How do I fix “for loop initial declaration used ou

Question

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?

Answer 1

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.

Answer 2

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!

Answer 3

Just compile in C++ mode. You don't NEED to use classes to use C++. I basically use C++ as a "nicer C" :)

I almost never use classes and never use method overiding.

Answer 4

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

QMAKE_CFLAGS_DEBUG = \
    -std=gnu99

QMAKE_CFLAGS_RELEASE = \
    -std=gnu99

Answer 5

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;
        }

}

Answer 6

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...