dc is a desk calculator program that takes input in reverse Polish notation (see Reverse Polish Notation later in this reference page). If you do not specify a file on the command line, dc reads input from the standard input; otherwise, it reads input from the file and then from the standard input (if there is no quit command in the file). dc sends output to the standard output.
There are several types of input:
- Numbers are sequences of digits, possibly containing a decimal point. Numbers can also contain the uppercase characters A through F standing for the hexadecimal (base 16) digits greater than ten; for more on hexadecimal, see the section on Numbers in Different Bases. Do not break up a number with spaces or commas; for example, you must write 1000000, not 1,000,000. To create a negative number, put an underscore (_) immediately before the first digit of the number. Do not use a minus sign (-) to indicate a negative number; the minus sign has an entirely different meaning in dc input.
- Strings are sequences of characters, enclosed in square brackets. For example, [abc] is a string which contains the characters abc.
- Operators are symbols or characters telling dc to perform some operation; for example, adding two numbers together.
- Register names are single characters. You may use any character as a register name. An uppercase letter is not the same as the corresponding lowercase one, so register a is different from register A. A register is a place where dc can store a number or a string; it is similar to a variable in a programming language. Typically, you use registers to store values that you want to remember for later use.
- Array names follow the same rules as register names. See Array Operations later in this reference page for more details.
You must separate adjacent numbers with at least one white space character. (The white space characters for dc are the blank, the horizontal tab and the newline.) You do not need to separate other pieces of input from one another, but putting in white space characters makes the input more readable. As exceptions, register names and array names must immediately follow the operator that tells what you want to do with the register or array (as described later in this reference page). If you put a white space character after an operator that expects a register or array name, dc assumes the white space character to be the name.
To use dc you must understand reverse Polish notation. This is a way to write arithmetic expressions. The form is a bit tricky for people to understand, since it is geared towards making it easy for the computer to perform calculations; however, most people can get used to the notation with a bit of practice.
Reverse Polish notation stores values in a stack. A stack of values is just like a stack of books: one value is placed on top of another. When you want to perform a calculation, the calculation uses the top numbers on the stack.
For example, here's a typical addition operation:
1 2 +
When dc reads a number or a string, it just puts the value onto the stack. Thus 1 goes on the stack, then 2 goes on the stack. When you put a value onto the stack, we say that you push it onto the stack. When dc reads the operator +, it takes the top two values off the stack, adds them, then pushes the result back onto the stack. After this addition, the stack contains
As another example, consider
2 3 4 + *
(The * stands for multiplication.) dc begins by pushing the three numbers onto the stack. When it finds the +, it takes the top two numbers off the stack and adds them. (Taking a value off the stack is called popping the stack.) dc then pushes the result of the addition back onto the stack in place of the two numbers. Thus the stack contains
When dc finds the * operator, it again pops the top two values off the stack. It multiplies them, then pushes the result back onto the stack, leaving
The following list gives a few more examples of reverse Polish expressions. After each, we show the contents of the stack, in parentheses.
7 2 - (5) 2 7 - (-5) 12 3 / (4) _12 3 / (-4) 4 5 + 2 * (18) 4 5 2 + * (28) 4 5 2 * - (-6)
If you are experimenting with dc to see how this works, you can type p to print out the top value on the stack and f to print out the full stack.
One of dc's great virtues is its ability to deal with numbers of arbitrary size and precision -- dc is not constrained by the hardware's restrictions on number size or precision.
Many arithmetic calculations use a scaling factor, an integer greater than or equal to zero, and strictly less than 100. The scaling factor affects how many decimal places dc uses when making calculations.
The default scaling factor begins at zero (no decimal places). This can be confusing; for example, if you try
1 2 / p
to divide 1 by 2 and print the result, dc prints 0. The real answer is 0.5 but a scaling factor of 0 tells dc not to keep track of fractions when doing arithmetic.
