How do I run the same code for multiple .csv files with a for loop in R?

Question

I have written a code to filter, group, and sort my large data files. I have multiple text files I have to analyze. I know I can copy the code and run it with new data but I was wondering if there was a way to put this in a for loop that would open the text files one by one and run and store the results. I use the following to load all my text files. In the next steps, I select columns and filter them to find the desired values. But at the moment it only reads one file. I want to obtain results from all data files.

Samples <- Sys.glob("*.csv")
for (filename in Samples) {
try <- read.csv(filename, sep = ",", header = FALSE)
shear <- data.frame(try[,5],try[,8],try[,12])
lane <- shear[which(shear$Load == "LL-1"),]
Ext <- subset(lane, Girder %in% c("Left Ext","Right Ext"))
Max.Ext <- max(Ext$Shear)
}

Use `lapply`. Start with `alldat <- lapply(list_of_filenames, read.csv, stringsAsFactors=FALSE)`, then for whatever you need to do to each frame, use `lapply(alldat, function(dat) ...)`. Other than that, please provide something concrete as in a reproducible question: https://stackoverflow.com/questions/5963269, https://stackoverflow.com/help/mcve, and https://stackoverflow.com/tags/r/info. — r2evans, Mar 14 '20 at 04:34
I just updated the question with a part of my code to clarify. My goal is to run multiple steps to get specific values out of my data files. I want to add this to a for loop so I do not have to run the code for each data file separately. — Maral Dorri, Mar 14 '20 at 04:56

score 1 · Accepted Answer · answered Mar 14 '20 at 05:12

1

You can put everything that you want to apply to each file in a function :

apply_fun <- function(filename) {

  try <- read.csv(filename, sep = ",", header = FALSE)
  shear <- data.frame(try[,5],try[,8],try[,12])
  lane <- shear[which(shear$Load == "LL-1"),]
  Ext <- subset(lane, Girder %in% c("Left Ext","Right Ext"))
  return(max(Ext$Shear, na.rm = TRUE))
}

and here it seems we want only one number (max) from each file, we can use sapply to apply the function to each file.

Samples <- Sys.glob("*.csv")
sapply(Samples, apply_fun)

answered Mar 14 '20 at 05:12

Ronak Shah

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The full function that I would like to run would result in two tables with 2 rows and 4 columns of maximum values from the data files – Maral Dorri Mar 14 '20 at 05:29
@MaralDorri In that case, change `sapply` to `lapply`. – Ronak Shah Mar 14 '20 at 05:32

How do I run the same code for multiple .csv files with a for loop in R?

1 Answers1