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Are you doing something like for (int i = 0; ...; ...)? If so, you need to declare i outside the for statement.

loaf wrote:

So I need to either change the for loops to while loops or not write for loops how I've learned them, I guess?

Excerpted from the GCC man page:

Options Controlling C Dialect

       The following options control the dialect of C (or languages derived
       from C, such as C++, Objective-C and Objective-C++) that the compiler
       accepts:

       -std=
           Determine the language standard.  This option is currently only
           supported when compiling C or C++.  A value for this option must be
           provided; possible values are

           c99
           c9x
           iso9899:1999
           iso9899:199x
               ISO C99.  Note that this standard is not yet fully supported;
               see <http://gcc.gnu.org/gcc-4.0/c99status.html> for more
               information.  The names c9x and iso9899:199x are deprecated.

           gnu89
               Default, ISO C90 plus GNU extensions (including some C99
               features).

           gnu99
           gnu9x
               ISO C99 plus GNU extensions.  When ISO C99 is fully implemented
               in GCC, this will become the default.  The name gnu9x is
               deprecated.

So you need to either:
- Add -std=c99 or -std=gnu99 to your compilation options
- Write your for loop like:

int i;
for (i = 0; ...)

loaf wrote:

Yep.

So I need to either change the for loops to while loops or not write for loops how I've learned them, I guess?

You would still have to pre-declare any variable you used with a while loop.  C++ added the ability to do spontaneous variable declaration (although I believe the compiler still declares the variables beforehand, it just parses your code with more sophistication).

akb825 wrote:

because the Microsoft compiler doesn't support C99, so it's more portable.

Well, in class we're using Microsoft Visual Studio, so I guess I just need to avoid for loops as I've been taught them then.  Because if I use C99 at home, the code won't compile at school.

If you use the -pedantic flag on GCC, it will complain more but your code will be more "standard". Code that compiles under GCC with the -pedantic flag should compile under Visual Studio. (that is, barring the differences in the standard library) You can enable this in XCode in the build settings for your project. Note that these will be issued as warnings rather than errors. (since it can obviously still compile the code, it's just telling you it isn't standard) Regardless, it's good practice to always fix any warnings you have. You should definitely still compile it under Visual Studio, but this should help alleviate some of the headache of making it work.