You can set a different default scaling factor with the k operation. This pops the top value from the stack and sets that value to the new default scaling factor. For example,
sets the default scaling factor to 4. Now if you try
1 2 / p
the result is .5000.
The number of decimal places in the operands also affects the number of decimal places in the answer. Thus the scaling factor is not the only influence on the precision of the calculations.
The K operation pushes the current default scaling factor onto the stack.
The following list shows the operators recognized by dc and the effects that they have.
- pops the top two values from the stack, adds them, then pushes the result onto the stack. The number of decimal places in the result is the maximum number of decimal places in the two operands; the scaling factor has no effect.
- pops the top two values from the stack, subtracts the first popped from the second, then pushes the result onto the stack. The number of decimal places in the result is the maximum number of decimal places in the two operands; the scaling factor has no effect.
- pops the top two values from the stack, multiplies them, then pushes the result onto the stack. dc normally sets the number of decimal places in the result to the sum of the decimal places in the two operands; if this is larger than the scaling factor and also larger than the number of decimal places in both individual operands, the number of decimal places in the result is the largest of the scaling factor or the number of decimal places in either operand.
- pops the top two values from the stack, divides the second value by the first, then pushes the result onto the stack. The number of decimal places in the result is equal to the scaling factor.
- pops the first two values from the stack, divides the second value by the first, then pushes the remainder onto the stack. (Mathematically, A B % calculates A modulo B.) dc determines the number of decimal places in the result by the result of the division.
pops the first two values from the stack, calculates the value of the second
number to the power of the first, then pushes the result onto the stack.
2 3 ^
leaves the value 8 on the stack. The exponent value must be an integer (that is, with no decimal places). The scaling factor of the result is the scaling factor you get if the base was multiplied the appropriate number of times.
- clears the stack (that is, pops all the values off and discards them).
duplicates the value on top of the stack.
duplicates the top value, then does a multiplication. The result is that you square the value on top of the stack. As another example, you can use d to save the value on top of the stack in a register while keeping a copy of the value on the stack; in this case, you'd use d to duplicate the top value, then use s to pop the duplicate value into a register.
- prints all values on the stack, from top to bottom. (It does not print the contents of the registers.)
- pushes the current default scaling factor onto the stack.
- pops the top value off the stack and uses it as the default scaling factor (see the section on The Scaling Factor).
- pops the top value off the register stack x (see the S command) and pushes that value onto the main stack. If the register has never contained a value, dc treats this as an error. (Contrast this behavior with the way that the lx operator works.) This operator also pops the array component of the specified register. See the Array Operations section for more information.
- takes the value from register x and pushes it onto the stack. This does not change the value of the register. If the register has never contained a value, dc puts a value of zero on the stack.
- pops the top value off the stack, prints it as a string and discards it. If the value is a string, dc prints it as such. If it is a number, dc prints the ASCII character with that value. If the number doesn't have an ASCII value (is greater than 256), dc prints the value of the number modulo 256.
- prints the top value on the stack. The value remains on the stack.
- quits a dc session; see the Executing Strings section for an exception.
- pops the top value off the stack and pushes this value onto the register x as if the register itself were another stack. In this way, you can use a single register to hold a sequence of values. This operation also pushes the array component of the register onto the register's stack. See the Array Operations section for more information.
- pops the top value off the stack and stores it in the register x. For example, sa pops the stack and stores the value in register a.
- replaces the top value on the stack with its square root. dc ignores the scaling factor when performing calculations to find the square root. The number of decimal places in the result is the maximum of the number of decimal places in the original value or the scaling factor.
- replaces the value on the top of the stack with the number of decimal places in the number.
- executes a string. See Executing Strings.
replaces the number on the top of the stack with its length (that is,
the number of digits in the number).
dc ignores the minus sign and decimal point when calculating this value, so that 12345 and _123.45 have the same length.
- determines how many values are currently on the stack, then pushes that number onto the stack.
