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277 lines
8.7 KiB
Markdown
277 lines
8.7 KiB
Markdown
# Calculating with dc
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## Introduction
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dc(1) is a non standard, but commonly found, reverse-polish Desk
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Calculator. According to Ken Thompson, "dc is the oldest language on
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Unix; it was written on the PDP-7 and ported to the PDP-11 before Unix
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\[itself\] was ported".
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Historically the standard bc(1) has been implemented as a *front-end to
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dc*.
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## Simple calculation
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In brief, the *reverse polish notation* means the numbers are put on the
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stack first, then an operation is applied to them. Instead of writing
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`1+1`, you write `1 1+`.
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By default `dc`, unlike `bc`, doesn't print anything, the result is
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pushed on the stack. You have to use the "p" command to print the
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element at the top of the stack. Thus a simple operation looks like:
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$ dc <<< '1 1+pq'
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2
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I used a "here string" present in bash 3.x, ksh93 and zsh. if your shell
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doesn't support this, you can use `echo '1 1+p' | dc` or if you have GNU
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`dc`, you can use `dc -e '1 1 +p`'.
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Of course, you can also just run `dc` and enter the commands.
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The classic operations are:
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- addition: `+`
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- subtraction: `-`
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- division: `/`
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- multiplication: `*`
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- remainder (modulo): `%`
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- exponentiation: `^`
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- square root: `v`
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GNU `dc` adds a couple more.
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To input a negative number you need to use the `_` (underscore)
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character:
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$ dc <<< '1_1-p'
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2
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You can use the *digits* `0` to `9` and the *letters* `A` to `F` as
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numbers, and a dot (`.`) as a decimal point. The `A` to `F` **must** be
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capital letters in order not to be confused with the commands specified
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with lower case characters. A number with a letter is considered
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hexadecimal:
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dc <<< 'Ap'
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10
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The **output** is converted to **base 10** by default
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## Scale And Base
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`dc` is a calulator with abitrary precision, by default this precision
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is 0. thus `dc <<< "5 4/p"` prints "1".
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We can increase the precision using the `k` command. It pops the value
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at the top of the stack and uses it as the precision argument:
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dc <<< '2k5 4/p' # prints 1.25
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dc <<< '4k5 4/p' # prints 1.2500
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dc <<< '100k 2vp'
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1.4142135623730950488016887242096980785696718753769480731766797379907\
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324784621070388503875343276415727
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dc supports *large* precision arguments.
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You can change the base used to output (*print*) the numbers with `o`
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and the base used to input (*type*) the numbers with `i`:
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dc << EOF
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20 p# prints 20, output is in base 10
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16o # the output is now in base 2 16
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20p # prints 14, in hex
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16i # the output is now in hex
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p # prints 14 this doesn't modify the number in the stack
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10p # prints 10 the output is done in base 16
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EOF
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Note: when the input value is modified, the base is modified for all
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commands, including `i`:
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dc << EOF
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16i 16o # base is 16 for input and output
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10p # prints 10
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10i # ! set the base to 10 i.e. to 16 decimal
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17p # prints 17
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EOF
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This code prints 17 while we might think that `10i` reverts the base
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back to 10 and thus the number should be converted to hex and printed as
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11. The problem is 10 was typed while the input base 16, thus the base
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was set to 10 hexadecimal, i.e. 16 decimal.
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dc << EOF
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16o16o10p #prints 10
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Ai # set the base to A in hex i.e. 10
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17p # prints 11 in base 16
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EOF
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## Stack
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There are two basic commands to manipulate the stack:
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- `d` duplicates the top of the stack
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- `c` clears the stack
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<!-- -->
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$ dc << EOF
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2 # put 2 on the stack
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d # duplicate i.e. put another 2 on the stack
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*p # multiply and print
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c p # clear and print
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EOF
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4
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dc: stack empty
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`c p` results in an error, as we would expect, as c removes everything
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on the stack. *Note: we can use `#` to put comments in the script.*
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If you are lost, you can inspect (i.e. print) the stack using the
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command `f`. The stack remains unchanged:
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dc <<< '1 2 d 4+f'
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6
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2
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1
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Note how the first element that will be popped from the stack is printed
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first, if you are used to an HP calculator, it's the reverse.
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Don't hesitate to put `f` in the examples of this tutorial, it doesn't
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change the result, and it's a good way to see what's going on.
