# # Math

## # Math using dc

`dc` is one of the oldest language on Unix.

It is using the reverse polish notation (opens new window), which means that you are first stacking numbers, then operations. For example `1+1` is written as `1 1+`.

To print an element from the top of the stack use command `p`

``````echo '2 3 + p' | dc
5

or

dc <<< '2 3 + p'
5

``````

You can print the top element many times

``````dc <<< '1 1 + p 2 + p'
2
4

``````

For negative numbers use `_` prefix

``````dc <<< '_1 p'
-1

``````

You can also use capital letters from `A to F` for numbers between `10 and 15` and `.` as a decimal point

``````dc <<< 'A.4 p'
10.4

``````

`dc` is using abitrary precision (opens new window) which means that the precision is limited only by the available memory. By default the precision is set to 0 decimals

``````dc <<< '4 3 / p'
1

``````

We can increase the precision using command `k`. `2k` will use

``````dc <<< '2k 4 3 / p'
1.33

dc <<< '4k 4 3 / p'
1.3333

``````

You can also use it over multiple lines

``````dc << EOF
1 1 +
3 *
p
EOF
6

``````

`bc` is a preprocessor for `dc`.

## # Math using bc

`bc` (opens new window) is an arbitrary precision calculator language. It could be used interactively or be executed from command line.

For example, it can print out the result of an expression:

``````echo '2 + 3' | bc
5

echo '12 / 5' | bc
2

``````

For floating-post arithmetic, you can import standard library `bc -l`:

``````echo '12 / 5' | bc -l
2.40000000000000000000

``````

It can be used for comparing expressions:

``````echo '8 > 5' | bc
1

echo '10 == 11' | bc
0

echo '10 == 10 && 8 > 3' | bc
1

``````

## # Math using bash capabilities

Arithmetic computation can be also done without involving any other programs like this:

Multiplication:

``````echo \$((5 * 2))
10

``````

Division:

``````echo \$((5 / 2))
2

``````

Modulo:

``````echo \$((5 % 2))
1

``````

Exponentiation:

``````echo \$((5 ** 2))
25

``````

## # Math using expr

`expr` or `Evaluate expressions` evaluates an expression and writes the result on standard output

Basic arithmetics

``````expr 2 + 3
5

``````

When multiplying, you need to escape the `*` sign

``````expr 2 \* 3
6

``````

You can also use variables

``````a=2
expr \$a + 3
5

``````

Keep in mind that it only supports integers, so expression like this

``````expr 3.0 / 2

``````

will throw an error `expr: not a decimal number: '3.0'`.

It supports regular expression to match patterns

``````expr 'Hello World' : 'Hell\(.*\)rld'
o Wo

``````

Or find the index of the first char in the search string

This will throw `expr: syntax error` on Mac OS X, because it uses BSD expr which does not have the index command, while expr on Linux is generally GNU expr

``````expr index hello l
3

expr index 'hello' 'lo'
3

``````