Programmers often find it useful to perform arithmetic with numbers in bases other than ten, for example, octal (base 8) or hexadecimal (base 16) numbers. Several commands help make this possible.
- pushes the current input base onto the stack.
pops the top value of the stack and uses this as the base when
interpreting further input.
tells dc that from now on, it is to interpret input numbers as octal values. For example, if you type 10 as input, dc interprets it as an octal number, equal to 8 (base ten).
You can use the characters A through F to input hexadecimal digits regardless of the base.
- pushes the current output base onto the stack.
pops the top value of the stack and uses this as the base when printing output.
tells dc to print subsequent numbers in hexadecimal format.
The input and output bases can be different; for example, you may find this convenient if you want to convert input in one base to output in another.
You can make the output base larger than 16. In this case, dc prints each digit as a decimal value and separates them with a single space. For example,
1000 o 123456789 p
123 456 789
This sets the output base to 1000, where digits are decimal values from 0 through 999. As a result, dc breaks up all values into one or more chunks with three digits per chunk. Using output bases that are large powers of ten, you can put your output in columns; for example, many users find that 100000 makes a good output base because dc groups numbers into chunks of five digits each.
dc outputs long numbers with a maximum of 70 characters per line. If a number is longer than this, dc puts a backslash \ at the end of the line, indicating that the number continues on the next line.
dc always prints a value of zero as 0, regardless of the output base and regardless of the number of decimal places that are normally attached to the value.
Some people have trouble figuring out how to put the input base back to base ten after working in some other base.
always works, since A stands for the hexadecimal digit ten.
The maximum output base is the maximum integer value that the hardware can represent.
A string is any sequence of characters. In particular, a string may consist of a sequence of dc commands.
The x command pops the top value from the stack and executes it as if it were a string containing dc commands. For example, consider the following code:
This pushes the string inside the square brackets onto the stack, and then pops it into register z. From this point onward, the command
pushes the string in z onto the stack, then executes the commands inside the string. The sequence of commands lapP pushes the value of register a onto the stack, prints it, then pops the value off again. The sequence of commands lbpP does the same for register b. The result is that we can use lzx to print the current contents of registers a and b any time we want.
There are several other commands for executing strings:
pops two values off the stack. If the first popped value is greater than the second, dc executes the contents of register x as a string of commands. As an example,
la lb >z
executes the string in register z if the contents of register b are greater than the contents of register a.
pops two values off the stack. If the first popped value is not greater than the second, dc executes the contents of register x as a string of commands.
pops two values off the stack. If the first popped value is less than the second, dc executes the contents of register x as a string of commands.
pops two values off the stack. If the first popped value is not less than the second, dc executes the contents of register x as a string of commands.
pops two values off the stack. If the first popped value is equal to the second, dc executes the contents of register x as a string of commands.
pops two values off the stack. If these two values are not equal, dc executes the contents of register x as a string of commands.
One string may execute another. For example, a string being executed via the x command may contain a > construction to execute a register string if the condition holds true. In this case, dc executes the new string, then returns to the old string to continue executing where it left off. A string may execute a string which executes another string, and so on. Because of this possibility, dc keeps a stack of the strings that it is currently executing.
When dc finds a q command inside a string being executed, it doesn't quit dc. Instead, it quits executing the current string, plus the string that caused the execution of the current string. In other words, it pops two strings off the currently executing stack.
To see why you want to quit the two most recent strings, consider the following example.
loads a quit command into register y. Now, we might use something like
[... la lb >y ...]
to quit in the middle of the string if the value in register b is greater than the value in register a. The command >y executes the command string in register y if the condition is true; if the quit command in y only stopped one command string, it would quit executing the commands from y and go right back to executing the main command string. To be able to use this technique to quit the main command string, the q command must pop two command strings.
The Q (uppercase) command is a variation on the simple q command. Q pops the top number off the (value) stack and stops execution of that many currently executing strings. For example,
stops the three most recent executing strings.
