Assume:

ri!n = 5

1. Outer forEach

items: enumerate(ri!n) + 1

enumerate(5) gives:

{0, 1, 2, 3, 4}

Adding 1:

{1, 2, 3, 4, 5}

So the outer loop runs 5 times.

2. fv!item represents the current number

During the outer loop:

1 → first iteration

2 → second iteration

3 → third iteration

4 → fourth iteration

5 → fifth iteration

So when fv!item = 3, the inner loop needs to generate 1, 2, 3.

3. Inner forEach

a!forEach(

items: enumerate(fv!item) + 1,

expression: fv!item

)

If the current outer value is 3:

enumerate(3) → {0,1,2}

+ 1 → {1,2,3}

The inner loop returns:

{1,2,3}

4. joinarray()

joinarray(

{1,2,3},

" "

)

converts the list into:

"1 2 3"

The same happens for every outer iteration.

Final result

1

1 2

1 2 3

1 2 3 4

1 2 3 4 5

In one sentence

The outer forEach() determines how many rows to create, while the inner forEach() generates numbers from 1 to the current row number, and joinarray() formats them as a space-separated string.