How to use Printf in Julia?

In Julia, printf is not a built-in function but a macro provided by the Printf module of Julia's standard library. This macro allows you to format strings similarly to the C programming language's printf function. The Printf.@printf macro takes a format string followed by zero or more arguments and outputs a formatted string to the standard output.

Syntax:

@printf([io::IO], "%Fmt", args...)

Print arguments using C style format specification string. Optionally, an IO may be passed as the first argument to redirect output.

Note:

1. There is no requirement to import the Printf module in Julia like other modules.
2. using Printf is necessary for using the @printf macro.
3. Both the macros in the Printf module can also be used with a full path like "Printf.@printf()".
4. Ensure that 'P' in Printf is a capital letter and everything else is small.

First type the 'using Printf' function in Julia REPL so that we can use the @printf macro ahead:

Format Specifiers in Julia

1. String Format Specifier %s

First type @printf then formatted string and format specifier in inverted commas - " " and then type arguments in inverted commas.

Note:

Do not use any ',' - comma like C language in Julia and if you want to use comma than make sure that you also use round brackets after @printf.

Without brackets:

@printf "Geeks for %s" "Geeks"

Output:

With brackets:

@printf("Geeks for %s","Geeks")

Output:

2. Character Input Format Specifier %c

Only a single character will be printed even if you provide a string as an argument.

@printf "Hello %c" "world"

Output:

Note:

Starting in Julia 1.8, %s (string) and %c (character) widths are computed using textwidth, which e.g. ignores zero-width characters (such as combining characters for diacritical marks) and treats certain "wide" characters (e.g. emoji) as width 2.

3. Floating Point Number Format Specifiers %f and %e

Below is the Julia program to implement the floating point numbers using %f:

@printf "Decimal digits: %f" 1.23456

Output:

Below is the Julia program to implement limiting float to two digits in %f:

@printf "Decimal two digits: %.2f" 1.23456

Output:

Below is the Julia program to implement the scientific notation using %e:

@printf "Scientific notation: %e" 1.234

Output:

Below is the Julia program to implement limiting scientific notation to three digits in %e:

@printf "Scientific notation three digits: %.3e" 1.23456

Output:

4. Padding to Length Format Specifier %i

Below is the Julia program for Padding to length 5 using %i:

@printf "Padded to length 5 %5i" 123

Output:

Below is the Julia program for Padding with zeros to length 6 using %i:

@printf "Padded with zeros to length 6 %06i" 123

Output:

All C language format specifiers can be similarly used in Julia.

5. Use of Dynamic Width and Precision

Dynamic width specifiers like %*s and %0*.*f can print width and precision together.

@printf "Use dynamic width and precision  %*.*f" 10 2 0.12345

Output:

Use dynamic width and precision        0.12

Note:

Dynamic width specifiers like %*s and %0*.*f require Julia 1.10.

Caveats in Julia

Inf and NaN are printed consistently as Inf and NaN for flags %a, %A, %e, %E, %f, %F, %g, and %G. Furthermore, if a floating point number is equally close to the numeric values of two possible output strings, the output string further away from zero is chosen.

@printf("%f %F %f %F", Inf, Inf, NaN, NaN)

Output:

@printf "%.0f %.1f %f" 0.5 0.025 -0.0078125

Output:

sprintf in Julia

Similarly, like the @printf macro of the Printf module, Julia also has an @sprintf macro in the Printf module. @sprintf macro is used to return the formatted output of @printf as a string.

Syntax:

@sprintf("%Fmt", args...)

@sprintf "This is a %s %15.1f" "number using @sprintf:" 34.567

Output: