Linux dc command

On Unix-like operating systems, the dc command is an arbitrary-precision arithmetic calculator.

Description

dc is a desk calculator which supports arbitrary, unlimited-precision arithmetic and reverse-polish (postfix) notation. It also lets you define and call macros. Normally dc reads from the standard input; if any command arguments are given to it, they are file names, and dc reads and executes the contents of the files before reading from standard input. All normal output is to standard output; all error output is to standard error.

A reverse-polish calculator stores numbers on a stack. Entering a number pushes it on the stack. Arithmetic operations pop arguments off the stack and push the results.

To enter a number in dc, type the digits (using uppercase letters A through F as "digits" when working with input bases greater than ten), with an optional decimal point. Exponential notation is not supported. To enter a negative number, begin the number with "_" (an underscore). "-" (dash or hyphen) cannot be used for this, as it is a binary operator for subtraction instead. To enter two numbers in succession, separate them with spaces or newlines. These have no meaning as commands.

Syntax

dc [-V] [--version] [-h] [--help] [-e scriptexpression] 
   [--expression=scriptexpression] [-f scriptfile] [--file=scriptfile] 
   [file ...]

Options

If any command-line parameters remain after processing the above, these parameters are interpreted as the names of input files to be processed. A file name of "-" (dash or hyphen) refers to the standard input stream. The standard input process if no script files or expressions are specified.

Printing commands

Arithmetic

Most arithmetic operations are affected by the "precision value", which can be set with the k command. The default precision value is zero, which means that all arithmetic except for addition and subtraction produces integer results.

Stack control

Registers

dc provides at least 256 memory registers, each named by a single character. You can store a number or a string in a register and retrieve it later.

Each register also contains its own stack. The current register value is the top of the register's stack.

Parameters

dc has three parameters that control its operation: the precision, the input radix, and the output radix. The precision specifies the number of fraction digits to keep in the result of most arithmetic operations. The input radix controls the interpretation of numbers typed in; all numbers typed in use this radix. The output radix is used for printing numbers.

The input and output radices are separate parameters; you can make them unequal, which can be useful or confusing. The input radix must be between 2 and 16 inclusive. The output radix must be at least 2. The precision must be zero or greater. The precision is always measured in decimal digits, regardless of the current input or output radix.

Strings

dc has a limited ability to operate on strings and on numbers; the only things you can do with strings are print them and execute them as macros (which indicates the contents of the string are processed as dc commands). All registers and the stack can hold strings, and dc always knows whether any given object is a string or a number. Some commands such as arithmetic operations demand numbers as arguments and print errors if given strings. Other commands can accept either a number or a string; for example, the p command can accept either and prints the object according to its type.

Macros are often stored in registers; [1p]sa stores a macro to print 1 into register a, and lax invokes this macro.

Status inquiry

Miscellaneous

Note that each stacked instance of a register has its own array associated with it. Thus 1 0:a 0Sa 2 0:a La 0;ap prints 1, because the 2 was stored in an instance of 0:a that was later popped.

Examples

First, at the command line, let's enter the desktop calculator.

dc

Nothing appears to have happened. dc's prompt is a blank line, and it's ready to take our commands. First, let's push a number onto the stack. How about 1234:

1234

Again, nothing seems to have happened. Let's use f to look at the entire contents of the stack:

f

This prints the following:

1234

So 1234 is on the stack, and ready to be operated on. Here we're going to multiply it by 2, and print the result. dc uses reverse-polish (postfix) notation, which places the operands first and the operators second. The command below in English says, "Take the top item on the stack, operate on it with the number two, using the operation multiplication, and print the result." The command is:

2 * p

This takes our top stack item 1234, multiplies it by 2, and prints the resulting value:

2468

Great. Now let's subtract 468 and print the result:

468 - p

2000

Keep in mind that if we weren't putting p at the end of the command, the same thing would be happening, but without any output. Okay, now let's divide ("/") our result 2000 by 2, and print the result:

2 / p

1000

And now let's take the square root:

v p

31

Because the default precision is 0 (calculate zero places after the decimal point), and we haven't changed it, the result was rounded down to the nearest integer. The answer is actually closer to 31.62. To get a more precise answer, we have to tell dc to use greater precision. Let's tell it to calculate to ten places using the k command:

10 k 1000 v p

31.6227766016

Here, we combined several operations into one command. The command did the following, in the following order:

1. pushed 10 onto the stack,
2. told k to pop that number from the stack and set the precision to that many decimal places,
3. pushed 1000 onto the stack,
4. told v to pop 1000 off the stack, calculate the square root, and push the result onto the stack, and
5. printed the result with p.

If we wanted to, we could set the precision to something ridiculous like 100000000, but that might take hours to compute, and we have better things to do.

Let's look at the stack a bit closer. Remember, the stack is like a pile of items. If you push an item onto the stack, it's on top. If you pop an item from the stack, you take the top (most recently pushed) item off the stack.

First, let's clear the stack:

c

To make sure it was cleared, let's print the top item on the stack with the p command:

p

dc: stack empty

Now, let's push two items onto the stack. Let's say 1 and 2.

1
2

If we use the p command, it shows the item on top of the stack. Here, that is the most recently-pushed item, 2:

p

2

Or, we can use the f command to display the entire contents of the stack. This shows the items top-to-bottom, in other words most-recently-pushed-first:

f

2
1

Now let's operate on the stack. Let's add the top two items and print the result:

+ p

3

Now let's look at the full contents of the stack using f:

f

3

The numbers 1 and 2 are gone. They were both popped off the stack by the addition operation, which added them together and then pushed the result, 3, onto the stack.

When adding numbers together, as we did, the order of the operands doesn't matter (1 + 2 is the same as 2 + 1). Let's use division now, to illustrate the order where the items on the stack are used. First, let's set the precision to two decimal points:

2 k

Now let's clear the stack, and then push 1 and then 2 onto the stack:

c 1 2

Let's check the stack real quick:

f

2
1

Now let's divide, and print the result:

/ p

.50

As you can see, this is the result of 1 divided by 2, not the other way around.

We're done for now, so let's quit dc:

q

Which returns us to the shell's command prompt.

Related commands

bc — A calculator.