For loop with dynamic input-CodePudding

I am trying to create a dynamic input that I can easily change that will be used inside a for loop. Inside a for loop that I am using, there is a variable, let's call it b, that is used multiple times, and every time I want to slightly change the value stored in b, I have to go through and find each instance of b and change it. I am hoping to create an input outside of the for loop that I can use to update b inside the for loop.

This is a very simplified example of what I am currently doing.

df <- data.frame(replicate(3,sample(0:100,10,rep=TRUE)))
for (i in c(1:nrow(df))){
  nm <- i
  if (nm == nrow(df)) {
    next
  } else {
    x <- df[c((nm):(nm 1)),]
    b <- x$X1[1]
    print(b > x$X2[2])
  }
}

Here is an example of what I am trying to do, however, in this example the input does not work because dateframe x is not yet created. Some more examples of the inputs could be b <- x$X1[2], b <- x$X2[1], b <- x$X3[2], etc

df <- data.frame(replicate(3,sample(0:100,10,rep=TRUE)))

##input

b <- x$X1[1]

###

for (i in c(1:nrow(df))){
  nm <- i
  if (nm == nrow(df)) {
    next
  } else {
    x <- df[c((nm):(nm 1)),]
    print(b > x$X2[2])
  }
}

I was looking at this post here and it looks like there is a way to achieve what I am trying to do, however, in the post, they assign a value to the variable. In my example, the value stored in b will change in every iteration of the for loop so this method will not work.

CodePudding user response：

It seems like essentially you want to delay the evaluation of the expression for b. As written, x$X1[1] will be evaluated (or at least attempted to be) when b is defined. You can mark it as an expression and evaluate it later. There are some base functions for this but I find rlang often easier to use

##input

b <- rlang::expr(x$X1[1])

###

for (i in c(1:nrow(df))){
  nm <- i
  if (nm == nrow(df)) {
    next
  } else {
    x <- df[c((nm):(nm 1)),]
    print(rlang::eval_bare(b) > x$X2[2])
  }
}