Function substring 

pub fn substring(
    array: &dyn Array,
    start: i64,
    length: Option<u64>,
) -> Result<ArrayRef, ArrowError>

Returns an [ArrayRef] with substrings of all the elements in array.

Arguments

• start - The start index of all substrings. If start >= 0, then count from the start of the string, otherwise count from the end of the string.

• length(option) - The length of all substrings. If length is None, then the substring is from start to the end of the string.

Attention: Both start and length are counted by byte, not by char.

Basic usage

let array = StringArray::from(vec![Some("arrow"), None, Some("rust")]);
let result = substring(&array, 1, Some(4)).unwrap();
let result = result.as_any().downcast_ref::<StringArray>().unwrap();
assert_eq!(result, &StringArray::from(vec![Some("rrow"), None, Some("ust")]));

Error

• The function errors when the passed array is not a [GenericStringArray], [GenericBinaryArray], [FixedSizeBinaryArray] or [DictionaryArray] with supported array type as its value type.
• The function errors if the offset of a substring in the input array is at invalid char boundary (only for [Large]String array). It is recommended to use substring_by_char if the input array may contain non-ASCII chars.

Example of trying to get an invalid utf-8 format substring

let array = StringArray::from(vec![Some("E=mc²")]);
let error = substring(&array, 0, Some(5)).unwrap_err().to_string();
assert!(error.contains("invalid utf-8 boundary"));