How to use a string variable to select a data frame column using $ notation

From the reading I've been doing with R, I can select a column in a data frame by either of these two methods: `frame[,column]` or `frame$column`. However, when I have a string as a variable, it works only in the first. In other words, consider the following: I have a data frame, tmp, a subset of a larger data frame of question responses. V1 is the responder's id, Q5.3 is the response, a 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, I define a variable x, that holds the string of the name of one of the columns. ``` x<-"Q5.3" ``` tmp[,x] returns what I think it should return: ``` tmp[,x] [1] 1 0 1 1 1 ``` tmp$"Q5.3" returns what I think it should return: ``` tmp$"Q5.3" [1] 1 0 1 1 1 ``` tmp$x however returns ``` tmp$x NULL ``` How can I tell R to interpret tmp$x as tmp$"Q5.3".