Conditional displaying values in R - r

Question

I'd like to see which values have a particular entry issue, but I'm not getting things done right.
For instance, I need to print on screen values from column "c" but conditional of a given value from "b" say where [b==0].
Finally, I need to add a new string for those whose condition is true.
df<- structure(list(a = c(11.77, 10.9, 10.32, 10.96, 9.906, 10.7,
11.43, 11.41, 10.48512, 11.19), b = c(2, 3, 2, 0, 0, 0, 1, 2,
4, 0), c = c("q", "c", "v", "f", "", "e", "e", "v", "a", "c")), .Names = c("a",
"b", "c"), row.names = c(NA, -10L), class = "data.frame")
I tried this without success:
if(df[b]==0){
print(df$c)
}
if((df[b]==0)&(df[c]=="v")){
df[c] <-paste("2")
}
Thanks for helping.

Answer 1

The correct syntax is like df[rows, columns], so you could try:
df[df$b==0, "c"]
You can accomplish changing values using ifelse:
df$c <- ifelse(df$b==0 & df$c=="v", paste(df$c, 2, sep=""), df$c)

Answer 2

Does this help?
rows <- which(df$b==0)
if (length(rows)>0) {
print(df$c[rows])
df$c[rows] <- paste(df$c[rows],'2')
## maybe you wanted to have:
# df$c[rows] <- '2'
}

Answer 3

There are several ways to subset data in R, like e.g.:
df$c[df$b == 0]
df[df$b == 0, "c"]
subset(df, b == 0, c)
with(df, c[b == 0])
# ...
To conditionally add another column (here: TRUE/FALSE):
df$e <- FALSE; df$e[df$b == 0] <- TRUE
df <- transform(df, c = ifelse(b == 0, TRUE, FALSE))
df <- within(df, e <- ifelse(b == 0, TRUE, FALSE))
# ...