{"id":7141,"date":"2025-09-08T00:01:36","date_gmt":"2025-09-08T07:01:36","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=7141"},"modified":"2025-09-06T14:14:06","modified_gmt":"2025-09-06T21:14:06","slug":"the-reversing-words-filter-solution","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=7141","title":{"rendered":"The Reversing Words Filter – Solution"},"content":{"rendered":"

The task for this month’s C programming Exercise<\/a> is to code a filter that reverses single words. As a filter, the code deals with standard input and output, but it must store this input in word-sized chunks for the reversing process to work.
\n
\nI asked Google how long the longest word is in the English language. It replied that pneumonoultramicroscopicsilicovolcanoconiosis is 45 letters long. So for a word-reversing filter to work, I need a buffer that holds at least 46 characters (pneumonoultramicroscopicsilicovolcanoconiosis plus the null character). Other human languages may require a larger buffer, but the technique works the same.<\/p>\n

Here is my solution:<\/p>\n

2025_09-Exercise.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <ctype.h>\r\n\r\nint main()\r\n{\r\n    \/* the longest legitimate word in English\r\n       is about 45 characters *\/<\/span>\r\n    const int longest = 64;\r\n    int ch,offset;\r\n    char word[longest];\r\n\r\n    offset = 0;\r\n    while( (ch=getc(stdin)) != EOF )\r\n    {\r\n        \/* store alphabetic words *\/<\/span>\r\n        if( isalnum(ch) || ch=='\\'' )\r\n        {\r\n            word[offset] = ch;\r\n            offset++;\r\n            \/* check for overflow *\/<\/span>\r\n            if( offset==longest )\r\n            {\r\n                fprintf(stderr,\"\\nInput overflow\\n\");\r\n                return 1;\r\n            }\r\n        }\r\n        else\r\n        {\r\n            \/* if the buffer has content, output\r\n               it backwards *\/<\/span>\r\n            while(offset>0)\r\n            {\r\n                offset--;\r\n                putc(word[offset],stdout);\r\n            }\r\n            \/* output the non-alpha character *\/<\/span>\r\n            putc(ch,stdout);\r\n        }\r\n    }\r\n\r\n    return 0;\r\n}<\/pre>\n
Constant int<\/em> variable longest<\/code> sets the size of the buffer, with more than enough room for pneumonoultramicroscopicsilicovolcanoconiosis. Integer variable ch<\/code> gobbles input. Variable offset<\/code> tracks word length. Buffer word[]<\/code> holds words as they’re input.<\/p>\n
The while<\/em> loop fetches standard input until the EOF (end-of-file) is encountered, which is a typical construction for a filter. The loop’s if-else<\/em> structure tracks words input, storage, and output.<\/p>\n
The if<\/em> condition uses the isalnum()<\/em> ctype function to test for alphanumeric characters. The ||<\/code> (OR) test checks for the apostrophe to catch contractions and possessives. When the if<\/em> condition is true, the character input is stored in the word[]<\/code> buffer. Variable offset<\/code> is incremented. A nested if<\/em> test checks for overflow just in case. If so, the program terminates.<\/p>\n
The else<\/em> part of the construction deals with non-alphanumeric character input. First, a while<\/em> loop empties any text stored in the word[]<\/code> buffer. The looping condition is offset>0<\/code>, so this loop is skipped when the buffer is empty. Otherwise, text in word[]<\/code> is output one character at a time, using variable offset<\/code> to work backwards through the buffer.<\/p>\n
Whether the while<\/em> loop outputs a backwards word or not, the next statement outputs the non-alpha\/apostrophe character caught by the else<\/em> condition. Variable offset<\/code> is equal to zero at this point, which primes the next spin of the main while<\/em> loop to potentially store the next word.<\/p>\n
Here’s a sample run:<\/p>\n
Here's a sample run:
\ns'ereH a elpmas nur:<\/code><\/p>\n
And, of course:<\/p>\n
pneumonoultramicroscopicsilicovolcanoconiosis
\nsisoinoconaclovociliscipocsorcimartluonomuenp<\/code><\/p>\n
Finally:<\/p>\n
The word pneumonoultramicroscopicsilicovolcanoconiosis is a long word
\nehT drow sisoinoconaclovociliscipocsorcimartluonomuenp si a gnol drow<\/code><\/p>\n
This code took me a few tries to get perfect. One of my drafts output null characters after each reversed word. This output was related to the value of variable offset<\/code> being decremented after the character was output. Null characters don’t appear on the screen. To ensure that your code doesn’t output null characters, run it through a program like hexdump<\/em> and watch for those little 00 bytes.<\/p>\n
I hope that your solution met with success!<\/p>\n","protected":false},"excerpt":{"rendered":"
The task for this month’s C programming Exercise is to code a filter that reverses single words. As a filter, the code deals with standard input and output, but it must store this input in word-sized chunks for the reversing … 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":[5],"tags":[],"class_list":["post-7141","post","type-post","status-publish","format-standard","hentry","category-solution"],"_links":{"self":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7141","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=7141"}],"version-history":[{"count":4,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7141\/revisions"}],"predecessor-version":[{"id":7159,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7141\/revisions\/7159"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7141"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7141"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7141"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}