Databases
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  1. PO
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    <p>Your recent update states that the file is really a database, in which you are looking for a word. This is very important.</p> <p>If you have enough memory to hold the whole database, you should do just that (read the whole database and arrange it for efficient searching), so you should probably <em>not</em> ask about searching in a file.</p> <p>Good database designs involve data structures like <a href="http://en.wikipedia.org/wiki/Trie" rel="nofollow">trie</a> and <a href="http://en.wikipedia.org/wiki/Hash_table" rel="nofollow">hash table</a>. But for a start, you could use the most basic improvement of the database - holding the words in alphabetical order (use the somewhat tricky <a href="http://en.cppreference.com/w/c/algorithm/qsort" rel="nofollow">qsort</a> function to achieve that).</p> <pre><code>struct Database { size_t count; struct Entry // not sure about C syntax here; I usually code in C++; sorry { char *word; char *explanation; } *entries; }; char *find_explanation_of_word(struct Database* db, char *word) { for (size_t i = 0; i &lt; db-&gt;count; i++) { int result = strcmp(db-&gt;entries[i].word, word); if (result == 0) return db-&gt;entries[i].explanation; else if (result &gt; 0) break; // if the database is sorted, this means word is not found } return NULL; // not found } </code></pre> <p>If your database is too big to hold in memory, you should use a trie that holds just the beginnings of the words in the database; for each beginning of a word, have a file offset at which to start scanning the file.</p> <pre><code>char* find_explanation_in_file(FILE *f, long offset, char *word) { fseek(f, offset, SEEK_SET); char line[100]; // 100 should be greater than max line in file while (line, sizeof(line), f) { char *word_in_file = strtok(line, " "); char *explanation = strtok(NULL, ""); int result = strcmp(word_in_file, word); if (result == 0) return explanation; else if (result &gt; 0) break; } return NULL; // not found } </code></pre>
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    1. POHow would I compare a string (entered by the user) to the first word of a line in a file?
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    1. COVery good solution to the underlying problem, with excellent points about efficiency etc - but I think it's actually not what the OP was looking for. I got the sense that "basic string manipulation" was where he was getting stuck (based on the first line of the question). Still giving this my vote for the pointers about database use.
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