R中向量的for循环如何工作？

Question

I am trying to understand the working of for loop for a vector in R. I figured out the solution for my problem but left with a doubt about its fundamental working.

In the process of creating a function, I came across this problem. The problem is that the for loop is looping through the elements of the vector but till a certain index.

## the output is partially complete, seems like it didn't loop through all the values however the loop counter is perfect
temp_vector<- c(1, NA,Inf, NaN,3,2,4,6,4,6,7,3,2,5,NaN, NA, 3,3,NaN, Inf, Inf, NaN, NA, 3,5,6,7)
ctr<- 0
for(i in  temp_vector){

  temp_vector[i]<- ifelse((!is.na(temp_vector[i])) & (!is.infinite(temp_vector[i])), temp_vector[i], 0  )
  ## replace the element of vector by 0 if they are Inf or NA or NaN
  ctr<- ctr+1
}
temp_vector
print(ctr)

# output
> temp_vector
[1]   1   0   0   0   3   2   4   6   4   6   7   3   2   5 NaN  NA   3   3 NaN Inf Inf NaN  NA   3   5   6   7
> print(ctr)
[1] 27

## this is generating correct output
temp_vector<- c(1, NA,Inf, NaN,3,2,4,6,4,6,7,3,2,5,NaN, NA, 3,3,NaN, Inf, Inf, NaN, NA, 3,5,6,7)
for(i in 1:length(temp_vector)){

  temp_vector[i]<- ifelse((!is.na(temp_vector[i])) & (!is.infinite(temp_vector[i])), temp_vector[i], 0  )
  ## replace the element of vector by 0 if they are Inf or NA or NaN
}
temp_vector

# output
> temp_vector
[1] 1 0 0 0 3 2 4 6 4 6 7 3 2 5 0 0 3 3 0 0 0 0 0 3 5 6 7

Below are few variants of for loops that I tried which generate different output, I am trying to understand how it basically works. It would be helpful, if you could shed some light on it. Thanks!

## variant-0
y <- c(2,5,3,9,8,11,6)
count <- 0
for (val in y) {
  if(val %% 2 == 0) 
    count = count+1
}
print(count)  

# output
[1] 3

## variant-1
x<- c(2,4,6,4,6,7,3,2,5,6)
for(i in x){

  x[i]<- ifelse(x[i]==6, NaN, x[i])
}
x

# output, Last element of the vector is not a NaN
[1]   2   4 NaN   4 NaN   7   3   2   5   6



## variant-2
x<- c(2,4,6,4,6,7,3,2,5,6)
ctr<- 0
for(i in x){

  x[i]<- ifelse(x[i]==6, NaN, x[i])
  ctr<- ctr+1
}
x
print(ctr)

# output, Note: Last element of the vector is not a NaN
> x
[1]   2   4 NaN   4 NaN   7   3   2   5   6
> print(ctr)
[1] 10



## variant-3
x<- c(2,4,6,4,6,7,3,2,5,6)
ctr<- 0
for(i in x){

  x[ctr]<- ifelse(x[ctr]==6, NaN, x[ctr])
  ctr<- ctr+1
}
x
print(ctr)


# output. Note: the counter is perfect
> x
[1]   2   4 NaN   4 NaN   7   3   2   5   6
> print(ctr)
[1] 10


## variant-4
x<- c(2,4,6,4,6,7,3,2,5,6)
ctr<- 0
for(i in x){

  i<- ifelse(i==6, NaN, i)
  ctr<- ctr+1
}
x
print(ctr)

# output
> x
[1] 2 4 6 4 6 7 3 2 5 6
> print(ctr)
[1] 10

Answer 1

Consider the following example:

> y <- c(2, 5, 3, 9, 8, 11, 6)

for loops over the vector you provide. In the first case you iterate over the elements of vector y :

> for (val in y) {
+     print(val)
+ }
[1] 2
[1] 5
[1] 3
[1] 9
[1] 8
[1] 11
[1] 6

In the second case you are iterating over the elements of vector 1:length(y) , meaning c(1, 2, 3, 4, 5, 6, 7) :

> for (val in 1:length(y)) {
+     print(val)
+ }
[1] 1
[1] 2
[1] 3
[1] 4
[1] 5
[1] 6
[1] 7

You got this mixed up in your code above. Hope this clears things up!

Answer 2

The for loop that you are using works in this way (I will take up the first variant ie variant-0)

This is normal definition part

y <- c(2,5,3,9,8,11,6)
count <- 0

Here is where the business begins:

for (val in y) 

Here, val will contain values of vector y which will be changing in each iteration.

For example:

val for iteration 1: 2;
val for iteration 2: 5;
val for iteration 3: 3;

and so on.

{
  if(val %% 2 == 0) 
    count = count+1
}
print(count) 

So, here, count will be incremented when val is even ie for iteration: 1,5,7

So, value of count is 3.