Xjyuk

Most of the books I read give examples using printf and scanf. At some point the students know perfectly how to use these two functions but they don't know about stdin, stdout and argv.

To me and according to many programming principles (e.g. KISS) a program should not interrupt the execution for prompting the user. Instead, and this is a much clever approach, the developer should learn to use the power of pipes and use the options and the arguments.

I think this:

$ whatdaywas 1982 02 16
Monday

Is much better than:

$ whatdaywas
Enter a year: 1982
Enter a month: 2
Enter a day: 16
It was a Monday. 

Is there any rationale behind this pedagogical approach?

I think that the rationale is so you don't have to introduce arrays and array notation on day 1. Typically, arrays enter into the picture at roughly the same time as loops, which would take place a few weeks or months later, depending on the pacing of your course.

That said, if you want to start with command line programs, it's not a huge problem to do so. At the very, very beginning of programming, there are always a few magical incantations that students must type, but won't fully explore until later. args[0] could easily be such an incantation.

As a side benefit to this approach, your students may have an easier time when they come to arrays, since they've already used them a bit.

Ultimately they need to understand both ways. Which you do first seems to be a matter of preference - or maybe just following the textbook.

They need to understand at some point that 'main' in C is an interface with the OS. But, since you can't teach everything at once, you have do pick the order of instruction. But teaching command line params later can give you a way to talk about "elegance" in program design.

Of course, the program written to accept args needs to behave properly if none (or too few) are presented. Often this defaults to prompting for data in simple programs.

Because, traditionally, programs iterate over data. See JSP.

Admittedly, prompting the user for data is weird, however we're talking about beginners here, having them try to remember what the different fields of data they have to enter isn't the point of the exercise, and is likely to make them view interacting with computers as needlessly complicated. As a bonus, prompting for the values helps make the beginner programs self documenting - the text of the prompt is an explanation of the variable that is to be read - so this will hopefully help our novice programmers understand the simple program without introducing the complex topic of comments.

(fun fact: originally in pascal it wasn't legitimate code to output before input, so you couldn't prompt for input. This was quickly abandoned)