Expressions

Now that we have functions, we should clarify what we mean by an expression.
In C, an expression is anything you can build up from:
- Constant values like 7 or 3.14,
- Function calls like power(2,16),
- Two expressions operated on by +, -, *, /, % (mod),
- Two expressions compared by <, >, <=, >=, ==, or !=,
- Two expressions combined with &&, or ||,
- An expression operated on by - or ! (not),
- A variable name with ++ or -- to either side,
- Or an expression surrounded by paranthesis.
[I've included both logical and arithmetic expressions together because as we said before, C treats them the same way.]
Here are some of the places were you can put expressions in C:
- In the test part of if, while, switch, or for statements.
- After the return in the return statement.
- On a line by itself with a semicolon after it.
That last place to put expressions might strike you as a bit odd (because it is). Simply put, C considers expressions to be statements in their own right. This is what allows you to write
```
printf( "Hello, world\n" );
```
because printf is just a function which is just an expression, but it also allows such oddities as
```
3 + 4;
```
C does not specify the order in which the different functions in an expression are evaluated, so avoid putting more than one function with a side effect in the same expression. I.e., avoid things like:
```
printf( "foo" ) + printf( "bar" );
i++ = i++;
```
because you will get different answers on different compilers (and maybe even on different runs!)

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