Python and Viv · Lists and growable arrays

Growing a list

What to notice

squares.append(x) is squares = append(squares, x). The append gives back the grown array and the program rebinds the name; a second name taken before the append keeps the old array.
The comprehensions are the same in both. A comprehension of numbers makes a Viv array; squares[:HEAD] / 2 would divide every element too.
squares[-1] and squares[:-1] count from the end in both.
extend is a function that returns the joined array, as append is.
v in squares and a if c else b read the same in both. On an array of numbers in compares as == does, so a NaN is in no array, as in NumPy.
A list of lists is a matrix in Viv. Appending a row to [] makes it 1 by 3, shape(table) gives its size, and for row in rows(table) walks its rows (for row in table would walk its numbers).
Viv's print of an array writes whole numbers without .0; fmt(halves) writes each element in Python's form, [0.0, 0.5, 2.0, 4.5].

Open the Viv in the workbench The atlas of the Viv py2viv refuses this Python (the reason is in its column).

Python growable.py · 39 lines

Viv growable.viv · 48 lines

py2viv refuses growable.py

1 start empty and append

4squares = []

5N = 10

6for i in range(N):

7 squares.append(i * i)

8print(squares)

2squares = []

3N = 10

4for i to N {

5 squares = append(squares, i * i)

7print(squares)

py2viv refuses this program:

growable.py:7:5: not numeric Python: list.append (lifted by R17)
        squares.append(i * i)
        ^^^^^^^^^^^^^^^^^^^^^
  py2viv changes a list (append, pop, insert, extend, remove, clear, reverse, sort) only when nothing else can hold it: bound only to new lists (`[]`, a list literal, a comprehension, list(...), sorted(...)) and read nowhere an alias could be taken (a second name, an argument, a container, a nested def); otherwise allocate the final size with np.zeros(n) and write by index.
growable.py:8:7: not numeric Python: printing a value of unknown type (it comes from a construct refused elsewhere)
    print(squares)
          ^^^^^^^
  Fix the refusal that produced it.
growable.py:11:21: not numeric Python: iterating over a value of unknown type
    evens = [s for s in squares if s % 2 == 0]
                        ^^^^^^^
  Fix the refusal that produced it.
growable.py:12:7: not numeric Python: printing a value of unknown type (it comes from a construct refused elsewhere)
    print(evens, len(evens))
          ^^^^^
  Fix the refusal that produced it.
growable.py:15:26: not numeric Python: iterating over a value of unknown type
    halves = [s / 2 for s in squares[:4]]
                             ^^^^^^^^^^^
  Fix the refusal that produced it.
growable.py:16:7: not numeric Python: printing a value of unknown type (it comes from a construct refused elsewhere)
    print(halves)
          ^^^^^^
  Fix the refusal that produced it.
growable.py:20:1: not numeric Python: list.extend (lifted by R17)
    squares.extend(more)
    ^^^^^^^^^^^^^^^^^^^^
  py2viv changes a list (append, pop, insert, extend, remove, clear, reverse, sort) only when nothing else can hold it: bound only to new lists (`[]`, a list literal, a comprehension, list(...), sorted(...)) and read nowhere an alias could be taken (a second name, an argument, a container, a nested def); otherwise allocate the final size with np.zeros(n) and write by index.
growable.py:21:21: not numeric Python: printing a value of unknown type (it comes from a construct refused elsewhere)
    print(len(squares), squares[-1])
                        ^^^^^^^^^^^
  Fix the refusal that produced it.
growable.py:22:1: not numeric Python: an empty list grown and also bound to something else
    squares = squares[:-1]
    ^^^^^^^
  py2viv grows a list bound only to new lists and read only by `len`, indexing, loops, conditions, `return` and its own methods; Viv's `lst = append(lst, x)` gives a new array, so an alias would not see the change.
growable.py:23:21: not numeric Python: printing a value of unknown type (it comes from a construct refused elsewhere)
    print(len(squares), squares[-1])
                        ^^^^^^^^^^^
  Fix the refusal that produced it.
growable.py:27:34: not numeric Python: `in` of a value of unknown type
        print(v, "is in the list" if v in squares else "is not in the list")
                                     ^^^^^^^^^^^^
  Test membership in a list or an array.
growable.py:38:8: not numeric Python: `==` of a Python list or a tuple
    assert evens == [0, 4, 16, 36, 64]
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  Python compares lists and tuples as a whole (in order, or raises TypeError against a number); make it a NumPy array to compare entry by entry.
12 constructs outside the numeric subset (tools/py2viv/SUBSET.md)

2 a comprehension with a condition

11evens = [s for s in squares if s % 2 == 0]

12print(evens, len(evens))

10evens = [s for s in squares if s % 2 == 0]

11print(evens, len(evens))

(refused)

3 a comprehension that maps

15halves = [s / 2 for s in squares[:4]]

16print(halves)

14HEAD = 4

15halves = [s / 2 for s in squares[:HEAD]]

16print(fmt(halves))

(refused)

4 extend, and drop the last element

19more = [100, 121]

20squares.extend(more)

21print(len(squares), squares[-1])

22squares = squares[:-1]

23print(len(squares), squares[-1])

19more = [100, 121]

20squares = extend(squares, more)

21print(len(squares), squares[-1])

22squares = squares[:-1]

23print(len(squares), squares[-1])

(refused)

5 membership and sums

26for v in [49, 50]:

27 print(v, "is in the list" if v in squares else "is not in the list")

28print(sum(squares), sum(squares) / len(squares))

26for v in [49, 50] {

27 print(v, "is in the list" if v in squares else "is not in the list")

28}

29print(sum(squares), fmt(sum(squares) / len(squares)))

(refused)

6 rows onto a table

31table = []

32for i in range(1, 4):

33 table.append([i, i * i, i ** 3])

34for row in table:

35 print(row)

32table = []

33ROWS = 4

34for i from 1 to ROWS {

35 table = append(table, [i, i * i, i ^ 3])

36}

37for row in rows(table) {

38 print(row)

39}

(refused)

7 checks

38assert evens == [0, 4, 16, 36, 64]

39assert len(table) == 3 and len(table[0]) == 3

42test "growable" {

43 assert_true(all(evens == [0, 4, 16, 36, 64]))

44 assert_true(shape(table)[0] == 3 and shape(table)[1] == 3)

45}

(refused)

What they print

identical — the two programs print the same text.

Dimmed lines are left out of the comparison. Each Viv test block prints one PASS line; the Python checks the same facts with assert, which prints nothing.

$ python3 growable.py

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
[0, 4, 16, 36, 64] 5
[0.0, 0.5, 2.0, 4.5]
12 121
11 100
49 is in the list
50 is not in the list
385 35.0
[1, 1, 1]
[2, 4, 8]
[3, 9, 27]

$ viv run growable.viv

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
[0, 4, 16, 36, 64] 5
[0.0, 0.5, 2.0, 4.5]
12 121
11 100
49 is in the list
50 is not in the list
385 35.0
[1, 1, 1]
[2, 4, 8]
[3, 9, 27]
PASS growable

Growing a list

What to notice

What they print

$ python3 growable.py

$ viv run growable.viv

Viv compiled to wasm