Structure of a for loop

I am learning how to create a function in R, but I am struggling to understand how to write for loop. My understanding is that

for (item I list_items) {

do_something(itemn)

}

I would like to write a for loop to replace with 333 the cells that are equal with 123. So the item is 123 and the list of items is the df from sec1 till sec4.

Could somebody explain this to me, please? And how this can be included in a function?

Sample code:

structure(list(sec1 = c(1, 123, 1), sec2 = c(123, 1, 1), sec3 = c(123, 
0, 0), sec4 = c(1, 123, 1)), spec = structure(list(cols = list(
    sec1 = structure(list(), class = c("collector_double", "collector"
    )), sec2 = structure(list(), class = c("collector_double", 
    "collector")), sec3 = structure(list(), class = c("collector_double", 
    "collector")), sec4 = structure(list(), class = c("collector_double", 
    "collector"))), default = structure(list(), class = c("collector_guess", 
"collector")), delim = ","), class = "col_spec"),  row.names = c(NA, 
-3L), class = c("spec_tbl_df", "tbl_df", "tbl", "data.frame"))

>Solution :

Here’s how it would work for one column of your data:

dat <- structure(list(sec1 = c(1, 123, 1), 
                      sec2 = c(123, 1, 1), 
                      sec3 = c(123, 0, 0), 
                      sec4 = c(1, 123, 1)), 
                 spec = structure(list(cols = list(                                                                              
                      sec1 = structure(list(),   
                      class = c("collector_double", "collector")),   
                      sec2 = structure(list(), 
                      class = c("collector_double","collector")),
                      sec3 = structure(list(), 
                      class = c("collector_double", "collector")), 
                      sec4 = structure(list(), 
                      class = c("collector_double","collector"))), 
                 default = structure(list(), 
                      class = c("collector_guess","collector")), 
                      delim = ","), class = "col_spec"),  
                      row.names = c(NA,-3L), class = 
                 c("spec_tbl_df", "tbl_df", "tbl", "data.frame"))


for(i in 1:nrow(dat)){
  dat$sec1[i] <- ifelse(dat$sec1[i] == 123, 333, dat$sec1[i])
}
dat
#>   sec1 sec2 sec3 sec4
#> 1    1  123  123    1
#> 2  333    1    0  123
#> 3    1    1    0    1

^{Created on 2022-01-31 by the reprex package (v2.0.1)}

To replace all of them, using for loops, you could do a double loop over columns and rows.

for(j in names(dat)){
  for(i in 1:nrow(dat)){
    dat[[j]][i] <- ifelse(dat[[j]][i] == 123, 333, dat[[j]][i])
  }
}

Of course, as others have identified, you certainly don’t need a for loop to accomplish this.