JuliaLang/julia

Printf for complex numbers

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#58,617 opened on Jun 3, 2025

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 (4 comments) (0 reactions) (0 assignees)Julia (48,709 stars) (5,773 forks)batch import
complexdisplay and printingfeaturehelp wanted

Description

Currently, @sprintf "%.2f" (1+2im) errors, but it would make sense for it to produce "1.00 + 2.00im" I think. Currently, printfing a complex number is not really convenient, requiring a manual split into real + complex, and keeping the format string in sync between them.

Contributor guide

Tech stack
None
Domain
backend
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Familiarity with Julia's printf implementation
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60
Research direction
The issue requests that complex numbers be supported in printf format strings, e.g., @sprintf "%.2f" (1+2im) should produce "1.00 + 2.00im". Start by examining the current printf implementation in Julia's source, likely in base/printf.jl or similar. Understand how real numbers are formatted and how the Complex type's show method works (in base/complex.jl). There are no linked PRs, so a new implementation would need to extend the printf machinery to handle complex numbers, possibly by formatting real and imaginary parts separately and inserting a sign. Tests would need to be added in test/printf.jl.