> For the complete documentation index, see [llms.txt](https://dev.ubidots.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://dev.ubidots.com/synthetic-variables/examples/special-functions.md). # Special functions ### $$fill$$\_$$missing()$$

Syntax	Example
`fill_missing (expression, <first_fill>, <last_fill>, <fill_value>)`	fill_missing(A+B)

To better understand this function, suppose you have a set of variables that you want to sum. Variables A, B, C, and D do not have data for all timestamps, as shown in the table below. Use the `fill_missing()` function to fill the gaps and perform the calculation. The following examples are based on this table:

Date	A	B	C	D	Output
07/12/2020 00:00:04	5	-	4	5	16
07/12/2020 00:00:03	3	2	3	4	12
07/12/2020 00:00:02	-	4	-	2	15
07/12/2020 00:00:01	4	3	5	-	-

$$fill$$\_$$missing(A + B + C + D)$$ By default, the `fill_missing()` expression sets the *`first_fill`* parameter to *`"ffill"`*. This means the function fills the gaps forward, starting at a point where it has enough data to fill the gaps for all involved time series. The result is as follows:

Date	A	B	C	D	Output
07/12/2020 00:00:04	5	2	4	5	16
07/12/2020 00:00:03	3	2	3	4	12
07/12/2020 00:00:02	4	4	5	2	15
07/12/2020 00:00:01	4	3	5	-	-

#### $$fill$$\_$$missing(A + B + C + D, first$$\_$$fill="bfill")$$ You can also set *`first_fill`* to *`"bfill"`*. In that case, the gaps are filled as follows:

Date	A	B	C	D	Output
07/12/2020 00:00:04	5	-	4	5	-
07/12/2020 00:00:03	3	2	3	4	12
07/12/2020 00:00:02	3	4	3	2	12
07/12/2020 00:00:01	4	3	5	2	14

#### $$fill$$\_$$missing(A + B + C + D, first$$\_$$fill="ffill",last$$\_$$fill="bfill")$$ Suppose you need to fill all gaps in the variables. In that case, set both `first_fill` and `last_fill` in the expression. The result is as follows:

Date	A	B	C	D	Output
07/12/2020 00:00:04	5	2	4	5	16
07/12/2020 00:00:03	3	2	3	4	12
07/12/2020 00:00:02	4	4	5	2	15
07/12/2020 00:00:01	4	3	5	2	14

$$fill$$\_$$missing(A + B + C + D, fill$$\_$$value=0)$$ You can also fill the gaps with a fixed value. In this case, the function sets `0` in every gap in the variables.

Date	A	B	C	D	Output
07/12/2020 00:00:04	5	0	4	5	14
07/12/2020 00:00:03	3	2	3	4	12
07/12/2020 00:00:02	0	4	0	2	6
07/12/2020 00:00:01	4	3	5	0	12

Syntax Example Output

Syntax	Example	Output
`where(condition, operation if true, operation if false)`	Step function: Assume a variable x = [-1, 2, 1]where(x >= 0, 1, 0)	[0, 1, 1]
	Interval function: Assume a variable x = [-1, 2, 1]where(x < 0, 0, where(x < 1, 1, 2))	[0, 2, 2]

where(condition, operation if true, operation if false)

Step function:

Assume a variable

x = [-1, 2, 1]where(x >= 0, 1, 0)

[0, 1, 1]

Interval function:

Assume a variable

x = [-1, 2, 1]where(x < 0, 0, where(x < 1, 1, 2))

[0, 2, 2]

Syntax Example Output

Syntax	Example	Output
`shift(<variable>,<step>)`	Assume a variable x = [-2, -1, 0, 1, 2]shift(x, -1) This lets you retrieve the previous value of the variable by shifting the series to the left.	[-1, 0, 1, 2]
	Assume a variable x = [-2, -1, 0, 1, 2]shift(x, 2) Shifts the series to the right.	[x, x, -2, -1, 0]

shift(<variable>,<step>)

Assume a variable x = [-2, -1, 0, 1, 2]shift(x, -1)

This lets you retrieve the previous value of the variable by shifting the series to the left.

[-1, 0, 1, 2]

Assume a variable x = [-2, -1, 0, 1, 2]shift(x, 2)

Shifts the series to the right.

[x, x, -2, -1, 0]

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