How can i read a line of numbers from stdin & compute their average?

Question

How may I count & extract a number of elements from stdin?

For instance, len() is easily available in Python, but not in C?

Im trying to compute an average, when user enters for eg: 1 3 5 7 8 10 11 -1

I've tried the following:

while (user_input != -1)
{
    total   += user_input;
    scanf ("%d", &user_input);

    //cannot work               
    average = total /  ...
}       
printf("Average = %f\n", average);  

Answer 1

You'll have to maintain a counter to do what you're trying to do. its just int counter = 0; and in the loop: counter++

int counter = 0;
while (user_input != -1)
{
    total   += user_input;
    counter ++;
    scanf ("%d", &user_input);
}       
average = total / counter;
printf("Average = %f\n", average);  

obviously, you'll have to check if scanf() returned atleast 1

--- EDIT --- the following program(that corresponds to the previous program) is VALID and works as required. People who do not understand how scanf() works, should stay the damn out:

#include <stdio.h>

int main(int argc, char *argv[])
{
   int total = 0;
   float average = 0.0f;
   int userinput = 0;
   int counter = -1;
   while(userinput != -1){
      counter ++;
      if(scanf("%d",&userinput) == 1 && userinput != -1){
         total += userinput;
      }
   }
   average = ((float)total/(float)counter);
   printf("Average = %f", average);
   return 0;
}

Input: 10 20 30 40 50 60 -1 Output: Average = 35

Answer 2

From your modified question & comments, it seems what you really want to do is read a line of input from the user and then extract numbers from that line of input.
The answer is you cannot do that using scanf , You need to use fgets to read the line from stdin and then you need to extract numbers from that read line. Here is a,
Quick sample program :

#include<stdio.h>
int main()
{
    char line[1024], *beg, *end;
    long readvalue;
    while (fgets(line, sizeof(line), stdin)) 
    {
        beg = line;
        for (beg = line; ; beg = end) 
        {
            readvalue = strtol(beg, &end, 10);
            if (beg == end)
                break;
                printf("[%d]",readvalue);
        }
    }
    return 0;
}

Input:

1 3 5 7 8 10 11 -1

Output:

[1][3][5][7][8][10][11][-1]