CS156 Increment

Shortcut Operators

big_ugly_variable = big_ugly_variable + 1 is tedious. We have a better way:

a+=b is the same as a = a+b
a-=b is the same as a = a-b
a*=b is the same as a = a*b
a/=b is the same as a = a/b
a%=b is the same as a = a%b

The expressions can be arbitrarily complex:

a+=a+b*c is the same as a = 2*a + b*c

Increment and Decrement Operators

Adding or subtracting one (incrementing/decrementing) is so common, that we have special notations for just that:

++ : Increment a variable
-- : Decrement a variable

The operators can be placed before, ++i, or after, i++, the variable. These two expressions have different behaviors.

Pre- and Post- Increment

Pre-incrementing, ++i, causes the variable to be incremented before it is used in the expression that contains it.
Post-incrementing, i++, causes the variable to be incremented after it is used in its containing expression.
Depending on context, you may see the same or different behaviors from these two expressions.

Increment example

int foo, i=2;
foo = i++; printf("foo=%d i=%d\n", foo, i);
foo = ++i; printf("foo=%d i=%d\n", foo, i);
foo = i--; printf("foo=%d i=%d\n", foo, i);
foo = --i; printf("foo=%d i=%d\n", foo, i);

foo=2 i=3
foo=4 i=4
foo=4 i=3
foo=2 i=2

The variable i takes the values 2, 3, 4, 3, 2 as expected, but its value is updated at different times depending on pre-increment or post-increment.

Side Effects in C

The use of the increment and decrement operators in expressions can result in undefined (unpredictable) behavior, because post-increment and post-decrement take place sometime after the value is returned.
Stick to using these operators as the only part of an expression.
Don’t do things like this:

    i = i++;
i++ * i++;
i = --i - i--;

These are okay:

    i++;
j = i++;
for (i=0; i<5; i++)

Unpredictable, really!

I'm serious about this—statements like i = i++;, or using an uninitialized variable, are undefined behavior.
Showing me that it works won’t impress me, either.
Yes, it happens to produce the result that you want on that computer, with the current version of that particular compiler.
That doesn’t mean that it’ll work on the GTA’s computer, with a different compiler.
I want you to write code that works everywhere, always.

CS156: Intro to C, Part I

Spring 2018