Developing Custom Q Functions

Introduction

Kdb+'s power and flexibility are significantly enhanced by the ability to create custom functions. These functions allow you to encapsulate complex logic, improve code readability, and optimize performance. This chapter explores various aspects of function development in kdb+.

Basic Function Definition

Functions are defined using the colon (:) operator.

Code snippet

f:{[x] x*2}  // Double the input

Function Parameters

Functions can take multiple parameters.

Code snippet

add:{[x;y] x+y}  // Add two numbers

Return Values

Functions can return any data type.

Code snippet

max:{[x] last x where x = max x}  // Find the maximum value in a list

Recursive Functions

Kdb+ supports recursive functions.

Code snippet

factorial:{[n] if[n<=1; 1; n*factorial[n-1]]}

Higher-Order Functions

Functions can take other functions as arguments.

Code snippet

apply:{[f;x] f[x]}  // Apply a function to a value

Anonymous Functions

Lambda functions provide a concise way to define functions.

Code snippet

{x*2}[5]  // Square the input

Function Attributes

Attributes provide metadata about functions.

Code snippet

f:{[x] x*2}
f[`:attr] `type:`function

Function Overloading

Kdb+ supports function overloading based on argument types.

Code snippet

add:{[x;y] if[type[x] in `float`int; x+y; x,'+',y]}  // Add numbers or concatenate strings

Error Handling

Use [;] to handle errors gracefully.

Code snippet

divide:{[x;y] $[y=0; 0; x%y]}  // Handle division by zero

Performance Optimization

• Vectorization: Leverage vectorized operations for performance gains.

• Avoid unnecessary copies: Use references where possible.

• Profiling: Use \ts to identify performance bottlenecks.

Code snippet

// Vectorized function
sum_squares:{[x] sum x*x}

// Avoid unnecessary copies
f:{[x] x:x+1; x*2}  // Avoid creating a copy of x

Advanced Function Techniques

• Currying: Create new functions by partially applying arguments.

• Function composition: Combine multiple functions into a single function.

• Tail recursion: Optimize recursive functions for performance.

Custom Aggregations

Define custom aggregations for specific calculations.

Code snippet

weighted_avg:{[x;w] sum x*w % sum w}

User-Defined Types (UDTs)

Create custom data structures for complex objects.

Code snippet

person:{
  name:`symbol$()
  age:int
  address:`symbol$()
}

Conclusion

Custom functions are essential for tailoring kdb+ to specific needs. By mastering function creation, optimization, and advanced techniques, you can significantly enhance the power and flexibility of your kdb+ applications.