How do I fix “for loop initial declaration used outside C99 mode” GCC error?

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'd try to declare i outside of the loop!

Good luck on solving 3n+1 :-)

Here's an example:

#include <stdio.h>

int main() {

   int i;

   /* for loop execution */
   for (i = 10; i < 20; i++) {
       printf("i: %d\n", i);
   }   

   return 0;
}

Read more on for loops in C here .

Answer 2

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.

Answer 3

To switch to mode in , follow the next steps: 切换到模式，请以下步骤：

Click , then in tab choose subtab , and place -std=c99 in the text area, and click . ，然后在“ 选项卡中选择“子卡 “ ，然后在文本区域中放置-std=c99 ，然后单击“ 。

This will turn mode on for your Compiler. 模式。

I hope this will help someone!

Answer 4

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 5

@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

Answer 6

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

Answer 7

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"

Answer 8

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 9

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...

Answer 10

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] '

Answer 11

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

QMAKE_CFLAGS_DEBUG = \
    -std=gnu99

QMAKE_CFLAGS_RELEASE = \
    -std=gnu99