Practice Questions

189 hands-on questions, each with a worked solution. Pick a topic to focus on one area.

Core Concepts

Data Analysis

System Features

Operations on Lists (Iterators & Amend)

General

01Calculate Final PnL Using Over 02Calculate Running PnL Using Scan 03Calculate Compounded Returns Using Scan 04Calculate Price Changes Using Each-Prior 05Get the Unique Elements of a List 06Remove Consecutive Duplicates from a List 07Pack Consecutive Duplicates into Sublists 08Drop Every Third Letter from a List 09Reverse the Order of Underscore-Separated Words 10Replace Nulls with a Constant Value

Dictionaries

Creating Dictionaries

01Create a dictionary - 3 ways 02Unique Keys 03Keys of same type

Dictionary Arithmetic

01Key-Union Arithmetic

Converting Dictionaries to Tables

01Column Dictionary 02Conforming Dictionaries 03Constructing a Table from Non-Conforming Dictionaries 04Create a table from conforming dictionaries

Inserting Dictionary Into Table

01Append Dictionary to Table

Updating & Merging Dictionaries

01Select and Remove Keys 02Dictionary Value Replace 03Merging Dictionaries

Lookup By Key

01Improving Lookup Performance in Large Dictionaries 02Multiple Key Lookup and .Q.fu

Lookup By Value

01Extract the first key by value 02Filter Dictionary Keys by Value 03Inverting a Dictionary 04Lookup by even values 05Extract the last key by value

Step Dictionary

01Step Dictionary Table Update 02Step Dictionary Lookup

In Memory Tables

Creating Tables

01Defining table schemas 02Table with Random Data 03Generate Random Market Data

Selecting Data

01Return Last N Rows of Table 02Select Rows Where a Column Matches a Pattern 03Delete a Row by Index 04Return a Table Without a Given Column 05Select Specific Columns Using a Functional Select 06Functional Select with Aggregations 07Select a Range of Rows, Ordered by a Column

Attributes

01Add, Confirm, and Remove a Grouped Attribute 02Sort a Table and Apply the Parted Attribute

Updating Tables

01Add a Column of Random Booleans 02Add a Column Derived from Another Column 03Add a Boolean Flag Column from a Condition 04Rename a Table Column

Saving Tables

01Save a Table to a CSV File 02Save a Table to a Custom CSV File Without Headers 03Save a Table in Splayed Format

Keyed Tables

01Keyed Tables and Key-Based Lookups

Functions

Defining Functions

01Implicit vs Explicit Arguments 02Local vs Global Variable Assignment 03Write a Recursive Function

Projections

01Project a Built-In Operator 02Project a User-Defined Function 03When to Use a Projection vs a Lambda

Default Arguments

01Simulate a Default Argument With a Null Check 02Use a Projection to Bake In Defaults

Attributes

The Four Attributes

01Name All the kdb+ Attributes and Describe Cases Where Each Is Used 02When Would You Use Each Attribute?03What Does It Mean That Attributes Are "Descriptive, Not Prescriptive"?04What Is the Memory Overhead of Each Attribute?05For Each Attribute, When Is It Preserved and When Is It Lost?

Attributes in the Tick Architecture

01Why Does the RDB Use `g#` While the HDB Uses `p#` on sym?02For a Window Join or As-Of Join, What Attribute Is Required?03How Can the Following aj Be Optimised? aj[`sym`time;t;q]04How Do Attributes Propagate From the Tickerplant to Subscribers?

Performance & Troubleshooting

01If You Append New Data to an Existing Partitioned Table, How Do You Maintain the Attributes?02How Would You Troubleshoot a Slow In-Memory Query?03How Would You Troubleshoot a Slow Historical Query?

Gotchas & Edge Cases

01How Does Applying `s#` Differ From Applying the Other Attributes?02In a Where Clause With Multiple Filters, How Many Attributed Columns Benefit?03What Is the Difference Between Plain Join and In-Place Append for Attributes?04What Operations Cause a List to Lose Its Attribute?05What Is a Step Function Dictionary and How Does `s#` Enable It?06What Does Applying `s#` to a Table Do?07How Do You Remove an Attribute From a List or Column?08How Does `s#` Enable Step-Function Lookup on a Keyed Table?09Why xasc on Two Columns Only Marks the First Column as Sorted

Performance Benchmarks

01What Is `\ts` and Why Is Workspace Important When Profiling Queries?02Which Attribute Has the Most Memory Overhead, and Which Has None?03At What Point Does the Unique Attribute Start Providing a Significant Benefit?04How Can the `group` Function Be Used as an Alternative to `select ... by sym`?

Iterators

Each

01Filter Sublists by Length 02Count Occurrences as a Dictionary 03Duplicate Each Element in a List 04Sum Even Values in Each Sublist 05Sum the Diagonal of a Matrix 06Repeat Each Number N Times to Build a Jagged List

Each-Left

01Remove Multiple Values from Every Sublist 02Append a Value to Every Sublist 03Sort Multiple Lists Together by a Key Row

Each-Right

01Find Sublists Containing a Value 02Find Sublists Containing Any of Several Values

Scan

01Build an Ever-Increasing List Using Scan 02Step Through a Backward-Fill Using Scan 03Generate Even Numbers Up to N

Over

01Backward-Fill Nulls with the Next Non-Null Value 02Search and Replace Multiple Substrings 03Sum the Rows and Columns of a Matrix 04Compute the Fibonacci Sequence

Amend

01Replace Even Values in a List of Lists with Zero 02Replace a Single Element in a Matrix 03Forward-Fill Nulls with the Previous Value Without Using fills

Interprocess Communication (IPC)

IPC Overview

01What is IPC Used for in a kdb+ System?02How Are Ports Typically Configured in a kdb+ Tick Setup?

