Compiled versus interpreted

Some languages ( C, C++ ) are compiled; others ( Python, Common Lisp ) are interpreted.

What's the difference?

It's worth noting that any language can be compiled -or- interpreted. All the interpreted languages I'm familiar with also have compilers. I've heard that there is a C interpreter out there, but that making a sane C++ interpreter is difficult, possibly impossible. Still, there -are- interpreters out there for what are traditionally compiled languages.

Also worth noting up front is that the end result is the same, either way. Your code-as-written will be broken down, parsed, turned into machine code, and then run by the computer's CPU.

So, back to the question. What's the difference?

A compiling language will produce an executable by default. The compiler will come along, analyze your code, make a symbol table where it will keep track of names of things, datatypes, and so on, and then it will create a whole bunch of machine code. This file that's produced can then be run by the OS later. Since your code was compiled, the symbol table is really easy for the computer to keep track of - when it runs across a symbol, for example, a function or variable name, it knows where to look for it.

For an interpreted program, life is a little more difficult. A lot of things that are handled by the symbol table now have to be handled on-the-fly by the interpreter. On the other hand, interpreted languages can use dynamic typing, which is a boon; dynamic scoping; and tend towards being more platform independent ( Java, for example, compiles to bytecode, which is then interpreted by the Java virtual machine, and is famous for its 'write once, run anywhere' manifesto ). They also have their downsides; the main disadvantage is that they have a bit more overhead than a compiled language. With a compiled language, the grunt work of turning your code-as-written into machine code is done at compile time, and then the compiler exits. It's no longer in memory space, taking up resources. An interpreted language, however, needs its interpreter. Otherwise, there's no machine code, and nothing gets done.

So, long-story-short, the interpreted languages need an interpreter. The compiled languages get compiled, but don't need the compiler once they've been made into an executable. Either way, your code is still getting turned into machine instructions for the CPU, there's just a difference in how those instructions get there.