Keyboard shortcuts

Press ← or → to navigate between chapters

Press S or / to search in the book

Press ? to show this help

Press Esc to hide this help

Selection & Filtering Documentation

This document explains how to use the selection, filtering, and sampling commands in tva: select, filter, slice, and sample. These commands allow you to subset your data based on structure (columns), values (rows), position (index), or randomly.

Introduction

Data analysis often begins with selecting the relevant subset of data:

  • select: Selects and reorders columns (e.g., “keep only name and email”).
  • filter: Selects rows where a condition is true (e.g., “keep rows where age > 30”).
  • slice: Selects rows by their position (index) in the file (e.g., “keep rows 10-20”).
  • sample: Randomly selects a subset of rows.

Field Syntax

All tools use a unified syntax to identify fields (columns). See Field Syntax Documentation for details.

select (Column Selection)

The select command allows you to keep only specific columns and reorder them.

Basic Usage

tva select [input_files...] --fields <columns>
  • --fields / -f: Comma-separated list of columns to select.
    • Names: name, email
    • Indices: 1, 3 (1-based)
    • Ranges: 1-3, start_col-end_col
    • Wildcards: user_*, *_id

Examples

1. Select by Name and Index

Consider the dataset docs/data/us_rent_income.tsv:

GEOID	NAME	variable	estimate	moe
01	Alabama	income	24476	136
01	Alabama	rent	747	3
02	Alaska	income	32940	508
...

To keep only the state name (NAME) and the estimate value (estimate):

tva select docs/data/us_rent_income.tsv -f NAME,estimate

Output:

NAME	estimate
Alabama	24476
Alabama	747
Alaska	32940
...

2. Reorder Columns

You can change the order of columns. Let’s move variable to the first column:

tva select docs/data/us_rent_income.tsv -f variable,estimate,NAME

Output:

variable	estimate	NAME
income	24476	Alabama
rent	747	Alabama
income	32940	Alaska
...

3. Select by Range and Wildcard

Consider docs/data/billboard.tsv which has many week columns (wk1, wk2, wk3…):

artist	track	wk1	wk2	wk3
2 Pac	Baby Don't Cry	87	82	72
2Ge+her	The Hardest Part	91	87	92

To select the artist, track, and all week columns:

tva select docs/data/billboard.tsv -f artist,track,wk*

Or using a range (if you know the indices):

tva select docs/data/billboard.tsv -f 1-2,3-5

filter (Row Filtering)

The filter command selects rows where a condition is true. It supports field-based tests, expressions, empty/blank checks, and field-to-field comparisons.

Basic Usage

tva filter [input_files...] [options]

Filter tests can be combined (default is AND logic, use --or for OR logic).

Filter Types

1. Expression Filter

Use the -E option to filter with an expression:

tva filter docs/data/us_rent_income.tsv -H -E '@estimate > 30000'

2. Empty/Blank Checks

  • --empty <field>: True if the field is empty (no characters)
  • --not-empty <field>: True if the field is not empty
  • --blank <field>: True if the field is empty or all whitespace
  • --not-blank <field>: True if the field contains a non-whitespace character
tva filter docs/data/us_rent_income.tsv --not-empty NAME

3. Numeric Comparison

Format: --<op> <field>:<value>

  • --eq, --ne, --gt, --ge, --lt, --le
tva filter docs/data/us_rent_income.tsv --gt estimate:30000

Output:

GEOID	NAME	variable	estimate	moe
02	Alaska	income	32940	508
04	Arizona	income	31614	242
06	California	income	33095	172
...

4. String Comparison

  • --str-eq, --str-ne: String equality/inequality
  • --str-gt, --str-ge, --str-lt, --str-le: String ordering
  • --istr-eq, --istr-ne: Case-insensitive string comparison
  • --str-in-fld, --str-not-in-fld: Substring test
  • --istr-in-fld, --istr-not-in-fld: Case-insensitive substring test
tva filter docs/data/us_rent_income.tsv --str-eq variable:rent

Output:

GEOID	NAME	variable	estimate	moe
01	Alabama	rent	747	3
02	Alaska	rent	1200	13
04	Arizona	rent	976	4
...

5. Regular Expression

  • --regex <field>:<pattern>: Field matches regex
  • --iregex <field>:<pattern>: Case-insensitive regex match
  • --not-regex <field>:<pattern>: Field does not match regex
  • --not-iregex <field>:<pattern>: Case-insensitive non-match
tva filter docs/data/billboard.tsv --regex track:"Baby"

Output:

artist	track	wk1	wk2	wk3
2 Pac	Baby Don't Cry	87	82	72
Beenie Man	Girls Dem Sugar	87	70	63
...

6. Length Comparison

  • --char-len-eq, --char-len-ne, --char-len-gt, --char-len-ge, --char-len-lt, --char-len-le: Character length
  • --byte-len-eq, --byte-len-ne, --byte-len-gt, --byte-len-ge, --byte-len-lt, --byte-len-le: Byte length
tva filter docs/data/billboard.tsv --char-len-gt track:10

7. Field Type Checks

  • --is-numeric <field>: True if field can be parsed as a number
  • --is-finite <field>: True if field is numeric and finite
  • --is-nan <field>: True if field is NaN
  • --is-infinity <field>: True if field is positive or negative infinity
tva filter docs/data/us_rent_income.tsv --is-numeric estimate

8. Field-to-Field Comparison

  • --ff-eq, --ff-ne, --ff-lt, --ff-le, --ff-gt, --ff-ge: Numeric field-to-field
  • --ff-str-eq, --ff-str-ne: String field-to-field
  • --ff-istr-eq, --ff-istr-ne: Case-insensitive string field-to-field
  • --ff-absdiff-le <f1>:<f2>:<num>: Absolute difference <= NUM
  • --ff-absdiff-gt <f1>:<f2>:<num>: Absolute difference > NUM
  • --ff-reldiff-le <f1>:<f2>:<num>: Relative difference <= NUM
  • --ff-reldiff-gt <f1>:<f2>:<num>: Relative difference > NUM
tva filter docs/data/us_rent_income.tsv --ff-gt estimate:moe

Common Options

  • --or: Evaluate tests as OR instead of AND
  • -v, --invert: Invert the filter, selecting non-matching rows
  • -c, --count: Print only the count of matching data rows
  • --label <header>: Label matched records instead of filtering (outputs 1/0)
  • --label-values <pass:fail>: Custom values for –label (default: 1:0)

slice (Row Selection by Index)

The slice command selects rows based on their integer index (position). Indices are 1-based.

Basic Usage

tva slice [input_files...] --rows <range> [options]
  • --rows / -r: The range of rows to keep (e.g., 1-10, 5, 100-). Can be specified multiple times.
  • --invert / -v: Invert selection (drop the specified rows).
  • --header / -H: Always preserve the first row (header).

Examples

1. Keep Specific Range (Head/Body)

To inspect the first 5 rows of docs/data/billboard.tsv (including header):

tva slice docs/data/billboard.tsv -r 1-5

Output:

artist	track	wk1	wk2	wk3
2 Pac	Baby Don't Cry	87	82	72
2Ge+her	The Hardest Part	91	87	92
...

2. Drop Header (Data Only)

Sometimes you want to process data without the header. You can drop the first row using --invert:

tva slice docs/data/billboard.tsv -r 1 --invert

Output:

2 Pac	Baby Don't Cry	87	82	72
2Ge+her	The Hardest Part	91	87	92
...

3. Keep Header and Specific Data Rows

To keep the header (row 1) and a slice of data from the middle (rows 10-15), use the -H flag:

tva slice docs/data/us_rent_income.tsv -H -r 10-15

This ensures the first line is always printed, even if it’s not in the range 10-15.

sample (Random Sampling)

The sample command randomly selects a subset of rows. This is useful for exploring large datasets.

Basic Usage

tva sample [input_files...] [options]
  • --rate / -r: Sampling rate (probability 0.0-1.0). (Bernoulli sampling)
  • --n / -n: Exact number of rows to sample. (Reservoir sampling)
  • --seed / -s: Random seed for reproducibility.

Examples

1. Sample by Rate

To keep approximately 10% of the rows from docs/data/us_rent_income.tsv:

tva sample docs/data/us_rent_income.tsv -r 0.1

2. Sample Exact Number

To pick exactly 5 random rows for inspection:

tva sample docs/data/us_rent_income.tsv -n 5

Output (example):

GEOID	NAME	variable	estimate	moe
35	New Mexico	rent	809	11
55	Wisconsin	income	32018	247
18	Indiana	rent	782	5
...