the book; e.g., see the summary on this blog.

Here, I consider the next step: an array of text, which we get when we have text data in a data set. This entry will describe where I wound up in trying to design a setup for dealing with grids of text data.

Last time, I talked about the straightforward gsl_vector and gsl_matrix functions, and the interface functions in Apophenia that smooth over some of the details. Similarly, the text element of the apop_data set is in no way special—it is declared as char ***text—but with the right interface functions, we can use this unremarkable structure reasonably gracefully.

OK, we're done with memory management of strings in C: just use those three functions, and it's all taken care of. Again, other systems do it in other ways, such as generating a struct that includes string metadata, thus forcing users to always use the accessor methods of the object.

Accessing the elements of a 2D grid of structs is also straightforward, once you have the rules down. Given a data set declared via apop_data *td = apop_text_alloc(NULL, 3, 3):

With the matrices and vectors, I was concerned with having a setup that gracefully dealt with matrices, vectors, and scalars. Why should I always have to specify two indices when I sometimes only need one or zero? We can do a similar thing using the rule that *x and x[0] are equivalent in C—you can think of one as syntactic sugar for the other see § of the C standard.

So there's the string-handling system via the apop_data struct. Write using the allocate and add functions, free with the free function, and read directly from the text and textsize elements.

Some bonus functions

Transpose via apop_data_transpose, which transposes both the matrix and text elements of the apop_data set that gets input. This is especially useful when you have a function that needs a list of strings, because the column of strings above was really a list of pointers-to-one-string, but each row of the text array is one pointer-to-strings as desired.

It might be nice to have a few view functions that view columns or subgrids. One could implement them the way that vector views were implemented in the GSL, by setting up a list of starting points and changing the textsizes accordingly. That's a hint, for anybody who's interested in contributing. It's nontrivial, though.

Use apop_text_paste to merge a grid of strings into a single string. This is an emulation of R's paste function, modified to more naturally handle a 2-D grid of text. With creative separators, you can produce all sorts of things with this. E.g., the documentation for the function prints a text grid as an HTML table, by putting the appropriate HTML tags between elements, between rows, and at the head and tail.

Conversely, if you need to split a string into parts, use apop_regex. Using parens in a regular expression tells the regex parser to return those parts of the regex to the user. The POSIX-standard regex parser will return a pointer to the end of the matched string, and we can pick up where that leaves off to parse again. The output from apop_regex will have columns of text corresponding to the parens in your regex, and columns corresponding to repeated application.

Here's an example that takes in a regex and a string, then parses it down and prints the results in a table. This would be pulling teeth in raw C, but with the right interface functions writing this in C is much like writing it in a pointer-free scripting language.

I wrote this as a shell script. Cut and paste it to your command line to have gcc produce a.out and run it as per the examples given.

gcc -xc - `pkg-config --libs --cflags apophenia` -g -Wall --std=gnu99 -O3 << "—-"
#include <apop.h>

int main(int argc, char **argv){
    Apop_stopif(argc == 2, return 1, 0, "Give two arguments: a regex and a string.");
    apop_data *txt;
    int returnval = apop_regex(argv[2], argv[1], &txt);
    Apop_stopif(!returnval, return 1, 0, "No matches found.");
    Apop_stopif(returnval==-1, return 2, 0, "Bad regex.");
    printf("%s", apop_text_paste(txt, .before="∙ [", .between_cols="] [", .between="]\n∙ [", .after="]\n"));

a.out "([[:alpha:]]+), *([-[:digit:].]+)" "{pants, 23} {plants,-14} {dirigibles, 12.81}"
echo —--
a.out "((<[^<]*>)([^<]*)</[^>]*>)" "<b>pants</b> <em>blazers</em>"
echo —--
a.out "([[:alnum:].])" " hello. "

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