r - Can somebody help me understand "for" functions?

Question

For a homework question I have the following code:

Could someone explain me what is happening in the code, and why it results in 22?

k <- 1    
for (i in 1:3){    
  k <- k + 1  
  for (j in 1:2){
    k = k * j
  }
}

Answer 1

An easy way to understand how loops work in any programming language is to follow, step by step what it is doing.

Here a simple example of how you can "debug" in paper. This could be useful in the future.

The table below represents each iteration of your loops and the values of each operation.

You should read it in this way:

1. Initially k = 1;
2. In the first iteration of the outer loop k becomes the current value of k (1) + 1, so k = 2.
3. Then the inner loop makes its first iteration, then j = 1 and k = k (2) * j (1), so k = 2
4. Then the second iteration of inner loop, j becomes 2 and k is still 2, so k = k (2) * j (2) so k = 4
5. Then returns to the outer loop where i becomes 2 and k = k (4) + 1 => k = 5
6. Then start again the inner loop (repeat steps 3 and 4)

and repeat until i is 3.