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A deeper introduction to Unix DC

72 points by ben165 5 years ago | 19 comments

arexxbifs 5 years ago
Related fun fact: bc is a dc preprocessor[0].
[0] https://en.wikipedia.org/wiki/Bc_(programming_language)#Hist...
staycoolboy 5 years ago
I love reading examples of these "great-great-grandparent" applications hidden in /bin. I've used dc in shell scripts for years, but for the simplest of simple computations (e.g., automating FDISK partitioning and doing sector math). Had no idea it could do this.
TIL this and a cool CSS trick.
c0l0 5 years ago
dc is worth having installed because of this CLI gem alone:
```
    echo "[q]sa[ln0=aln256%Pln256/snlbx]sb729901041524823122snlbxq" | dc
```
I've never tried to understand what's really going on to produce the result, but also wouldn't really know where to start - maybe someone in HN's audience can enlighten me? :)
- em500 5 years ago
  dc and bc are standard POSIX utils (and also part of busybox for embedded systems) so for most unix-like systems they're installed by default unless you take active measures to exclude them.
  - c0l0 5 years ago
    POSIX.1-2017 describes the `dc` program as "excluded" from the standard; check https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xcu... for details.
    Many GNU/Linux distros (at least Debian and RHEL) install neither `dc` nor `bc` by default, afaik.
    - yjftsjthsd-h 5 years ago
      > Many GNU/Linux distros (at least Debian and RHEL) install neither `dc` nor `bc` by default, afaik.
      Yeah, I recently wrote a script that used bc and immediately discovered that it didn't work on at least Arch Linux (and I think others, although I only seem to have added it to the Arch ansible config...); conveniently, I only needed the most trivial of calculations, so I just shifted to awk, which has better default availability.
    - akira2501 5 years ago
      Gentoo doesn't include it in the common 'stage3' tarball, either. Which is annoying because it's actually required to compile the linux kernel.
    - em500 5 years ago
      I stand corrected, only bc is included in POSIX.
  - 5 years ago
- na85 5 years ago
  https://stackoverflow.com/questions/37880810/how-come-is-thi...
- kps 5 years ago
```
    echo '1033333377708482P' | dc
```
  produces the same output using Unix dc but not in the GNU near-clone. (To understand why, you have to R the P as well as the M.)
- m463 5 years ago
```
  echo 'EGSZ%%%'|tr 'E-Z%' 'R-ZA-M!'
```
Hitton 5 years ago
Funny. Despite being long time Linux user this is the first time I remember hearing about dc. I only knew bc.
- m463 5 years ago
  I have a way of looking at this. More people are familiar with apples than okra. (there's a reason for that)

    [Monte Carlo approximation of Pi.
    Registers:
    u - routine : execute i if sum of squares less than 1
    i - routine : increment register x
    z - routine : iterator - execute u while n > m++
    r - routine : RANDU PRNG
    m - variable: number of samples
    x - variable: number of samples inside circle
    s - variable: seed for r
    k - variable: scale for division
    n - variable: number of iterations (user input)
    ]c
    [lrx 2^ lrx 2^ + 1>i]su
    [lx 1+ sx]si
    [lu x lm 1+ d sm ln>z]sz
    [0k ls 65539 * 2 31^ % d ss lkk 2 31 ^ /]sr
    ? sn
    5dksk
    1 ss
    lzx
    lx lm / 4*
    p

    $ dc pi.dc
    100000
    3.13372

azizsaya 5 years ago
Aah! command I encounter and ignore when I mistype cd
- yjftsjthsd-h 5 years ago
  At least it's not sl ;)
082349872349872 5 years ago
6581840dnP
- kps 5 years ago
```
    156 is unimplemented
```
  Is that a gnuism?
  - 082349872349872 5 years ago
    Yes, sorry. Try:
```
    6582352
    dpP
```
    (at least we now know the error is old enough to be in octal)