Using if-statement and for-loop in R

I have a data frame, which looks like:

pr     class
0,317   1
0,579   3
0,317   1
0,426   2
9,851   7
0,992   3
3,731   5
1,909   4
0,426   2
9,851   7
3,731   5
9,851   7

I need to change probability if it is not equal to particular value in the particular class. For example, if the class is 1 and the probability is not 0.317, I need to change it to 0.317.

I tried to use for one class for-loop and if-statement but for some reason it didn't work at all.

for(class in data_frame) {
  if(data_frame$class = 1) {
    data_frame$pr = 0.317
  }
}

I tried only with one class because I don't have any idea how I can do it for all classes in one code (may be another for-loop?).

Thanks a lot in advance!