PreIncrementBeforeLoop

Misconception:

Using a ++ pre-increment operator in a for loop’s update part, e.g., for (...; ...; ++i) ..., means that the increment happens before the loop body executes, while using a ++ post-increment means that the increment happens after the loop body executes.

Pre-increment in update part of for loop means increment before loop body

Pre-increment in update part of for loop means same as pre-increment anywhere else

Students incorrectly believe that:

for (int i=0; i<2; ++i) {
  print(i);
}

is equivalent to:

int i=0;
while (i<2) {
  ++i; // incorrect!
  print(i);
}

This is wrong!

The correct meaning of the above for loop is:

int i=0;
while (i<2) {
  print(i);
  ++i;
}

The ++i still happens after the end of the loop body. This code will print 0 and 1. It is no different from this:

for (int i=0; i<2; i++) {
  print(i);
}

The ++ happens before or after the value of i is read. In both cases above, the value of i is read, but is thrown away (i.e., it is not assigned to anything, like with a=i++, and it is not passed to anything, like with m(i++)).

In this misconception, students essentially extend the meaning of “pre” beyond the expression and to the scope of the entire loop.

Java

jls13

14.14.1.2 Iteration of for Statement

jls13

15.15.1 Prefix Increment Operator ++

jls13

15.14.2 Postfix Increment Operator ++

Operator Loop Control Flow

graph of for loop with pre-increment in update part