Tell me how you format output and I tell you what programming language you are

Since K&R times it’s traditional to make a programming language print “Hello, world!” as the introduction to it. Recently I had a thought that formatting a number in output tells you the essentials of the programming language itself.

So if you want to print not just a constant string but, say, a number expanded to a certain length (e.g. printf("%4d\n", val);), how can you do it?

Essentially there are three major approaches: using WRITE with optional specifiers for each element, using one format string with arguments to follow and having just basic output with utilities to format elements using some pattern and string concatenation. And there’s C++.

The first approach can be found in the old venerable languages like FORTRAN or ALGOL. It is also somewhat funny that in Pascal its writeln() can’t be implemented in the language itself so the compiler has to convert it to a sequence of formatting and writes or string concatenations (exactly the third way). The same is true for Rust as well (which belongs to the second category) but at least println!() outright tells you that it’s not a normal function but rather a macro or a compiler plug-in.

The second approach even if not invented got popularised by C. You have a template string with recognizable percent signs to signal where arguments should be substituted and in what form. Of course this is not the safest approach since you may pass a template with not enough arguments and read garbage from the stack (and that’s not counting the dreaded %n). In the recent years another template format got popular (probably thanks to Python, with an influence from bash or PHP): instead of percent sign and type there are braces with an optional format arguments or variable name inside. Speaking of Python, you can use print with the old printf-style or new f-string formatting and it will put a newline at the end by default (reminding once again that Python is all about whitespaces). As for the implementation, in some languages it’s a built-in function (i.e. you can’t implement it in the language itself but the compiler/interpreter takes care of it), in C it’s implemented by parsing the stack more or less directly, in many modern languages variable-argument function simply passes those arguments as an array of the most generic object types (so you can implement something like printf() yourself). I think I can also mention here the third pattern format mostly used for formatting binary output—the one used in Perl pack() function (and whatever languages borrowed it), its syntax reminds me of the classic languages from the first category.

The third approach is too primitive so usually languages try to use some syntactic sugar to make it easier to use (see Pascal or Rust mentioned above). I should probably mention INTERCAL where outputting a single character might pose a challenge for an uninitiated but that’s part of the charm of the language.

And finally C++. In the original version of the language you were supposed to use cout << setw(4) << val << endl; and while it demonstrates the advantages of the operator overloading, in the same time it suffers from the extreme verbosity and side effects (that width modifier will be applied to all the following argument in all following prints until you change it). In the modern C++ you should use std::cout, std::setw() and std::endl() instead. As the result people mostly use printf() or some third-party formatting library (you may even get it with one of your string class implementations for free).

As you can see, the way you can print a formatted number (both the syntax and the implementation) can tell you a lot about the programming language in a rather short time. Of course there are exceptions but the lack of such functionality tells you at least about the kind of language you're dealing with.

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