Groovy List Grep Function

last modified February 25, 2025

The grep function in Groovy is a versatile tool for filtering and searching elements in a list. It allows you to select elements that match a specific condition, such as a pattern, type, or custom logic. This tutorial covers the grep function with 15 practical examples.

Basic Usage of Grep

The grep function filters a list based on a condition. It returns a new list containing only the elements that match the condition.

def numbers = [1, 2, 3, 4, 5, 6]
def evenNumbers = numbers.grep { it % 2 == 0 }

println(evenNumbers)

This example filters even numbers from a list using the grep function.

Filtering by Type

You can use grep to filter elements of a specific type.

def mixedList = [1, "Groovy", 3.14, true, "Java"]
def strings = mixedList.grep(String)

println(strings)

This example filters only string elements from a mixed list.

Filtering by Regular Expression

grep can filter elements that match a regular expression.

def words = ["apple", "banana", "cherry", "date"]
def aWords = words.grep(~ /a/)

println(aWords)

This example filters words containing the letter "a".

Filtering by Range

You can use a range to filter elements within a specific range.

def numbers = [10, 20, 30, 40, 50]
def rangeFiltered = numbers.grep(20..40)

println(rangeFiltered)

This example filters numbers within the range 20 to 40.

Filtering by Custom Condition

You can use a closure to define a custom filtering condition.

def numbers = [1, 2, 3, 4, 5, 6]
def greaterThanThree = numbers.grep { it > 3 }

println(greaterThanThree)

This example filters numbers greater than 3.

Filtering Null Values

You can use grep to filter out null values from a list.

def mixedList = [1, null, "Groovy", null, 3.14]
def nonNullValues = mixedList.grep { it != null }

println(nonNullValues)

This example removes null values from a list.

Filtering by Class

You can filter elements that are instances of a specific class.

def mixedList = [1, "Groovy", 3.14, true]
def integers = mixedList.grep(Integer)

println(integers)

This example filters only integer values from a mixed list.

Filtering by Multiple Conditions

You can combine multiple conditions in a closure for filtering.

def numbers = [1, 2, 3, 4, 5, 6]
def filtered = numbers.grep { it % 2 == 0 && it > 3 }

println(filtered)

This example filters even numbers greater than 3.

Filtering by Collection

You can filter elements that are contained in another collection.

def numbers = [1, 2, 3, 4, 5, 6]
def filterList = [2, 4, 6]
def filtered = numbers.grep(filterList)

println(filtered)

This example filters elements that are present in another list.

Filtering by Closure with Index

You can use the index of elements in the filtering condition.

def numbers = [10, 20, 30, 40, 50]
def filtered = numbers.grep { it, index -> index % 2 == 0 }

println(filtered)

This example filters elements at even indices.

Filtering by Object Properties

You can filter objects based on their properties.

class Person {
    String name
    int age
}

def people = [
    new Person(name: "Alice", age: 30),
    new Person(name: "Bob", age: 25),
    new Person(name: "Charlie", age: 35)
]

def adults = people.grep { it.age >= 30 }

println(adults.name)

This example filters people aged 30 or older.

Filtering by Case Insensitive Match

You can filter strings using case-insensitive matching.

def words = ["Apple", "banana", "Cherry", "Date"]
def aWords = words.grep(~ /(?i)a/)

println(aWords)

This example filters words containing "a" or "A".

Filtering by Negative Condition

You can filter elements that do not match a condition.

def numbers = [1, 2, 3, 4, 5, 6]
def notEven = numbers.grep { !(it % 2 == 0) }

println(notEven)

This example filters numbers that are not even.

Filtering by Custom Object

You can filter elements based on custom logic in an object.

class Filter {
    boolean isEven(n) { n % 2 == 0 }
}

def numbers = [1, 2, 3, 4, 5, 6]
def filter = new Filter()
def evenNumbers = numbers.grep(filter.&isEven)

println(evenNumbers)

This example uses a custom object to filter even numbers.

Filtering by Multiple Patterns

You can filter elements that match any of multiple patterns.

def words = ["apple", "banana", "cherry", "date"]
def patterns = [~ /a/, ~ /e/]
def filtered = words.grep { word ->
    patterns.any { word =~ it }
}

println(filtered)

This example filters words that contain either "a" or "e".

Best Practices for Using Grep

• Use Descriptive Conditions: Write clear and concise conditions for better readability.
• Combine Conditions: Use logical operators to combine multiple conditions.
• Optimize Performance: Avoid complex conditions for large lists to improve performance.
• Leverage Regular Expressions: Use regex for powerful pattern matching.

Source

Groovy Grep Documentation

In this tutorial, we explored the grep function in Groovy with 15 practical examples. The grep function is a powerful tool for filtering and searching lists, making it easier to work with collections in Groovy.

Author

My name is Jan Bodnar, and I am a passionate programmer with many years of programming experience. I have been writing programming articles since 2007. So far, I have written over 1400 articles and 8 e-books. I have over eight years of experience in teaching programming.

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