Create a vector iteratively with conditional

[asolisc]

Hi guys, I have a very straightforward question:

My goal is to create a vector where the next value is equal to last value + a constant (call this add1). However, when we get to a certain length (call this threshold) we change the constant added to another one (call this add2). The vector starts at 1. Thus, for example:

Let: add1 = 1, threshold = 5, add2 = 4. We would have:

x
1 # start at 1
2 # since add1 = 1, add 1 to last value
3 # since add1 = 1, add 1 to last value
4 # since add1 = 1, add 1 to last value
8 # since threshold = 5,  and this is the 5th position, add `add2 = 4` to last value
9
10
11
12
16 # since this is the 10th position (a multiple of `threshold`), add 4 to last value.

So far, I've tried this, but it doesn't work:

x <- c()
x[1] <- 1

for (i in seq_along(1:30)) {
  
  if (i %% threshold == 0) {
    
    x[i+1] <- x[i] + add2
  }
  
  x[i+1] <- x[i] + add1
}

I'd love to do this with a map_* function, but I can't even do it with a for loop... let alone doing it with purrr

[Fer]

Not sure how well I understand, but

add1 = 1
threshold = 5
add2 = 4

x <- c()
x[1] <- 1

for (i in 2:30) {
  if (i <= threshold) {
  x[i] <- x[i-1] + add1    
  } else {
    x[i] <- x[i-1] + add2  
}
}

and then seems ok:

> x
 [1]   1   2   3   4   5   9  13  17  21  25  29  33  37  41  45  49  53  57  61
[20]  65  69  73  77  81  85  89  93  97 101 105

> diff(x)
 [1] 1 1 1 1 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4

cheers
Fer

[siddharthprabhu]

Good solution Fer, but if I understand the problem correctly, the OP wants to add add2 at every multiple of the threshold index. In the example above, 4 should be added at every 5th, 10th, 15th... index.

Just a small modification to your solution will give the desired result. I would also pre-allocate the result vector beforehand to make the loop more efficient.

add1 <- 1
threshold <- 5
add2 <- 4

input_vec <- 1:20

print(input_vec)
#>  [1]  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20

output_vec <- vector("numeric", length(input_vec))

for (i in seq_along(input_vec)) {
  if (i == 1) {
    output_vec[[i]] <- input_vec[[i]]
  } else if (i %% threshold == 0) {
    output_vec[[i]] <- output_vec[[i - 1]] + add2
  } else {
    output_vec[[i]] <- output_vec[[i - 1]] + add1
  }
}

print(output_vec)
#>  [1]  1  2  3  4  8  9 10 11 12 16 17 18 19 20 24 25 26 27 28 32

^{Created on 2020-10-05 by the reprex package (v0.3.0)}

[asolisc]

Many thanks to both of you!!! Now my next challenge is being able to do it with purrr haha.

This solved my issue, once again, thank you so much!!