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    <p>Your input seems <em>so</em> easy that I would probably just use a loop around <code>fgetc(3)</code>; I'll sketch some pseudo-code here:</p> <pre><code>fd = fopen("file", "r"); int c; while((c=fgetc(fd)) != EOF) { switch(c) { case '0': case '1': /* so on */ case '9': handle_digit(c); break; case ',': handle_comma(); break; case '\n': handle_newline(); break; default: fprintf(stderr, "mistaken input %c\n", c); break; } } fclose(fd); </code></pre> <p>You'll have to manage the input in the functions in a manner that <em>may</em> be a bit awkward if you're used to higher-level languages such as Ruby or Python where you'd just run <code>line.split(',')</code> to get a list of numbers, but that is pretty idiomatic C.</p> <p>Of course, if this were a <em>real</em> problem, I'd probably prefer <code>flex</code> and <code>bison</code>, and write a tiny lexer and grammar, mostly because it would be a <em>lot</em> easier to extend in the future as needs change.</p> <hr> <p><strong>Update</strong></p> <p>With some additional criteria to check, the <code>handle_{digit,comma,newline}()</code> routines are easier to sketch. I'll sketch using global variables, but you could just as easily stuff these into a <code>struct</code> and pass them around from function to function:</p> <pre><code>enum seen { NEWLINE, COMMA, DIGIT, }; enum seen last_seen = NEWLINE; handle_digit(int c) { if (last_seen == DIGIT) { /* error if numbers cannot have multiple digits or construct a larger number if numbers can have multiple digits */ } else if (last_seen == COMMA || last_seen == NEWLINE) { /* start a new entry */ } last_seen = DIGIT; } handle_comma() { if (last_seen == COMMA) { /* error */ } else if (last_seen == NEWLINE) { /* error */ } else if (last_seen == DIGIT) { /* end previous field */ } last_seen = COMMA; } handle_newline() { if (last_seen == NEWLINE) { /* error */ } else if (last_seen == COMMA) { /* error */ } else if (last_seen == DIGIT) { /* end previous field */ } last_seen = NEWLINE; } </code></pre> <p>Add whichever checks you need to validate the contents according to whichever rules you have. You might wish to standardize the order and contents of the tests to ensure that you never forget one, even if it means you write a <code>/* nop */</code> comment once or twice to remind yourself that something is fine.</p>
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    1. COThat's really helpful, thanks. I guess I am still a bit confused as to how I can tell the program that the required format is an integer then a comma then another integer (and so forth). If I have understood your code above correctly, the switch statement is just accepting all of those cases, but isn't actually specifying that they need to be in a certain format. Am I missing something here? I feel like I have read every possible thing, but I can't seem to find an example of what I need to do.
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    2. CO@Philip: Thanks for the edit, much better.
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    3. CO@Angela: well, it's not very sophisticated in the sense that this program inspects every character and ensures it is either a digit, a comma, or a newline. Spaces would appear to be invalid. Fixing that would be easy. What might be more difficult is if your numbers can be (or can't be) multiple digits long; `1,2` vs `1234,2` -- and if you're converting the digits into _integers_ in the `handle_digit(c)` routine (and emitting them in the `handle_comma()` or `handle_newline()` routines), then you may need to add additional protection against too-long numbers such as `12345678901234567890`.
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