How to convert an untyped array value to a string #5377

Timmmm · 2024-02-08T22:21:35Z

Timmmm
Feb 8, 2024

I'm using DuckDB's Rust interface and it returns a StructArray, containing a list of ArrayRef - one for each column.

I do not know the types of the ArrayRefs. Is there a generic way to just convert all types to strings? I tried this (took me literally 3 hours to get this far - documentation is... well):

    let results = stmt.step().ok_or(anyhow!("No results?"))?;
    let string_cols = results.columns().iter().map(|col| compute::cast(col, &DataType::Utf8)).collect::<Result<Vec<_>, _>>()?;
    let downcase_string_cols = string_cols.iter().map(|c| c.as_string::<i64>()).collect::<Vec<_>>();

    let mut output = "<!DOCTYPE HTML><html><body><table>\n".to_string();
    output += "<th>";
    for col in results.column_names() {
        output += "<td>";
        output += &escape_html(&col);
        output += "</td>";
    }
    output += "</th>\n";

    for row in 0..results.len() {
        output += "<tr>";
        for col in downcase_string_cols.iter() {
            let value = col.value(row);
            output += "<td>";
            output += &escape_html(&format!("{:?}", value));
            output += "</td>";
        }
        output += "</tr>\n";
    }

    output += "</table></body></html>";

    Ok(output)

Unfortunately it doesn't work. It says Cannot cast from struct to other types (turns out one of the columns has a struct type).

Is there some thing that will do a "best effort" of converting to strings? E.g. format structs/lists as JSON? This is for debugging, essentially a DuckDB UI.

Answered by Timmmm

Feb 11, 2024

Ah ArrayFormatter was exactly what I was looking for, thanks! For reference for future people:

use arrow::{
    array::ArrayRef,
    util::display::{ArrayFormatter, FormatOptions},
};

pub fn value_string(column: &ArrayRef, row: usize) -> String {
    let options = FormatOptions::default();
    match ArrayFormatter::try_new(column.as_ref(), &options) {
        Ok(f) => f.value(row).to_string(),
        Err(e) => e.to_string(),
    }
}

View full answer

tustvold · 2024-02-11T12:28:47Z

tustvold
Feb 11, 2024
Collaborator

You can format to JSON using the arrow-json package, or alternatively you can use ArrayFormatter from arrow-cast

1 reply

Timmmm Feb 11, 2024
Author

Ah ArrayFormatter was exactly what I was looking for, thanks! For reference for future people:

use arrow::{
    array::ArrayRef,
    util::display::{ArrayFormatter, FormatOptions},
};

pub fn value_string(column: &ArrayRef, row: usize) -> String {
    let options = FormatOptions::default();
    match ArrayFormatter::try_new(column.as_ref(), &options) {
        Ok(f) => f.value(row).to_string(),
        Err(e) => e.to_string(),
    }
}

Answer selected by Timmmm

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How to convert an untyped array value to a string #5377

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How to convert an untyped array value to a string #5377

Timmmm Feb 8, 2024

Replies: 1 comment · 1 reply

tustvold Feb 11, 2024 Collaborator

Timmmm Feb 11, 2024 Author

Timmmm
Feb 8, 2024

Replies: 1 comment 1 reply

tustvold
Feb 11, 2024
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Timmmm Feb 11, 2024
Author