RC27. The Walrus and the Char Pointer

Daily dispatches from my 12 weeks at the Recurse Center in Summer 2023

This short post has nothing to do with chars or pointers (I couldn’t resist). It does, however, have to do with walruses – walrus operators, to be exact.

THE WALRUS OPERATOR (?)

Like the Dall-e hallucination above, the walrus operator (:=) looks … sort of walrus-y. It’s a thing I’ve known exists and have probably looked up and then forgotten about a dozen times. Yesterday it came up again while working on Implement DNS in a Weekend. Now I’m going to write about it, and by virtue of doing so I will never have to look the walrus operator up again.

The Walrus . . .

Normally, you might assign a variable in Python like so:

x = 10      # Step 1: Assign

and then evaluate it in a REPL like so:

x           # Step 2: Evaluate

10

Or perhaps you prefer to assign and then print using the print() function:

x = 15      # Step 1: Assign
print(x)    # Step 2: Evaluate

15

The walrus operator allows you to assign and evaluate in a single step. That’s it.

(y := 100)  # Steps 1 and 2: Assign and evaluate!

100

Or:

print(z := "O Oysters, come and walk with us!") # Assign and evaluate!
print(z)

O Oysters, come and walk with us!
O Oysters, come and walk with us!

. . . and the Char Pointer

I guess the reference to chars and pointers is not wholy irreievalt, since the walrus operator turns out to enable very C-like syntax. In C, for instance, it is very natural to do something like this:

while ((c = getchar()) != EOF) {
    // do stuff with `c`
}

In this case we’re assignining c and evaluating the assignment expression in one go, so that c can be be used as a component of the predicate c != EOF.

While you would probably never read a file like this in Python, you could approximate this C-style syntax using the walrus:

while ((line := f.readline()) != '\n'):
    print(line)

The sun was shining on the sea,

    Shining with all his might:

He did his very best to make

    The billows smooth and bright—

And this was odd, because it was

    The middle of the night.

As another example, in C you could do things like this:

    int i = 0;
    char s[] = "The time has come";

    printf("%c\n", s[i]);           // s[0] => T
    printf("%c\n", s[++i]);         // s[1] => h
    printf("%c\n", s[(i += 1)]);    // s[2] => e

T
h
e

++i and (i += 1) both increment i and evaluate to new i in one go. You could walrus it up to accomplish a similar feat in Python:

s = "Of shoes and ships and sealing wax"
i = 0

print(s[i])
print(s[(i := i + 1)])
print(s[(i := i + 1)])
print(s[(i := i + 1)])

O
f

s

Other goings on

• Coffee-chat hat trick. Three damn fine coffee chats with a bunch of damn fine folks who are alums or in the last week of batch, all of whom offered some welcome perspective for someone at the halfway-point moment of reckoning. As a group the convos left me with the abiding sense that it’s okay not to build a React app or do whatever thing seems the most portfolio-ready.
• Continued on to Part 2 (which turned out to be Part 1 of Part 2) of the DNS project. Appreciating this introduction to all things DNS, which is piquing my curiosity about other network-y things.
• Spent a while redrawing CPU diagrams and talked CPU implementations at weekly nand2tetris meeting