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PODelete characters at positions within a string in R?
 

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  1. PODelete characters at positions within a string in R?
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    <p>I am looking for a way to delete the characters at certain positions within a string in R. For example, if we have a string <code>"1,2,1,1,2,1,1,1,1,2,1,1"</code>, I want to delete the third, fourth, 7th and 8th position. The operation would make the string: <code>"1,1,2,1,1,1,1,2,1,1"</code>.</p> <p>Unfortunately, breaking the string into a list using strsplit is not an option, because the strings I am working with are over 1 million characters long. Considering I have about 2,500 strings, it works out to be quite some time.</p> <p>Alternatively, finding a way to replace the characters with an empty string <code>""</code> would achieve the same purpose - I think. Looking into this line of thought, I came across this StackOverflow post:</p> <p><a href="https://stackoverflow.com/questions/6819573/r-how-can-i-replace-lets-say-the-5th-element-within-a-string">R: How can I replace let&#39;s say the 5th element within a string?</a></p> <p>Unfortunately, the solution suggested is hard to efficiently generalize and the following takes about 60 seconds per input string for a list of 2000 positions to remove:</p> <pre><code>subchar2 = function(inputstring, pos){ string = "" memory = 0 for(num in pos){ string = paste(string, substr(inputstring, (memory+1), (num-1)), sep = "") memory = num } string = paste(string, substr(inputstring,(memory+1), nchar(inputstring)),sep = "") return(string) } </code></pre> <p>Looking into the problem, I found a snippet of code, that seems to replace the characters at certain positions with <code>"-"</code>:</p> <pre><code>subchar &lt;- function(string, pos) { for(i in pos) { string &lt;- gsub(paste("^(.{", i-1, "}).", sep=""), "\\1-", string) } return(string) } </code></pre> <p>I don't quite understand regular expression (yet), but I have a strong suspicion something along these lines will be much better time-wise than the first code solution. Unfortunately, this subchar function seems to break when the values in pos gets high:</p> <pre><code>&gt; test = subchar(data[1], 257) Error in gsub(paste("^(.{", i - 1, "}).", sep = ""), "\\1-", string) : invalid regular expression '^(.{256}).', reason 'Invalid contents of {}' </code></pre> <p>I was also considering trying to read in the string data into a table using SQL, but I was hoping that there would be a elegant string solution. The SQL implementation in R to do this seems rather complicated.</p> <p>Any ideas? Thanks!</p>
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    1. USCommunity
    1. USGordon Freeman
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      1. USsebastian-c
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    1. COWhere are the strings coming from in the first place? It might be easier to preprocess the data in something other than R.
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      1. USDaniel Kinsman
    2. COThe strings come from an .RData file, but I can quickly write it out to a text file, so that opens the scope. Any language suggestions?
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      1. USGordon Freeman
    3. COAre they always comma separately numbers? If so, wouldn't it be easier to convert them each to a vector, subset, and then convert back to character? Then you can just use numerical indexing to drop elements.
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