As noted previously, arrays are similar to registers in that they have names consisting of a single character. However, a register's array values are independent of it's scalar value; the array element X for example, is different than the scalar X.
An array is just a list of values. Values in the list are referred to by number; for example, you can ask for the 12th value in the list. The numbers used to refer to values are called the subscripts of the array. The beginning of the list has the subscript 0 and the maximum subscript for any array is 2047.
There are two array operations:
stores a value in array x. The operation begins by popping a number off the stack and uses it as the subscript into the array. dc then pops another value off the stack and stores this value in the array using the given subscript.
obtains a value from the array x. dc pop the top number off the stack to use as a subscript into the array. It then places the value found at that subscript on the stack. The operation does not affect the value inside the array; it just takes a copy of the value.
If you use ; to obtain a value from an array, but you have not yet used : to store a value in that position, dc automatically puts a zero onto the stack (as if there were a zero in that position).
In an earlier section, the S and L operators were used to push and pop the scalar value of a register onto the register's stack. These operators also push and pop the array component of a register. This is done at the same time that the scalar values are being pushed or popped, with some differences in the details of how operations work. Where L popped the top of the register's scalar stack onto the main stack, the array operation simply pops the top of the register's array stack then discards the result. Where S popped the top of the main stack and pushed it onto the register's scalar stack, the array operation simply hides the current array values. Again, both the scalar and array operations are caused by the same operator at the same time. The following example shows how the S and L operations can be used to save or hide the scalar and array values of a register. The operations
11 sa 12 1 :a la p 1 ;a p c
finally clear the main stack. The register a now has the scalar value 11 and the array element a now has the value 12. Next, the operations
0 Sa la p 1 ;a p
save the current array and scalar values associated with the register and print the new values for a and a (which are now zero). The old array and scalar values of the register have been saved on the register's stack. You can change the value of the register or any of the array elements without affecting these saved values. To restore the old values, execute
La la p 1; a p
This pops the current array and scalar values off of the register stack thus making the old values visible again. The restored values are then printed.
executes the rest of the line as a system command. For example,
!cp file1 file2
executes the given cp command.
reads an input line from the input source (for example, the terminal) and executes that line. This is useful when you are executing a command string but want to obtain input in the middle of the string.
The following sequence of commands prints out the first 12 elements of the Fibonacci sequence. In this sequence, the first two values are 1, and each subsequent value is the sum of the previous two values. Registers a and b hold the two most recent values of the sequence; new values are calculated in the stack. Register z holds the code needed to calculate new values, and register c holds a count of how many values have been printed.
1 sa 1 sb 2 sc [la lb + p lb sa sb lc 1 + d sc 13 >z] sz la p sx lp p sx lz x
- The first three lines set up registers a and b with the value 1, and c with the count of values that have already been calculated (the first two).
The next line loads z with the main code to execute.
This code loads the values in a and b,
adds them and prints the result, moves the value of b to
a, then saves the newly calculated value in b.
The count in c is then incremented; the commands
make a duplicate copy of the count and save this duplicate back in c.
- The final part of the code checks the count to see if it is less than 13; if this is true, the contents of z are executed again to get the next value. The final line in the program prints the first two values of the sequence and then executes the code in z.
Possible exit status values are:
Failure due to any of the following:
- — input radix too big
- — output radix too big
- — scale too big
- — number expected, string found
- — negative argument to Q
- — command stack underflow
- — command too long
- — cannot execute number
- — specified operator is unimplemented
- — numerical constant is too long
- — string is too long
- — stack too deep
- — empty stack
- — negative index
- — index too big
- — divide by 0
- — exponent out of range
- — sqrt of negative number
- — out of memory
Maximum array index: 2047. Maximum exponent in an exponentiation operation: 9999. Maximum input buffer size (line length): 1000 characters. Maximum scaling factor: 99. Maximum stack depth: MAXINT (the largest positive integer that can be supported by the hardware).
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