Sum column based on condition in another columns in R

Question

I have to sum the values of a column based on an if statement. Here my code:

a <- c(1,2,3)
b <- c(2,2,3)
f <- c(1,2,3)
df <- data.frame(a,b,f)
df
for (i in 1:nrow(df)){
  if (df$a[i] == df$b[i]){
    w <- sum(df$f)
  }
}

My result is 6 while it should be 5, the sum of f[2]=2 + f[3]=3.

Thank you for help

Answer 1

We don't need a loop

with(df, sum(f[a == b]))
#[1] 5

---

Or for faster subset and sum, can use collapse

library(collapse)
fsum(fsubset(df, a == b)$f)
#[1] 5

---

In the loop, it can be changed to

w <- 0
for(i in seq_len(nrow(df))) {
    if(df$a[i] == df$b[i]) {
        w <- w + df$f[i]
   }
 }

w
#[1] 5