Loop through specific columns in dataframe

Question

I have a dataframe that I'm trying to loop through, printing out the values from each dataframe... I'm new to R and am more familiar with python and javascript so I'm gonna pseudo-code what I'm trying to say here:

df <- read.table(text="
              a     b    c    d     e      f       g
              1     5    6    7     3      4       7
              3     2    8    4     1      9       4
              6     5    2    5     4      6       1
         ",header=TRUE,stringsAsFactors = FALSE)

my javascript mentality...(I only want to select from columns 4 and on):

for (i = 3; i < df.length; i++) {
     console.log(i)
}

and it would return to me

              d     e      f       g
              7     3      4       7
              4     1      9       4
              5     4      6       1

to understand this better, I'm making a dashboard where when I hover over certain data points only values from certain columns are being displayed in a tooltip, which is why i still need to keep the original dataframe without shortening it, yet write an algorithm that will return values from specific columns in the dataframe just for the tooltip

Answer 1

Here are two different ways two subset a dataframe:

# by column numbers
df[, 4:7]

# by column names
names <- c("d", "e", "f", "g")
df[names]

Answer 2

Try this: df[,c(3,4,5,6)]

And check out this link: https://dzone.com/articles/learn-r-how-extract-rows

I'm not sure if this is what you are asking.

Answer 3

You can run a loop over with a conditional to loop:

for (column in dataFrame) {
if(column %% 2 == 0){
for (entry in rawdata[,column]) {
#run your code here, it will impact ever second column
  }
 }
}

Change the 0 to a 1 in order to do it for every odd column.