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## Registers
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The GNU `dc` manual says that dc has at least **256 registers**
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depending on the range of unsigned char. I'm not sure how you are
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supposed to use the NUL byte. Using a register is easy:
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dc <<EOF
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12 # put 12 on the stack
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sa # remove it from the stack (s), and put it in register 'a'
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10 # put 10 on the stack
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la # read (l) the value of register 'a' and push it on the stack
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+p # add the 2 values and print
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EOF
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The above snippet uses newlines to embed comments, but it doesn't really
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matter, you can use `echo '12sa10la+p'| dc`, with the same results.
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The register can contain more than just a value, **each register is a
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stack on its own**.
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dc <<EOF
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12sa #store 12 in 'a'
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6Sa # with a capital S the 6 is removed
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# from the main stack and pushed on the 'a' stack
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lap # prints 6, the value at the top of the 'a' stack
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lap # still prints 6
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Lap # prints 6 also but with a capital L, it pushes the value in 'a'
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# to the main stack and pulls it from the 'a' stack
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lap # prints 12, which is now at the top of the stack
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EOF
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## Macros
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`dc` lets you push arbitrary strings on the stack when the strings are
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enclosed in `[]`. You can print it with `p`: `dc <<< '[Hello World!]p'`
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and you can evalute it with x: `dc <<< '[1 2+]xp'`.
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This is not that interesting until combined with registers. First, let's
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say we want to calculate the square of a number (don't forget to include
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`f` if you get lost!):
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dc << EOF
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3 # push our number on the stack
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d # duplicate it i.e. push 3 on the stack again
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d**p # duplicate again and calculate the product and print
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EOF
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Now we have several cubes to calculate, we could use `dd**` several
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times, or use a macro.
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dc << EOF
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[dd**] # push a string
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sa # save it in register a
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3 # push 3 on the stack
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lax # push the string "dd**" on the stack and execute it
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p # print the result
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4laxp # same operation for 4, in one line
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EOF
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## Conditionals and Loops
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`dc` can execute a macro stored in a register using the `lR x` combo,
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but it can also execute macros conditionally. `>a` will execute the
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macro stored in the register `a`, if the top of the stack is *greater
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than* the second element of the stack. Note: the top of the stack
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contains the last entry. When written, it appears as the reverse of what
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we are used to reading:
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dc << EOF
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[[Hello World]p] sR # store in 'R' a macro that prints Hello World
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2 1 >R # do nothing 1 is at the top 2 is the second element
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1 2 >R # prints Hello World
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EOF
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Some `dc` have `>R <R =R`, GNU `dc` had some more, check your manual.
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Note that the test "consumes" its operands: the 2 first elements are
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popped off the stack (you can verify that
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`dc <<< "[f]sR 2 1 >R 1 2 >R f"` doesn't print anything)
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Have you noticed how we can *include* a macro (string) in a macro? and
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as `dc` relies on a stack we can, in fact, use the macro recursively
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(have your favorite control-c key combo ready ;)) :
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dc << EOF
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[ [Hello World] p # our macro starts by printing Hello World
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lRx ] # and then executes the macro in R
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sR # we store it in the register R
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lRx # and finally executes it.
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EOF
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We have recursivity, we have test, we have loops:
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dc << EOF
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[ li # put our index i on the stack
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p # print it, to see what's going on
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1 - # we decrement the index by one
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si # store decremented index (i=i-1)
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0 li >L # if i > 0 then execute L
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] sL # store our macro with the name L
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10 si # let's give to our index the value 10
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lLx # and start our loop
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EOF
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Of course code written this way is far too easy to read! Make sure to
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remove all those extra spaces newlines and comments:
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dc <<< '[lip1-si0li>L]sL10silLx'
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dc <<< '[p1-d0<L]sL10lLx' # use the stack instead of a register
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I'll let you figure out the second example, it's not hard, it uses the
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stack instead of a register for the index.
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## Next
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Check your dc manual, i haven't decribed everything, like arrays (only
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documented with "; : are used by bc(1) for array operations" on solaris,
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probably because *echo '1 0:a 0Sa 2 0:a La 0;ap' \| dc* results in
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//Segmentation Fault (core dump) //, the latest solaris uses GNU dc)
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You can find more info and dc programs here:
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- <http://en.wikipedia.org/wiki/Dc_(Unix)>
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And more example, as well as a dc implementation in python here:
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- <http://en.literateprograms.org/Category:Programming_language:dc>
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- <http://en.literateprograms.org/Desk_calculator_%28Python%29>
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The manual for the 1971 dc from Bell Labs:
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