dataframe - R how use a string variable to select a data frame column using $ notation -

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• dynamically select data frame columns using $ , vector of column names 4 answers

from reading i've been doing r, can select column in data frame either of these 2 methods: frame[,column] or frame$column. however, when have string variable, works in first. in other words, consider following:

i have data frame, tmp, subset of larger data frame of question responses. v1 responder's id, q5.3 response, 1 or 0:

            v1 q5.3 2 r_bdykkzwcvbxdftt    1 3 r_41wnkuqcm8muw2x    0 4 r_2ogeykkgbh2e4rl    1 5 r_8d4jzmbfyo0m0ux    1 6 r_3kpgp2pxwronip7    1  str(tmp)  'data.frame':   5 obs. of  2 variables:      $ v1  : factor w/ 364 levels "r_0039ornooowadqx",..: 256 116 70 201 95      $ q5.3: num  1 0 1 1 1 

now, define variable x, holds string of name of 1 of columns.

x<-"q5.3" 

tmp[,x] returns think should return:

tmp[,x]  [1] 1 0 1 1 1 

tmp$"q5.3" returns think should return:

tmp$"q5.3"  [1] 1 0 1 1 1 

tmp$x returns

tmp$x  null 

how can tell r interpret tmp$x tmp$"q5.3"

thanks!

if have variable x column name in tmp, tmp[,x] or tmp[[x]] correct way extract it. cannot r use treat tmp$x tmp$"q5.3". tmp$x refer item named "x" in "tmp".