# Incrementally pretty print a table, one row at a time

**URL:** <https://discourse.julialang.org/t/incrementally-pretty-print-a-table-one-row-at-a-time/128544>\
**Category:** Performance\
**Tags:** tables, prettytables\
**Created:** [April 29, 2025, 11:36pm UTC](https://discourse.julialang.org/t/incrementally-pretty-print-a-table-one-row-at-a-time/128544 "2025-04-29T23:36:56Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![luke-kiernan](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/luke-kiernan/32/214309_2.png) [@luke-kiernan](https://discourse.julialang.org/u/luke-kiernan)\
**Post date:** [April 29, 2025, 11:36pm UTC](https://discourse.julialang.org/t/incrementally-pretty-print-a-table-one-row-at-a-time/128544/1 "2025-04-29T23:36:56Z")

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I’m comparing some Newton’s method-type iterative solvers. I’d like to pretty print the progress of the iterative method, so I can tell if the search is making good progress. If I didn’t care about performance, I’d just store it all to one big `DataFrame`, then print it at the end. But I do care about performance. Is there a way to feed `PrettyTables.jl` data one row at a time, instead of all at once?

I can give it the headers and the max width of each column at the start. If I’m limited to printing strings of max width N and need to handle the rest myself (round/truncate), that would also be acceptable.

I could write my own crude plaintext table printer…but then I’d have to go think about buffers, performance, etc.

Edit: I guess this gets the job done. Have not profiled it, though. Allocating `words` here is potentially slow.

```julia
"""A simple plaintext table writer: always left-aligned.
N is number of columns and W the width of each column."""
struct TableWriter{N, W}
    bf::IOStream
end

function TableWriter{N, W}(bf::IOStream, headers::NTuple{N, String}) where {N, W}
    t = TableWriter{N, W}(bf)
    write_row_separator(t)
    write_row_content(t, headers)
    return t
end

function write_row_separator(t::TableWriter{N, W}) where {N, W}
    write(t.bf, '+') 
    for _ in 1:N
        for _ in 1:W
            write(t.bf, '-')
        end
        write(t.bf, '+')
    end
    write(t.bf, '\n')
end

function write_row_content(t::TableWriter{N, W}, words::NTuple{N, String}) where {N, W}
    write(t.bf, '|')
    for word in words
        write(t.bf, word)
        for _ in 1:(W - length(word))
            write(t.bf, ' ')
        end
        write(t.bf, '|')
    end
    write(t.bf, '\n')
end

function write_row(t::TableWriter{N, W}, words::NTuple{N, String}) where {N, W}
    write_row_separator(t)
    write_row_content(t, words)
end

function write_row(t::TableWriter{N, W}, nums::NTuple{N, Float64}) where {N, W}
    f = x -> round(x; sigdigits = 3)
    words = NTuple{N, String}(string.(f.(nums)))
    write_row_separator(t)
    write_row_content(t, words)
end

write_row(t::TableWriter{N, W}, nums::Tuple) where {N, W} = write_row(t, Float64.(nums))

open("test.txt", "w") do filehandle
    headers = NTuple{2,String}(["V_θ", "V_m"])
    t = TableWriter{2, 10}(filehandle, headers)
    write_row(t, (1.5, 2.5))
    write_row(t, (13, 5.0))
end

```

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**Author:** ![karei](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/karei/32/214809_2.png) [@karei](https://discourse.julialang.org/u/karei)\
**Post date:** [April 30, 2025, 1:55am UTC](https://discourse.julialang.org/t/incrementally-pretty-print-a-table-one-row-at-a-time/128544/2 "2025-04-30T01:55:38Z")

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Why not use `println` in the loop to print one line at a time?