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

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 2

@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 3

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 4

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 5

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.