Calculate sum, mean, sum of squares, and standard deviation of array elements
Let the compiler help you

The first thing to do is to compile with a decent compiler and look at the warnings. With gcc -O -Wall -W:

dassouki.c: In function ‘main’:
dassouki.c:70: warning: implicit declaration of function ‘rand’
dassouki.c:70: warning: implicit declaration of function ‘time’
dassouki.c:90: warning: control reaches end of non-void function


You're missing two headers, stdlib.h and time.h, where the functions rand and time are declared. Once you add them you see another error:

small_fixes.c: In function ‘main’:
small_fixes.c:72: error: too many arguments to function ‘rand’


For now, to get code that compiles, remove the arguments to rand, it doesn't take any. I'll explain about rand below.

The last warning is there because main function returns an int value; you neglected to do that. If your implementation conforms to C99 (few do, and the one I compiled with doesn't), there is a special dispensation for the main function: you can omit the return statement, and it's as if you'd written return EXIT_SUCCESS. For utmost portability, return EXIT_SUCCESS to indicate success, or EXIT_FAILURE to indicate failure. On unix and Windows, EXIT_SUCCESS is 0 and EXIT_FAILURE is 1, though any nonzero value (positive and up to 255, though you should stick to small values) indicates failure. If you don't return a value from main (and aren't using a C99 compiler), the effect on most platforms is that your program returns whatever was in a particular register when main finishes executing, which is not good.

General style

Generally speaking, your code is well-organized and well-presented, congratulations. Here are a few minor improvements:


The array x doesn't actually need to be global, you can make it local to main. If you make it global, give it a longer name — global variables should be used sparingly and should be easily noticeable and searchable.
Don't use abbreviations in the names of global variables or functions. Call your functions square_sum or sum_of_squares, standard_deviation.
The proper type for an array length is size_t. On some machines, you can have arrays whose size doesn't fit into an int. Note that size_t is an unsigned type, which has the advantage that you won't run into the occasional difficulties of signed arithmetic, but you need to be careful with downwards loops (for (i=n; i&gt;=0; i--) is an infinite loop if i is unsigned).


Output


You're missing newlines (\n) at the end of several printf calls.
The printf conversion specifier for a double is %f (or %e or %g), not %lf. Many implementations allow %lf as a synonym for %f, but this is not universal.
The printf conversion specified for a size_t is %z, but that only exists if your implementation conforms to C99, which is still not the norm today. In C89 (the prevalent standard today), your best bet is to cast to unsigned long, e.g.

printf("[%lu] %lf\n", (unsigned long)i, x[i]);

Why are you printing i+1 next to element number i? It's confusing. Print i and x[i] (as above).


Random number generation

rand() generates a random integer between 0 and RAND_MAX. On many platforms, RAND_MAX is 32767. Here, you're storing this into a floating point array, so it looks like you want to generate a random floating-point number. This is a difficult problem, and the solution depends on what distribution you want. On unix/POSIX platforms, there is a function drand48 which generates a random floating-point number in the range [0,1] with a uniform distribution. This issue is unrelated to the meat of your program, so you may be content with generating a random integer with rand().

A second issue is that rand() and friends use a pseudo-random number generator. This means that they will always return the same sequence of numbers if they are initialized in the same way. Initializing a random generator is called seeding it, and the function to seed the PRNG is srand(int). If you never call that function, your program will always fill the array with the same values. A common way of initializing the PRNG for testing is srand(time(NULL)); this will make your program use different values every second (on most machines).

Random number generation is a fairly complex topic; I recommend reading the clc FAQ, questions 13.15–13.20, especially Each time I run my program, I get the same sequence of numbers back from rand() and How can I generate floating-point random numbers?.

For the time being, add srand(time(NULL)); near the beginning of your main() function, and fix the call to rand().

Input validation

You read an integer with scanf. This function is convenient for quick tests but tricky to use correctly. It's difficult to react properly if the user enters garbage.

Since num should be a positive integer, make it an unsigned type. In C99, make it size_t and call scanf("%z", &amp;num). In C89, make it unsigned long and call scanf("%lu", &amp;num).

Perform at least some token validation of num. The number should be positive (if you want to allow 0, you need to add a special case to some of your calculations). And it must not be larger than the size of the array.

scanf("%lu", &amp;num);
if (num == 0 || num &gt;= sizeof(x)/sizeof(x[0])) {
  printf("Invalid number of elements, goodbye.\n");
  return EXIT_FAILURE;
}


sizeof(x)/sizeof(x[0]) (which can also be written sizeof(x)/sizeof(*x)) is an idiom to denote the number of elements in the array x: it's the size of the array divided by the size of one element. Note that this only works if x is an array, not if x is a pointer. Pointers to arrays don't contain any indication of the size of the array, and the size needs to be specified separately (hence your arr_count argument when you pass a pointer to the array to the various functions).