R：如何用条件将列中的字符串替换为第二列？

我有这样一个小数据框：

a<-c("green","green","green","green","green","green","green","green","green","green")
b<-c("blue","red","blue","red","blue","red","blue","red","blue","red")
df<-data.frame(a,b)
df
       a    b
1  green blue
2  green  red
3  green blue
4  green  red
5  green blue
6  green  red
7  green blue
8  green  red
9  green blue
10 green  red

或

---

>df$a[子集（df$a，df$b==“红色”）]df[b==“红色”，]$a您可以使用which功能。有关一些示例，请参见

df[which（df[，2]=“red”），1]尝试
df您还需要有两个条件：
df$b==“red”
和
df$a==“green”
，以防出现这种情况data@bouncyball这个解决方案很好用。谢谢
，这里实际上不需要
。删除该函数可以正常工作：
df[df[，2]='red'，]
> df[b=="red",]$a<-gsub("green","yellow",df[b=="red",])
Fehler in `$<-.data.frame`(`*tmp*`, "a", value = c("c(1, 1, 1, 1, 1)",  : 
  replacement has 2 rows, data has 5

> df$a[subset(df$a,df$b=="red")]<-"yellow"
Warnmeldung:
In `[<-.factor`(`*tmp*`, subset(df$a, df$b == "red"), value = c(NA,  :
  invalid factor level, NA generated
> df
       a    b
1   <NA> blue
2  green  red
3  green blue
4  green  red
5  green blue
6  green  red
7  green blue
8  green  red
9  green blue
10 green  red

> df[b=="red",]$a<-"yellow"
Warnmeldung:
In `[<-.factor`(`*tmp*`, iseq, value = c("yellow", "yellow", "yellow",  :
  invalid factor level, NA generated
> df
       a    b
1  green blue
2   <NA>  red
3  green blue
4   <NA>  red
5  green blue
6   <NA>  red
7  green blue
8   <NA>  red
9  green blue
10  <NA>  red

df[which(df[,2]=="red"),1] <- "yellow"