Why Do R Objects Not Print in a Function or a "For" Loop

R not printing the right thing in loop

Okay, you are trying to construct a variable name using paste and then do a table. You are simply passing the name of the variable to table, not the variable object itself. For this sort of approach you want to use get()

for (i in 1:4) {
    with(get(paste0("group", i), table(BMI_obese, A1.1))
}
#example saving as a list (using lapply rather than for loop)
group1 <- data.frame(x=LETTERS[1:10], y=(1:10)[sample(10, replace=TRUE)])
group2 <- data.frame(x=LETTERS[1:10], y=(1:10)[sample(10, replace=TRUE)])
result <- lapply(1:2, function(i) with(get(paste0("group", i)), table(x, y)))
#look at first six rows of each:
head(result[[1]])
head(result[[2]])

#example illustrating fetching objects from a string name
data(mtcars)
head(with(get("mtcars"), table(disp, cyl)))
head(with(get("mtcars"), table(disp, "cyl")))
#Error in table(disp, "cyl") : all arguments must have the same length
head(with(get("mtcars"), table(disp, get("cyl"))))

For Loop in R not printing what I need

if you are using for loop we need to mention jth interation equals to i.

for(i in list1){
  for(j in list2[list1==i]){
    print(i + j)
  }
}
[1] 3
[1] 6
[1] 9
[1] 12

Role of print() function in a for loop

The reason for this is, that all the stuff inside the loop gets evaluated, but never returned to somewhere. Though print deliver something I wouldn't advise it, because you can't use these values later, because they get returned to the console rather then the global environment. Instead you might want to assign them to something.
For example:

#example data
df <- data.frame(x = 1:10, y = rnorm(10))
#it is good to create the output in the desired length first
#it is much more efficient in terms of speed an memory beeing used but here it don't really matter
ret <- vector("list", NCOL(df)) 

for(x in seq_len(NCOL(df))){
  ret[[x]] <- c(mean(df[[x]]),
    var(df[[x]]),
    sd(df[[x]]))
}
ret

Though I don't advise that neither. For the most (if not all things) you can do with a for loop you can use the apply family of functions from base r or the map function from purrr. This would look that way:

library(purrr)
map(df, ~c(mean(.), var(.), sd(.)))
#or even save it with names
ret <- map(df, ~c(mean = mean(.), var = var(.), sd = sd(.)))
ret

The apply/map variants are faster & shorter, but more importantly for me easier to understand and have less room for errors. Though there are a hole bunch of other arguments why you might want to use apply/map.

Loop does not print function output

I'm using the example data from the function help for boot.ci since you haven't included any. Some functions need to be forced to print when they're inside other functions, often using the function print. Copy the format of this:

library(boot)
ratio <- function(d, w) sum(d$x * w)/sum(d$u * w)
city.boot <- boot(city, ratio, R = 999, stype = "w", sim = "ordinary")

for (i in letters[1:5]) {
   cat("This is number:\n", i, "\n")
   print(boot.ci(city.boot, conf = c(0.90, 0.95),type = c("bca")))
}

Notice that you don't need to paste inside of cat. But in general it is good to avoid cat as print is a slightly gentler function. Using cat can lead to annoying messages that are hard later.

In future, please supply reproducible examples! (And let us know which packages you're using!)

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