[c] How do I fix "for loop initial declaration used outside C99 mode" GCC error?

I'm trying to solve the 3n+1 problem and I have a for loop that looks like this:

for(int i = low; i <= high; ++i)
        {
                res = runalg(i);
                if (res > highestres)
                {
                        highestres = res;
                }

        }

Unfortunately I'm getting this error when I try to compile with GCC:

3np1.c:15: error: 'for' loop initial declaration used outside C99 mode

I don't know what C99 mode is. Any ideas?

I've gotten this error too.

for (int i=0;i<10;i++) { ..

is not valid in the C89/C90 standard. As OysterD says, you need to do:

int i;
for (i=0;i<10;i++) { ..

Your original code is allowed in C99 and later standards of the C language.

For Qt-creator: just add next lines to *.pro file...

QMAKE_CFLAGS_DEBUG = \
    -std=gnu99

QMAKE_CFLAGS_RELEASE = \
    -std=gnu99

To switch to C99 mode in CodeBlocks, follow the next steps:

Click Project/Build options, then in tab Compiler Settings choose subtab Other options, and place -std=c99 in the text area, and click Ok.

This will turn C99 mode on for your Compiler.

I hope this will help someone!

Jihene Stambouli answered OP question most directly... Question was; why does

for(int i = low; i <= high; ++i)
{
    res = runalg(i);
    if (res > highestres)
    {
        highestres = res;
    }
}

produce the error;

3np1.c:15: error: 'for' loop initial declaration used outside C99 mode

for which the answer is

for(int i = low...

should be

int i;
for (i=low...

There is a compiler switch which enables C99 mode, which amongst other things allows declaration of a variable inside the for loop. To turn it on use the compiler switch -std=c99

Or as @OysterD says, declare the variable outside the loop.

Enable C99 mode in Code::Blocks 16.01

• Go to Settings-> Compiler...
• In Compiler Flags section of Compiler settings tab, select checkbox 'Have gcc follow the 1999 ISO C language standard [-std=c99]'

@Blorgbeard:

New Features in C99

• inline functions
• variable declaration no longer restricted to file scope or the start of a compound statement
• several new data types, including long long int, optional extended integer types, an explicit boolean data type, and a complex type to represent complex numbers
• variable-length arrays
• support for one-line comments beginning with //, as in BCPL or C++
• new library functions, such as snprintf
• new header files, such as stdbool.h and inttypes.h
• type-generic math functions (tgmath.h)
• improved support for IEEE floating point
• designated initializers
• compound literals
• support for variadic macros (macros of variable arity)
• restrict qualification to allow more aggressive code optimization

http://en.wikipedia.org/wiki/C99

A Tour of C99

For anyone attempting to compile code from an external source that uses an automated build utility such as Make, to avoid having to track down the explicit gcc compilation calls you can set an environment variable. Enter on command prompt or put in .bashrc (or .bash_profile on Mac):

export CFLAGS="-std=c99"

Note that a similar solution applies if you run into a similar scenario with C++ compilation that requires C++ 11, you can use:

export CXXFLAGS="-std=c++11"

I had the same problem and it works you just have to declare the i outside of the loop:

int i;

for(i = low; i <= high; ++i)

{
        res = runalg(i);
        if (res > highestres)
        {
                highestres = res;
        }

}

if you compile in C change

for (int i=0;i<10;i++) { ..

to

int i;
for (i=0;i<10;i++) { ..

You can also compile with the C99 switch set. Put -std=c99 in the compilation line:

gcc -std=c99 foo.c -o foo

REF: http://cplusplus.syntaxerrors.info/index.php?title='for'_loop_initial_declaration_used_outside_C99_mode