loops - Syntax in R for prediction

Question

How do I perform this loop in RStudio using R library? Actually my dataset has over 100,000 rows and need some efficient syntax that can produce something similar to this for loop

Use previous row's value to predict for next all rows(in col d) after rows when data not available

# df is a dataframe with columns b,c,d,p.
        
    
    d = c(1, 2, 4, NA, NA)
    b = c(1,1,1,2,2) 
    c=c(1,1,1,1,1)
    
    df= data.frame(cbind(b,c,d))
    df$p <- c(0.1,0.2,0.1,0.1,0.3)
    
    for(i in 1:(nrow(df)-1)) {
      if (df$b[i + 1] > df$c[i + 1]) {
        df$d[i + 1] = df$d[i] * (1 - df$p[i + 1])
      } else{
        df$d[i + 1] = df$d[i+1]
      }
    }

question from:https://stackoverflow.com/questions/65840586/syntax-in-r-for-prediction

Answer 1

This vectorized code gives the same output as the question's for loop. And is much faster.

inx <- seq_along(df$a)[-1]
b_greater <- df$b[inx] > df$c[inx]
df$a[inx] <- df$d[inx - 1]
df$a[inx][b_greater] <- df$d[inx - 1][b_greater] * (1 - df$p[inx][b_greater])

df
#  b c  d   a   p
#1 1 1  1  NA 0.1
#2 1 1  2 1.0 0.2
#3 1 1  4 2.0 0.1
#4 2 1 NA 3.6 0.1
#5 2 1 NA  NA 0.3