请问，如下的执行错在哪里？怎么改正？命令跟原文一模一样。
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my practice:
> cc<-SRINC
> cc
[1] 3 2 3 2 2 NA 3 3 3 3 2 3 2 2 2 3 3 2 2 3 3 3 2 2 2
> for (i in 1:length(cc)){
+ if (cc==2) cc<-"Middle Income";
+ if (cc==3) cc<-"Low Income";}
Error in if (cc == 2) cc <- "Middle Income" :
missing value where TRUE/FALSE needed
> cc
[1] "Low Income" "Middle Income" "Low Income" "Middle Income"
[5] "Middle Income" NA "3" "3"
[9] "3" "3" "2" "3"
[13] "2" "2" "2" "3"
[17] "3" "2" "2" "3"
[21] "3" "3" "2" "2"
[25] "2"


谢谢！