Defining variables at the beginning of function and "fprintf" - c

Question

I'd like your help with couple of things I'm not sure of them, even-though the compiler doesn't complain about them:
Here I need to write a program which gets input and output, in the input file were stored integers which I don't know their amount divided by spaces, I need to read these numbers, sort them by the sum of digits comparison and print out the sorted numbers in the output file. this is what I wrote, after that a couple of short question about this code:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>
int myComp(const void *a, const void *b){
int x=(*(int*)a);
int y=(*(int*)b);
int sumForx=0;
int sumFory=0;
while (x){
sumForx=sumForx+(x%10);
x=(x-(x%10))/10;
}
while (y){
sumFory=sumFory+(y%10);
y=(y-(y%10))/10;
}
if (x>y) return 1;
else if (x<y) return -1;
else return 0;
}
int main(int argc, char** argv) {
FILE* inFile;
FILE* outFile;
int size=0;
int tmp;
if (argc!=3) {
printf("Please enter 3 arguments");
assert(0);
}
inFile=fopen(argv[1], "r");
if (inFile==NULL) {
printf("path to the input file has not found");
assert(0);
}
outFile=fopen(argv[2], "w");
if (outFile==NULL) {
printf("path to the output file has not found");
assert(0);
}
while (fscanf(inFile, "%d", &tmp)==1) {
size++;
}
int arr[size];
fseek(inFile, 0, SEEK_SET);
int i=0;
while (fscanf(inFile, "%d", &tmp)==1) {
arr[0]=tmp;
i++;
}
qsort(arr,size,sizeof(int),myComp);
int j;
for (j=0;j<size;j++){
fprintf(outFile,"%d",arr[j]);
fprintf(outFile,"%c",' ');
}
fclose(inFile);
fclose(outFile);
return 1;
}
I kept define new variables during the main, in different places- I can recall that I shouldn't do that since all variables must be defined at the beginning of the function, unless there are local variables of inner functions/brackets, and this is not the case here- but still the compiler is fine with this- what's the correct thing to do?
If the answer the (1) is "you must define all variables at the beginning of the function"- in my case, I must define dynamic int* arr and allocate space for it after I computed size, otherwise I can't use int arr[size] since size is computed after all the variables have defined already, include the integer array.
3.I want to enter a space between these numbers while being printed to the file, is fprintf(outFile,"%c",' '); correct after putting integer every time?
4.any other correction would be gladly welcome!

Answer 1

The requirement to declare all variables at the beginning of a function dates back the the pre-previous version of the standard (C89, which was outdated by C99, which was outdated by C11).
Since you are using a variable-length array (arr[size]), which wasn't possible in that pre-previous version of the standard, you are obviously using a halfway-decent compiler that doesn't stick to restrictions no longer applying since 1999. ;-)
As for printing a space, fprintf( outfile, " " ) or (even better) fputc( ' ', outfile ) would do.
As for further corrections, I have a habit of not reading / commenting on uncommented source. You have a coding style with which I violently disagree, but at least you're consistent in applying it. ;-)

Answer 2

This depends on the version of C you are using. If you use ANSI C (c89) you will get warnings or errors about this. But most (if not all) modern compilers support declarations almost anywhere in the code.
This again depends on your compiler. Old compilers couldn't allocate static arrays like that, modern compilers can.
fprintf(outFile, " "); is perfectly legal too.

Answer 3

If you are using gcc, it will allow you to place variables anywhere in the function. However, other compilers (e.g. MSVC 2008) follow ansi c i.e c89 and probably will complain about it. The correct thing to do depends on your application. If you want it to be portable across compilers, declarations should come before statements.
This is also compiler and C standards specific, so reasoning similar to point 1 applies.
For this, your code seems fine but I use something like:
fprintf(outFile, "%d ", arr[j]);
to achieve similar functionality in a single line.

Answer 4

printf("Please enter 3 arguments");
The executable name is usually not referred to as an argument so this would normally be
printf("Please enter 2 arguments - input filename and output filename e.g.:\n");
printf("%s file1.txt file2.txt\n",argv[0]);
This
arr[0]=tmp;
should be
arr[i]=tmp;

Answer 5

1: Declaring variables anywhere in the function body is fine as long as you are on a compiler that supports the C99 standard or later. Earlier compilers required that you declared variables at the beginning of a block scope, as you say.
You can do even better than in your code however, by limiting the scope of some of the variables even more. Instead of:
int j;
for (j = 0; j < size; j++) { ... }
You can simplify this to:
for (int j = 0; j < size; j++) { ... }
Which also limits the scope where j is accessible to the block following the for statement.
2: Dynamically sized arrays are only supported in C99 and later, so the code would not compile on a compiler that does not support the newer standard.
3: You can simply add the extra space to the initial format string:
fprintf(outfile, "%d ", arr[j]);