---
type: "fhirpath-function"
title: "FHIRPath Function: log"
function: "log"
category: "Math"
section: "5.8.6"
source: "fhirpath/functions.json"
---
# FHIRPath Function: log
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Returns the logarithm base `base` of the input number.
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Accepts Decimal input types. Integer and Long types are also accepted via implicit conversion to Decimal. 
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If the input is 0 or negative, the evaluation will end and signal an error to the calling environment.
If the base argument is 0 or negative, the evaluation will end and signal an error to the calling environment.
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If `base` is empty, the result is empty.
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If the input collection is empty, the result is empty.
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If the input collection contains multiple items, the evaluation of the expression will end and signal an error to the calling environment.
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``` fhirpath
16.log(2) // 4.0
100.0.log(10.0) // 2.0
```
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## Summary
- **Category**: Math
- **Section**: `5.8.6`
- **Return Type**: `Decimal`
- **Empty Input Result**: `empty`
- **Errors on Multiple Input**: `true`
## Description
Returns the logarithm base `base` of the input number.
## Arguments
- `base`: `Decimal` - Base of the logarithm
## Type Mapping
- `Integer-Decimal`
- `Long-Decimal`
- `Decimal-Decimal`
## Example
```fhirpath
16.log(2) // 4.0
100.0.log(10.0) // 2.0
```