Listening on a Port

01Instruct a q Process to Listen on a Port 02Stop a q Process from Listening on a Port 03Start a Server and Client Process on Different Ports

Opening Connections (hopen)

01Open a Connection to Another Process 02What Value Should a Handle Have, and Why?03Specify a Timeout When Opening a Connection 04Demonstrate hopen Timeout Behaviour 05Implement Error Handling on hopen

Message Handlers (.z Namespace)

01What Are the .z Message Handlers?02What Order Do the Authentication Handlers Run In?03What Do .z.pg and .z.ps Do By Default?04Restrict Which Functions Can Be Called Remotely 05Identify the Calling Process Inside a Handler 06Capture a Connecting User's IP Address and Username 07Detect When a Connection Has Closed 08If Process A Communicates With Process B, What Order Do the .z Functions Execute?09Implement .z.pw to Authenticate Connecting Users

Inspecting Handles

01List a Process's Open IPC Handles

Synchronous & Asynchronous Requests

01Insert Data into a Remote Table, Sync and Async 02Define and Call a Function on a Remote Process

Asynchronous Flush

01Demonstrate Asynchronous Flush

Asynchronous Callbacks

01Implement an Asynchronous Callback 02If You Send a Message Asynchronously, How Can You Verify the Other Process Received It?

Handle Lifecycle

01Detect When a Remote Connection Has Closed

Asynchronous Broadcast

01Broadcast an Async Message to Multiple Handles

Memory Management & Memory Mapping

Caches, Virtual Memory, and the Page Cache

01Why Mapping a Huge File Doesn't Use a Huge Amount of RAM 02How to Check a q Process's Memory Usage From the Command Line

Memory Mapping (mmap)

01Immediate vs Deferred Memory Mapping 02Detect How a File Was Written, and Why It Matters for Large Files 03Why the Same HDB Query Can Run at Very Different Speeds

Memory Allocation (q's Heap)

01Why a List Uses More Memory Than Its Raw Byte Count 02Why a Tiny List Still Costs More Than Its Raw Data

Garbage Collection & Reference Counting

01Reclaim Memory with .Q.gc and Check Usage with .Q.w 02Cap a Process's Memory Usage with -w

Optimising Memory Usage

01Reduce Memory Use by Selecting Only Needed Columns 02Why a Large Join Can Spike Memory Above Either Input

Anymap

01Why Anymap Speeds Up Compound Columns 02Anymap Is Flexible, But Not Always the Fastest

Persisting Tables

Writing Partitioned Tables to Disk

01Persist an In-Memory Table to Date-Partitioned HDB 02Sort and Apply Parted Attribute to On-Disk Partitions 03Sort In-Memory Before Saving with Parted Attribute 04Check Whether a Column Was Persisted with Compression 05Compress Existing Partitions Column by Column 06Append Missing Records Then Re-Sort and Re-Compress

qSQL

Select

01What Are the Components of a q Select Query?02Filter Rows Using Pattern Matching 03Select and Sort a Window of Rows 04Find the Times of the Minimum and Maximum Prices

Grouping & Aggregation

01Count Rows by Group 02xbar: Count Rows in Time Buckets 03fby: Find the Highest Price for Each Symbol Using fby 04fby: Select First,Last and Second Last Rows by Group 05Find the Symbol With the Most Appearances 06Count Rows per Symbol with a Filter 07Select the Maximum-Price Row per Symbol 08Select the Last N Rows per Symbol 09Count Rows per 15-Minute Bucket, per Symbol

Update

01Create a Boolean Flag Column 02Derive Columns from Boolean Flags 03Applying and Removing Attributes

Delete

01Delete a Column from a Table 02Delete a Row by Index

Exec

01exec vs select — Extracting Lists, Scalars, and Dictionaries

Functional Form

01Function Select 02Functional Select With Aggregations

Joins

Each-Left Join

01Extend a Table with New Null Columns

Join (,) and Union Join (uj)

01Join Two Tables with Shared Columns

Join-Each (,')

01Attach Equal-Length Columns with Join-Each

Left-Join (lj), Inner-Join (ij), Plus-Join (pj)

01Add Columns from a Keyed Table with Left-Join 02Left-Join Overwrite Behavior for Shared Columns 03Combine Shared Columns with Plus-Join 04Inner-Join vs Left-Join with Missing Matches

Window Join (wj)

01Compute a Lagging Moving Average with Window Join 02Inspect the Rows Captured by a Window Join 03Compute a Lagging VWAP with Window Join 04Prepare a Table to Maximise Window Join Performance

Asof

01Look Up the Previous Value with Asof

As-of Join (aj)

01Join Prevailing Quotes to Trades with aj 02Optimise the Performance of an As-of Join

File Parsing

General

01Decode Text 02Find Exchanges Supporting a Trading Pair 03Inspect Table Size in Memory