{"id":689,"date":"2014-05-17T00:01:21","date_gmt":"2014-05-17T07:01:21","guid":{"rendered":"http:\/\/c-for-dummies.com\/blog\/?p=689"},"modified":"2014-05-24T07:59:12","modified_gmt":"2014-05-24T14:59:12","slug":"a-word-wrap-filter","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=689","title":{"rendered":"A Word Wrap Filter"},"content":{"rendered":"

Unlike static text, streaming text feeds into a program one character at a time. To wrap that text, you need to know all the basics about wrapping text (covered in this Lesson<\/a>), but also how to concoct a filter that buffers its output.
\n
\nA filter is a basic I\/O machine. I write about them in my For Dummies<\/em> C language titles. The program gobbles up input one character at a time, does something to the character(s), then spews output.<\/p>\n

Here is a most basic filter:<\/p>\n

\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    int a;\r\n\r\n    while( (a = getchar()) != EOF)\r\n        putchar(a);\r\n\r\n    return(0);\r\n}<\/pre>\n
The code simply takes input from getchar()<\/em> and sends that to output via putchar()<\/em>. The while<\/em> loop keeps working until the input stream generates an EOF<\/em> (end of file) character.<\/p>\n
When using this code to read standard input, the EOF<\/em> character is Ctrl+D in Unix, Ctrl+Z in Windows. Or just type Ctrl+C to terminate the program.<\/p><\/blockquote>\n
The problem with the code above is that it doesn’t do anything to modify input, so it’s not a very useful filter.<\/p>\n
An example of a more useful filter would be one that wraps text. The code needs to know a wrapping point, what I call the right margin<\/em>. Then it also needs to determine the proper breaking point at which to wrap. The following code shows one way to handle this task.<\/p>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n#include <ctype.h>\r\n\r\nint main()\r\n{\r\n    char *buffer,*space;\r\n    int c,x,right_margin,position;\r\n\r\n    right_margin = 40;      \/* make the margin variable *\/<\/span>\r\n\r\n    \/* allocate space for the buffer *\/<\/span>\r\n    buffer = malloc(sizeof(char) * right_margin);\r\n    if(buffer==NULL)\r\n    {\r\n        perror(\"Unable to allocate buffer\");\r\n        exit(1);\r\n    }\r\n\r\n    *buffer = '\\0';         \/* initialize buffer *\/<\/span>\r\n    space = buffer;         \/* Initialize space *\/<\/span>\r\n    position = 0;           \/*  and position *\/<\/span>\r\n    c = 65;                 \/* Ensure that it loops *\/<\/span>\r\n    while(c)\r\n    {\r\n        c = getchar();          \/* fetch character *\/<\/span>\r\n        *(buffer+position) = c; \/* store character *\/<\/span>\r\n        \/* if c is white space, save its location *\/<\/span>\r\n        if(isspace(c))\r\n            space = buffer+position;\r\n        position++;             \/* buffer index *\/<\/span>\r\n        \/* check to see if the margin has been reached *\/<\/span>\r\n        if(position > right_margin)\r\n        {\r\n            \/* Display the buffer up to the space *\/<\/span>\r\n            for(x=0;x<right_margin;x++)\r\n            {\r\n                if( *(buffer+x)==EOF)   \/* exit on EOF *\/<\/span>\r\n                    exit(0);\r\n                if(buffer+x == space)   \/* stop at w\/s char *\/<\/span>\r\n                    break;\r\n                putchar(*(buffer+x));\r\n            }\r\n            putchar('\\n');\r\n            \/* copy the rest of the buffer to the start *\/<\/span>\r\n            position = 0;\r\n            space++;\r\n            while(space <= buffer+right_margin)\r\n            {\r\n                *(buffer+position) = *(space);\r\n                position++;\r\n                space++;\r\n            }\r\n            \/* Re-initialize the space pointer *\/<\/span>\r\n            space = buffer;\r\n        }\r\n    }\r\n\r\n    return(0);\r\n}<\/pre>\n
Yes! That’s a long chunk of code, but it’s doing a lot. The primary difference between this code and last week’s Lesson (on wrapping existing text) is that this example deals with stream input. That ratchets up the difficulty a notch.<\/p>\n
To demonstrate how the code works you can run the program and type text at the command prompt, or you can redirect input from a file. As an example, assume that the file preamble.txt<\/code> contains the Preamble to the Declaration of Independence. Figure 1 illustrates how this code processes that input.<\/p>\n
\"Figure
Figure 1. Processing a file redirected as input.<\/p><\/div>\n
Click here<\/a> to download a copy of the preamble.txt<\/code> file.<\/p>\n
Next week<\/a>, I’ll present the blow-by-blow of how this code works, along with a video that visually explains what’s going on. I’ll also offer an explanation of the two conditions that cause this code to malfunction.<\/p>\n","protected":false},"excerpt":{"rendered":"
Wrapping streaming text requires that you “look ahead” to see what’s coming up. Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[],"class_list":["post-689","post","type-post","status-publish","format-standard","hentry","category-main"],"_links":{"self":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/689","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=689"}],"version-history":[{"count":11,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/689\/revisions"}],"predecessor-version":[{"id":741,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/689\/revisions\/741"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=689"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=689"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=689"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}