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    <p>I'll pick on one loop, and leave it to you to rethink your implementation. Specifically this:</p> <pre><code>for(i = 0; isdigit(hexd[i]); i++) n = (16 * i) + (hexd[i] - '0'); </code></pre> <p>doesn't do what you probably think it does...</p> <ul> <li>It only processes the first span of characters where <code>isdigit()</code> is TRUE.</li> <li>It stops on the first character where <code>isdigit()</code> is FALSE.</li> <li>It doesn't run past the end because <code>isdigit('\0')</code> is known to be FALSE. I'm concerned that might be accidentally correct, though.</li> <li>It does correctly convert a hex number that can be expressed solely with digits 0-9.</li> </ul> <p>Things to think about for the whole program:</p> <ul> <li>Generally, prefer to not modify input strings unless the modification is a valuable side effect. In your example code, you are forcing the string to lower case in-place. Modifying the input string in-place means that a user writing <code>htoi("1234")</code> is invoking <em>undefined behavior</em>. You really <em>don't</em> want to do that.</li> <li>Only one of the loops over digits is going to process a non-zero number of digits.</li> <li>What happens if I send <code>0123456789ABCDEF0123456789ABCDEF</code> to <code>stdin</code>?</li> <li>What do you expect to get for <code>80000000</code>? What did you get? Are you surprised?</li> <li>Personally, I wouldn't use <code>NL</code> for <code>'\n'</code>. C usage pretty much expects to see <code>\n</code> in a lot of contexts where the macro is not convenient, so it is better to just get used to it now...</li> </ul>
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    1. POK&R Exercise 2-3 "Hex to int converter" Problem
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