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    <p>OK, so forget about C. Suppose I give you a number and ask you to determine if it's prime. How do you do it? Write down the steps clearly, <em>then</em> worry about translating them into code.</p> <p>Once you have the algorithm determined, it will be much easier for you to figure out how to write a program, and for others to help you with it.</p> <p><strong>edit:</strong> Here's the C# code you posted:</p> <pre><code>static bool IsPrime(int number) { for (int i = 2; i &lt; number; i++) { if (number % i == 0 &amp;&amp; i != number) return false; } return true; } </code></pre> <p>This is <em>very nearly</em> valid C as is; there's no <code>bool</code> type in C, and no <code>true</code> or <code>false</code>, so you need to modify it a little bit (edit: Kristopher Johnson correctly points out that C99 added the stdbool.h header). Since some people don't have access to a C99 environment (but you should use one!), let's make that very minor change:</p> <pre><code>int IsPrime(int number) { int i; for (i=2; i&lt;number; i++) { if (number % i == 0 &amp;&amp; i != number) return 0; } return 1; } </code></pre> <p>This is a perfectly valid C program that does what you want. We can improve it a little bit without too much effort. First, note that <code>i</code> is always less than <code>number</code>, so the check that <code>i != number</code> always succeeds; we can get rid of it.</p> <p>Also, you don't actually need to try divisors all the way up to <code>number - 1</code>; you can stop checking when you reach sqrt(number). Since <code>sqrt</code> is a floating-point operation and that brings a whole pile of subtleties, we won't actually compute <code>sqrt(number)</code>. Instead, we can just check that <code>i*i &lt;= number</code>:</p> <pre><code>int IsPrime(int number) { int i; for (i=2; i*i&lt;=number; i++) { if (number % i == 0) return 0; } return 1; } </code></pre> <p>One last thing, though; there was a small bug in your original algorithm! If <code>number</code> is negative, or zero, or one, this function will claim that the number is prime. You likely want to handle that properly, and you may want to make <code>number</code> be unsigned, since you're more likely to care about positive values only:</p> <pre><code>int IsPrime(unsigned int number) { if (number &lt;= 1) return 0; // zero and one are not prime unsigned int i; for (i=2; i*i&lt;=number; i++) { if (number % i == 0) return 0; } return 1; } </code></pre> <p>This definitely isn't the fastest way to check if a number is prime, but it works, and it's pretty straightforward. We barely had to modify your code at all!</p>
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    1. USStephen Canon
    1. USStephen Canon
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    1. COFYI, the C99 standard defines a <stdbool.h> header that provides `bool`, `true`, and `false`.
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      1. USKristopher Johnson
    2. COexcellent answer, thank you very much
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      1. USJimmy
    3. COalso forgot to mention that i don't need to worry about 0, 1, or negatives, this is just running through a list of 50 numbers if i remember correctly but thank you again
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