What are the historical reasons C languages have pre-increments and post-increments?

Question

(Note: I am not asking about the definitions of pre-increment vs. post-increment, or how they are used in C/C++. Therefore, I do not think this is a duplicate question.)

Answer 1

Incrementing and decrementing by 1 were widely supported in hardware at the time: a single opcode, and fast. This because "incrementing by 1" and "decrementing by 1" were a very common operation in code (true to this day).

The post- and predecrement forms only affected the place where this opcode got inserted in the generated machine code. Conceptually, this mimics "increase/decrease before or after using the result". In a single statement

i++;

the 'before/after' concept is not used (and so it does the same as ++i;), but in

printf ("%d", ++i);

it is. That distinction is as important nowadays as it was when the language C was designed (this particular idiom was copied from its precursor named "B").

From The Development of the C Language

This feature [PDP-7's "`auto-increment' memory cells"] probably suggested such operators to Thompson [Ken Thompson, who designed "B", the precursor of C]; the generalization to make them both prefix and postfix was his own. Indeed, the auto-increment cells were not used directly in implementation of the operators, and a stronger motivation for the innovation was probably his observation that the translation of ++x was smaller than that of x=x+1.

^{Thanks to @dyp for mentioning this document.}