The output is a list of length 1. We need to extract ([[) the element before applying the grep
unique (grep(paste(rv,collapse="|"),
links[[1]], value=TRUE))
Note that the rv includes numbers 0 to 9 and it can match a digit if it is present anywhere in the link. If the intention is to subset the digits following the questions
grep("questions/\\d+", links[[1]], value = TRUE)
-output
[1] "/questions/72859976/recognizing-and-keeping-elements-containing-certain-patterns-in-a-list"
[2] "/questions/72843570/combing-two-selections-together"
[3] "/questions/72840913/selecting-rows-from-a-table-based-on-a-list"
[4] "/questions/72840624/even-out-table-in-r"
[5] "/questions/72840548/creating-a-dictionary-reference-table"
[6] "/questions/72837147/sequentially-replacing-factor-variables-with-numerical-values"
[7] "/questions/72822951/scanning-and-replacing-values-of-rows-in-r"
[8] "/questions/72822781/alternative-to-do-callrbind-data-frame-for-combining-a-list-of-data-frames"
[9] "/questions/72738885/referencing-a-query-in-another-query"
[10] "/questions/72725108/defining-cte-common-table-expressions-in-r"
[11] "/questions/72723768/creating-an-id-variable-on-the-spot"
[12] "/questions/72720013/selecting-data-using-conditions-stored-in-a-variable"
[13] "/questions/72717135/effecient-ways-to-append-sql-results-in-r"
...
If there are multiple pages, add the page= with paste or sprintf
urls <- c(url, sprintf("%s&page=%d", url, 2:3))
out_lst <- lapply(urls, function(url)
{
page <-read_html(url)
resource <- GET(url)
parse <- htmlParse(resource)
links <- list(xpathSApply(parse, path="//a", xmlGetAttr, "href"))
grep("questions/\\d+", links[[1]], value = TRUE)
})
-output
> out_lst
[[1]]
[1] "/questions/72859976/recognizing-and-keeping-elements-containing-certain-patterns-in-a-list"
[2] "/questions/72843570/combing-two-selections-together"
[3] "/questions/72840913/selecting-rows-from-a-table-based-on-a-list"
[4] "/questions/72840624/even-out-table-in-r"
[5] "/questions/72840548/creating-a-dictionary-reference-table"
[6] "/questions/72837147/sequentially-replacing-factor-variables-with-numerical-values"
[7] "/questions/72822951/scanning-and-replacing-values-of-rows-in-r"
[8] "/questions/72822781/alternative-to-do-callrbind-data-frame-for-combining-a-list-of-data-frames"
[9] "/questions/72738885/referencing-a-query-in-another-query"
[10] "/questions/72725108/defining-cte-common-table-expressions-in-r"
[11] "/questions/72723768/creating-an-id-variable-on-the-spot"
[12] "/questions/72720013/selecting-data-using-conditions-stored-in-a-variable"
[13] "/questions/72717135/effecient-ways-to-append-sql-results-in-r"
[14] "/questions/72710448/removing-files-from-global-environment-with-a-certain-pattern"
[15] "/questions/72710203/r-sql-is-the-default-option-sampling-with-replacement"
[16] "/questions/72695401/allocating-max-memory-in-r"
[17] "/questions/72681898/randomly-delete-columns-from-datasets"
[18] "/questions/72663516/are-rds-files-more-efficient-than-csv-files"
[19] "/questions/72625690/importing-files-using-list-files"
[20] "/questions/72623856/second-most-common-element-in-each-row"
[21] "/questions/72623744/counting-the-position-where-a-pattern-is-completed"
[22] "/questions/72620501/bulk-import-export-files-from-r"
[23] "/questions/72613413/counting-every-position-where-a-pattern-appears"
[24] "/questions/72612577/counting-the-position-of-the-first-0-in-each-row"
[25] "/questions/72607160/taking-averages-across-lists"
