Formatting numbers and strings

The @printf macro from the Printf package (we'll look deeper into macros in Chapter 7, Metaprogramming in Julia) takes a format string and one or more variables to substitute into this string while being formatted. It works in a manner similar to printf in C. You can write a format string that includes placeholders for variables, for example, as follows:

julia> name = "Pascal" 
julia> using Printf 
julia> @printf("Hello, %s \n", name) # returns Hello, Pascal 

Because @printf now lives in another package, you have to do this using Printf first (prior to 1.0, it belonged to Base).

If you need a string as the return value, use the macro @sprintf.

The following formatting.jl script shows the most common formats:

using Printf 
# d for integers: 
@printf("%d\n", 1e5) #> 100000 
x = 7.35679 
# f = float format, rounded if needed: 
@printf("x = %0.3f\n", x) #> 7.357 
aa = 1.5231071779744345 
bb = 33.976886930000695 
@printf("%.2f %.2f\n", aa, bb) #> 1.52 33.98 
# or to create another string: 
str = @sprintf("%0.3f", x) 
show(str) #> "7.357" 
println() 
# e = scientific format with e: 
@printf("%0.6e\n", x) #> 7.356790e+00 
# c = for characters: 
@printf("output: %c\n", 'α') #> output: α 
# s for strings: 
@printf("%s\n", "I like Julia") 
# right justify: 
@printf("%50s\n", "text right justified!") 
# p for pointers: 
@printf("a pointer: %p\n", 1e10) #> a pointer: 0x00000002540be400 
 

The following output is obtained upon running the preceding script:

100000 
x = 7.357 
1.52 33.98 
"7.357" 
7.356790e+00 
output: α 
I like Julia 
                   text right justified! 
a pointer: 0x00000002540be400 

A special kind of string is VersionNumber, which the form v"0.3.0" (note the preceding v), with optional additional details. They can be compared, and are used for Julia's versions, but also in the package versions and dependency mechanism of Pkg (refer to the Packages section of Chapter 1, Installing the Julia Platform). If you have the code that works differently for different versions, use something as follows:

if v"0.5" <= VERSION < v"0.6-" 
# do something specific to 0.5 release series 
end