R 用更有效的方式替换嵌套循环来遍历 2 个数据帧

Question

I want to match state and district code between 2 dataframes and thus retrieve the production and yield of crop for the first dataframe.

Can somebody please help me in replacing the nested loop below for a more efficient way:

for (i in 1:length(og_county2$State)){
  for (j in 1:length(km2$State)){    
      if (is.na(og_county2$`Value.CORN, GRAIN - ACRES HARVESTED`[i]) == "TRUE" & (as.character(og_county2$State[i]) == as.character(km2$State[j])) & (as.character(og_county2$District.code[i]) == as.character(km2$Ag.District.Code[j]))& km2$County[j] =="OTHER (COMBINED) COUNTIES"){

        og_county2$`Value.CORN, GRAIN - ACRES HARVESTED`[i] = km2$`Value.CORN, GRAIN - ACRES HARVESTED`[j]
      }
    if (is.na(og_county2$`Value.CORN, GRAIN - PRODUCTION, MEASURED IN BU`[i]) == "TRUE" & (as.character(og_county2$State[i]) == as.character(km2$State[j])) & (as.character(og_county2$District.code[i]) == as.character(km2$Ag.District.Code[j]))& km2$County[j] =="OTHER (COMBINED) COUNTIES"){

      og_county2$`Value.CORN, GRAIN - PRODUCTION, MEASURED IN BU`[i] = km2$`Value.CORN, GRAIN - PRODUCTION, MEASURED IN BU`[j]
    }
    if (is.na(og_county2$`Value.CORN, GRAIN - YIELD, MEASURED IN BU / ACRE`[i]) == "TRUE" & (as.character(og_county2$State[i]) == as.character(km2$State[j])) & (as.character(og_county2$District.code[i]) == as.character(km2$Ag.District.Code[j]))& km2$County[j] =="OTHER (COMBINED) COUNTIES"){

      og_county2$`Value.CORN, GRAIN - YIELD, MEASURED IN BU / ACRE`[i] = km2$`Value.CORN, GRAIN - YIELD, MEASURED IN BU / ACRE`[j]
    }
    }
   }

Answer 1

Look at the 'merge()' function: http://www.datasciencemadesimple.com/join-in-r-merge-in-r/