Passing a String Via Index in R - r

Question

I'm trying to use a for loop to pull subsets of data out of a dataframe with R.
I have a little vector to hold all the possible occurences of the names in that column
meter_class<-c("one_s_120","nine_s_120", "nine_s_480","fortyfive_s_120", "fortyfive_s_480")
Whenever I try to address it by index reference, it fails. Either nothing in the data subset survives (NULLs everywhere), or R complains about not passing the right argument by using meter_class[1]
attach(meter_class[1])
Error in attach(meter_class[1]) : file 'one_s_120' not found
subset(cal, cal$Form==as.character(meter_class[1]))
[1] Test Amps Type Accuracy Voltage Form
<0 rows> (or 0-length row.names)
Also, here's the output of dput on the datafram cal:
structure(list(Test = structure(c(3L, 3L, 3L, 3L, 3L, 3L, 3L,
3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
1L, 1L, 1L, 1L, 1L, 1L, 1L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L,
4L, 4L, 4L, 4L, 4L, 4L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L,
2L, 2L, 2L, 2L, 2L), .Label = c("Burst", "ESD", "Inspection",
"Surge"), class = "factor"), Amps = c(15, 15, 1.5, 2.5, 2.5,
0.25, 2.5, 2.5, 0.25, 2.5, 2.5, 0.25, 2.5, 2.5, 0.25, 15, 15,
1.5, 2.5, 2.5, 0.25, 2.5, 2.5, 0.25, 2.5, 2.5, 0.25, 2.5, 2.5,
0.25, 15, 15, 1.5, 2.5, 2.5, 0.25, 2.5, 2.5, 0.25, 2.5, 2.5,
0.25, 2.5, 2.5, 0.25, 15, 15, 1.5, 2.5, 2.5, 0.25, 2.5, 2.5,
0.25, 2.5, 2.5, 0.25, 2.5, 2.5, 0.25), Type = structure(c(2L,
1L, 3L, 2L, 1L, 3L, 2L, 1L, 3L, 2L, 1L, 3L, 2L, 1L, 3L, 2L, 1L,
3L, 2L, 1L, 3L, 2L, 1L, 3L, 2L, 1L, 3L, 2L, 1L, 3L, 2L, 1L, 3L,
2L, 1L, 3L, 2L, 1L, 3L, 2L, 1L, 3L, 2L, 1L, 3L, 2L, 1L, 3L, 2L,
1L, 3L, 2L, 1L, 3L, 2L, 1L, 3L, 2L, 1L, 3L), .Label = c("Lag - 0.5",
"Unity - Full", "Unity - Light"), class = "factor"), Accuracy = c(-0.011,
0.012, 0.027, 0.033, 0.076, 0.006, 0.052, 0.046, -0.016, 0.021,
0.008, 0.023, 0.034, 0.036, 0.038, 0.002, 0.012, 0.097, 0.055,
0.093, 0.033, 0.068, 0.048, -0.016, 0.042, 0.03, 0.035, 0.041,
0.024, 0.027, 0.004, -0.012, 0.002, 0.038, 0.084, 0.015, 0.049,
0.045, -0.009, 0.025, 0.002, 0.029, 0.03, 0.032, 0.064, 0.011,
0.024, 0.033, 0.054, 0.085, 0.027, 0.071, 0.059, 0.01, 0.051,
0.012, 0.051, 0.048, 0.04, 0.051), Voltage = c(120, 120, 120,
120, 120, 120, 480, 480, 480, 120, 120, 120, 480, 480, 480, 120,
120, 120, 120, 120, 120, 480, 480, 480, 120, 120, 120, 480, 480,
480, 120, 120, 120, 120, 120, 120, 480, 480, 480, 120, 120, 120,
480, 480, 480, 120, 120, 120, 120, 120, 120, 480, 480, 480, 120,
120, 120, 480, 480, 480), Form = structure(c(3L, 3L, 3L, 2L,
2L, 2L, 2L, 2L, 2L, 1L, 1L, 1L, 1L, 1L, 1L, 3L, 3L, 3L, 2L, 2L,
2L, 2L, 2L, 2L, 1L, 1L, 1L, 1L, 1L, 1L, 3L, 3L, 3L, 2L, 2L, 2L,
2L, 2L, 2L, 1L, 1L, 1L, 1L, 1L, 1L, 3L, 3L, 3L, 2L, 2L, 2L, 2L,
2L, 2L, 1L, 1L, 1L, 1L, 1L, 1L), .Label = c("45S", "9S", "i210plus"
), class = "factor")), .Names = c("Test", "Amps", "Type", "Accuracy",
"Voltage", "Form"), class = "data.frame", row.names = c(NA, -60L
))
I know this is a simple thing to do if you know how to do it...Can anyone light the way?
Thanks!

Answer 1

It seems that none of the values of "meter_class" are represented in "Form" in your data frame.
unique(df$Form)
# [1] i210plus 9S 45S
meter_class %in% unique(df$Form)
# [1] FALSE FALSE FALSE FALSE FALSE
Just try two forms of subsetting, using values of "Form" actually present in the data:
subset(df, Form == "9S")
df[df$Form == "9S", ]
I also note that you wish to "pull subsets of data out of a dataframe". Not knowing the full story and your objectives of doing so, but please note that there are loads of functions that allow you to perform calculations, plotting, or whatever, on subsets of your data.
Update following comment
You can subset a data frame by combining logical conditions with logical operators (see e.g. ?Extract, ?&)
meter_class <- c("i210plus", "9S", "45S")
df[df$Form == "9S" & df$Voltage == 120, ]
# or
subset(df, Form == "9S" & Voltage == 120)