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POFaster parsing of numbers on .NET
 

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  1. POFaster parsing of numbers on .NET
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    <p>I have written two functions that convert a string of whitespace-separated integers into an int array. The first function uses <code>Substring</code> and then applies <code>System.Int32.Parse</code> to convert the substring into an <code>int</code> value:</p> <pre class="lang-ml prettyprint-override"><code>let intsOfString (s: string) = let ints = ResizeArray() let rec inside i j = if j = s.Length then ints.Add(s.Substring(i, j-i) |&gt; System.Int32.Parse) else let c = s.[j] if '0' &lt;= c &amp;&amp; c &lt;= '9' then inside i (j+1) else ints.Add(s.Substring(i, j-i) |&gt; System.Int32.Parse) outside (j+1) and outside i = if i &lt; s.Length then let c = s.[i] if '0' &lt;= c &amp;&amp; c &lt;= '9' then inside i (i+1) else outside (i+1) outside 0 ints.ToArray() </code></pre> <p>The second function traverses the characters of the string in-place accumulating the integer without creating a temporary substring:</p> <pre class="lang-ml prettyprint-override"><code>let intsOfString (s: string) = let ints = ResizeArray() let rec inside n i = if i = s.Length then ints.Add n else let c = s.[i] if '0' &lt;= c &amp;&amp; c &lt;= '9' then inside (10*n + int c - 48) (i+1) else ints.Add n outside(i+1) and outside i = if i &lt; s.Length then let c = s.[i] if '0' &lt;= c &amp;&amp; c &lt;= '9' then inside (int c - 48) (i+1) else outside (i+1) outside 0 ints.ToArray() </code></pre> <p>Benchmarking on space-separated integers 1 to 1,000,000, the first version takes 1.5s whereas the second version takes 0.3s.</p> <p>Parsing such values can be performance critical so leaving 5x performance on the table by using temporary substrings can be undesirable. Parsing integers is easy but parsing other values such as floating point numbers, decimals and dates is considerably harder.</p> <p>So, are there built-in functions to parse directly from a substring within a string (i.e. using the given start and length of a string) in order to avoid generating a temporary string? If not, are there any libraries that provide efficient functions to do this?</p>
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    1. USJon Harrop
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    1. COHave you tried using regular expressions instead of using Substring? A compiled regular expression can be much faster than string operations
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      1. USPanagiotis Kanavos
    2. CO@PanagiotisKanavos Can you explain how a regular expression can be used to parse a string into an array of ints?
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      1. USJon Harrop
    3. COI had a similar problem recently and couldn't find any when I searched, had to write the decimal parsing code myself. It's not as hard as you might think as the Decimal class has a constructor that takes a scale factor so you can do pretty much the same as integer parsing and just keep track of where the decimal point is. Dates weren't too hard either, however I did have tight control over the formats in both cases. I wouldn't want to write general parsing code...
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      1. USMrKWatkins
 

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