pretty printing of numerical arrays

[lindahua]

See below

julia> x = qr(randn(3,3))[1];

julia> c = x'x;

julia> c
3x3 Array{Float64,2}:
 1.0          5.55112e-17  5.55112e-17
 5.55112e-17  1.0          2.22045e-16
 5.55112e-17  2.22045e-16  1.0        

julia> println(c)
[1.0 5.551115123125783e-17 5.551115123125783e-17
 5.551115123125783e-17 0.9999999999999996 2.220446049250313e-16
 5.551115123125783e-17 2.220446049250313e-16 0.9999999999999998]

julia> show(c)
[1.0 5.551115123125783e-17 5.551115123125783e-17
 5.551115123125783e-17 0.9999999999999996 2.220446049250313e-16
 5.551115123125783e-17 2.220446049250313e-16 0.9999999999999998]

The REPL printing is quite decent, while the version produced by println is almost unreadable.

I am wondering why println or show does not do the pretty version.

One consequence of the current behavior is that it makes debugging difficult, since println(a) always outputs something that I have hard time comprehending, and it also affects @test_approx_eq.

Also, in application scripts, when I do want to print an array, I always ended up writing for-loops using @printf.

[jiahao]

cc: @alanedelman, who usually defines custom rounding for printing out arrays.

[JeffBezanson]

Many issues about this: #25, #29, #5709 etc.