[26] "/questions/72589276/functions-for-finding-out-the-midpoint-interpolation"
[27] "/questions/72587298/sandwiching-values-between-rows"
[28] "/questions/72569338/integration-error-lengthlower-1-is-not-true"
[29] "/questions/72568817/synchronizing-nas-in-r"
[30] "/questions/72568661/finding-the-loser-in-each-row"
[[2]]
[1] "/questions/72566170/making-a-race-between-two-variables"
[2] "/questions/72418723/making-a-list-of-random-numbers"
[3] "/questions/72418364/random-uniform-numbers-without-runif"
[4] "/questions/72353102/integrate-normal-distribution-between-2-values"
[5] "/questions/72174868/placing-commas-between-names"
[6] "/questions/72163297/simulate-flipping-french-fries-in-r"
[7] "/questions/71982286/alternatives-to-the-partition-by-statement-in-sql"
[8] "/questions/71970960/converting-lists-into-data-frames"
[9] "/questions/71970672/random-numbers-are-too-similar-to-each-other"
[10] "/questions/71933753/making-combinations-of-items"
[11] "/questions/71874791/sorting-rows-in-specified-order"
[12] "/questions/71866097/hiding-the-legend-in-this-graph"
[13] "/questions/71866048/understanding-the-median-in-this-graph"
[14] "/questions/71852517/nas-produced-when-number-of-iterations-increase"
[15] "/questions/71791906/assigning-unique-colors-to-multiple-lines-on-a-graph"
[16] "/questions/71787336/finding-identical-rows-in-multiple-datasets"
[17] "/questions/71758983/multiple-replace-lookups"
[18] "/questions/71758648/create-ascending-id-in-a-data-frame"
[19] "/questions/71731208/webscraping-data-which-pokemon-can-learn-which-attacks"
[20] "/questions/71728273/webscraping-pokemon-data"
[21] "/questions/71683045/identifying-smallest-element-in-each-row-of-a-matrix"
[22] "/questions/71671488/connecting-all-nodes-together-on-a-graph"
[23] "/questions/71641774/overriding-colors-in-ggplot2"
[24] "/questions/71641404/applying-a-function-to-a-data-frame-lapply-vs-traditional-way"
[25] "/questions/71624111/sending-emails-from-r"
[26] "/questions/71623019/sql-joining-tables-from-2-different-servers-r-vs-sas"
[27] "/questions/71429265/overriding-sql-errors-during-r-uploads"
[28] "/questions/71429129/splitting-a-dataset-into-uneven-portions"
[29] "/questions/71418533/multiplying-and-adding-values-across-rows"
[30] "/questions/71417489/tricking-an-sql-server-to-accept-a-file-from-r"
[[3]]
[1] "/questions/71417218/splitting-a-dataset-into-arbitrary-sections"
[2] "/questions/71398804/plotting-vector-fields-and-gradient-fields"
[3] "/questions/71387596/animating-the-mandelbrot-set"
[4] "/questions/71358405/repeat-a-set-of-ids-for-every-n-rows"
[5] "/questions/71344822/time-series-graphs-with-different-symbols"
[6] "/questions/71341865/creating-a-data-frame-with-commas"
[7] "/questions/71287944/converting-igraph-to-visnetwork"
[8] "/questions/71282863/fixing-the-first-and-last-numbers-in-a-random-list"
[9] "/questions/71282403/adding-labels-to-graph-nodes"
[10] "/questions/71262761/understanding-list-and-do-call-commands"
[11] "/questions/71261431/adjusting-graph-layouts"
[12] "/questions/71255038/overriding-non-existent-components-in-a-loop"
[13] "/questions/71244872/fixing-cluttered-titles-on-graphs"
[14] "/questions/71243676/directly-adding-titles-and-labels-to-visnetwork"
[15] "/questions/71232353/removing-all-edges-in-igraph"
[16] "/questions/71230273/writing-a-function-that-references-elements-in-a-matrix"
[17] "/questions/71227260/generating-random-graphs-according-to-some-conditions"
[18] "/questions/71087349/adding-combinations-of-numbers-in-a-matrix"
