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copied!<p>(This explains the idea hinted by <strong>Spudd86</strong>).</p> <p>You need to implement a finite state machine. There are the following states:</p> <ul> <li>General state</li> <li>Inside a file name</li> <li>Inside the <code>&&</code> token</li> <li>Inside the <code>||</code> token</li> </ul> <p>For each state and next input character, you have to decide what is the next state, and whether to output a token. For example:</p> <ul> <li>Current state: <strong>General</strong>; character: <strong>x</strong> => next state: <strong>inside-file-name</strong></li> <li>Current state: <strong>inside-file-name</strong>; character: <strong>space</strong> => next state: <strong>General</strong>; output the token</li> <li>Current state: <strong>inside-file-name</strong>; character: <strong>&</strong> => next state: <strong>inside-&&</strong>; output the token</li> <li>Current state: <strong>inside-&&</strong>; character: <strong>&</strong> => next state: <strong>General</strong>; output the token</li> <li>Current state: <strong>inside-&&</strong>; character: <strong>x</strong> => next state: <strong>General</strong>; syntax error</li> <li>... (ad nauseum)</li> </ul> <p>It's much boring work to work out all the rules (the fun starts when you must debug the resulting code), so most people use code generators to do that.</p> <hr> <p>Edit: some code (sorry if the syntax is messed-up; i usually program in C++)</p> <pre><code>enum state { STATE_GENERAL, STATE_IN_FILENAME, ... }; // Many characters are treated the same (e.g. 'x' and 'y') - so use categories enum character_category { CHAR_GENERAL, // can appear in filenames CHAR_WHITESPACE = ' ', CHAR_AMPERSAND = '&', CHAR_PIPE = '|', CHAR_EOF = EOF, ... }; character_category translate(int c) { switch (c) { case '&': return CHAR_AMPERSAND; case ' ': case '\t': case '\n': return CHAR_WHITESPACE; ... default: return CHAR_GENERAL; } } void do_stuff() { character_category cat; state current_state = STATE_GENERAL; state next_state; char token[100]; char token_length = 0; do { int c = getchar(); cat = translate(c); // The following implements a switch on 2 variables int selector = 1000 * current_state + cat; switch (selector) { case 1000 * STATE_GENERAL + CHAR_GENERAL: next_state = STATE_IN_FILENAME; token[token_length++] = c; // append a character to a filename token break; case 1000 * STATE_GENERAL + CHAR_WHITESPACE: next_state = STATE_GENERAL; // do nothing break; case 1000 * STATE_GENERAL + CHAR_PIPE: next_state = STATE_IN_OR_TOKEN; // the first char in '||' or just '|' break; // Much repetitive code already; define a macro for the case constants? // Have to cover all states and all character categories; good luck... case 1000 * STATE_IN_FILENAME + EOF: case 1000 * STATE_IN_FILENAME + CHAR_WHITESPACE: next_state = STATE_GENERAL; printf("Filename token: %s\n", token); break; default: printf("Bug\n"); // forgot one of the cases? } current_state = next_state; } while (cat != CHAR_EOF); } </code></pre>

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