Elektra  0.8.26
Plugin: mathcheck

Compares a key value to a mathematical expression using polish prefix notation defined in the check/math metakey. Operations are + - / * . Operants are keys with names relative to the parent key. How the values are compared is specified at the beginning of the metakey using the conditions <, <=, ==, !=, =>, >, := := is used to set key values. All values are interpreted as double floating point values.

Keynames

Keynames are all either relative to to-be-tested key (starting with ./ or ../), relative to the parentkey (starting with @/) or absolute (e.g. system/key).

Examples

check/math = "== + ../testval1 + ../testval2 ../testval3" compares the keyvalue to the sum of testval1-3 and yields an error if the values are not equal. check/math = "\<= - \@/testval1 * \@/testval2 \@/testval3" tests if the keyvalue is less than or equal to testval1 - (testval2 * testval3) and yields an error if not.

Full example:

1 # Backup-and-Restore:/tests/mathcheck
2 
3 sudo kdb mount mathcheck.dump /tests/mathcheck mathcheck
4 
5 kdb set /tests/mathcheck/a 3.1
6 kdb set /tests/mathcheck/b 4.5
7 kdb set /tests/mathcheck/k 7.6
8 kdb setmeta /tests/mathcheck/k check/math "== + ../a ../b"
9 
10 # should fail
11 kdb set /tests/mathcheck/k 7.7
12 # RET:5
13 # ERROR:123
14 # Set string to "7.7"
15 # The command set failed while accessing the key database with the info:
16 # Error (#123) occurred!
17 # Description: invalid value
18 # Ingroup: plugin
19 # Module: mathcheck
20 # At: /home/thomas/Dev/Elektra/libelektra/src/plugins/mathcheck/mathcheck.c:399
21 # Reason: 7.7 != 7.6
22 # Mountpoint: /tests/mathcheck
23 # Configfile: /home/thomas/.config/mathcheck.dump.25680:1478749409.938013.tmp

To calculate values on-demand you can use:

1 kdb setmeta /tests/mathcheck/k check/math ":= + @/a @/b"
2 kdb set /tests/mathcheck/a 8.0
3 kdb set /tests/mathcheck/b 4.5
4 
5 kdb get /tests/mathcheck/k
6 #> 12.5
7 
8 kdb set /tests/mathcheck/a 5.5
9 
10 kdb get /tests/mathcheck/k
11 #> 10

It also works with constants:

1 kdb setmeta /tests/mathcheck/k check/math ":= + ../a '5'"
2 kdb set /tests/mathcheck/a 5.5
3 
4 kdb get /tests/mathcheck/k
5 #> 10.5
6 
7 kdb set /tests/mathcheck/a 8.0
8 
9 kdb get /tests/mathcheck/k
10 #> 13
11 
12 #cleanup
13 kdb rm -r /tests/mathcheck
14 sudo kdb umount /tests/mathcheck