Partially filled array and output variable

Hi everyone. I've got example of partially filled array where two sizes of array are used, declared size and the actual size. What I'm confused about is the output variable *dbl_sizep. Is it pointer to in_use in main function? If it is indeed pointer to in_use variable then why it stands that *dbl_sizep = i if there will be condition in main function that i<in_use?

This is the program I talk about:

I - The fill_to_sentinel function

insert

Code:

---

1. /*
2.  * Gets data to place in dbl_arr until value of sentinel is encountered in 
3.  * the input.
4.  * Returns number of values stored through dbl_sizep. 
5.  * Stops input prematurely if there are more than dbl_max data values before 
6.  * the sentinel or if invalid data is encountered.
 7.  * Pre:  sentinel and dbl_max are defined and dbl_max is the declared size 
8.  *      of dbl_arr 
9.  */
10. void
11. fill_to_sentinel(int    dbl_max,    /* input - declared size of dbl_arr      */
12.                  double sentinel,  /* input - end of data value in
13.                                        input list                            */
14.                  double dbl_arr[],  /* output - array of data                */
15.                  int  *dbl_sizep)  /* output - number of data values
16.                                                stored in dbl_arr            */
17. {
18.      double data;
19.      int    i, status;
20.
21.      /* Sentinel input loop                                                */
22.      i = 0;
23.      status = scanf("%lf", &data);
24.      while (status == 1 && data != sentinel && i < dbl_max) {
25.          dbl_arr[i] = data;
26.          ++i;
27.          status = scanf("%lf", &data);
28.      }
29.
30.      /* Issues error message on premature exit                              */
31.      if (status != 1) {
32.            printf("\n*** Error in data format ***\n");
33.            printf("*** Using first %d data values ***\n", i);
34.  } else if (data != sentinel) {
35.            printf("\n*** Error: too much data before sentinel ***\n");
36.            printf("*** Using first %d data values ***\n", i);
37.      }
38.
39.      /* Sends back size of used portion of array                            */
40.      *dbl_sizep = i;

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II - Driver for testing fill_to_sentinel function

insert

Code:

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1. /* Driver to test fill_to_sentinel function */ 
2. 
3. #define A_SIZE 20 
4. #define SENT -1.0 
5. 
6. int 
7. main(void)
8. { 
9.      double arr[A_SIZE];
10.      int    in_use,    /* number of elements of arr in use */
11.              i;
12.
13.      fill_to_sentinel(A_SIZE, SENT, arr, &in_use);
14.
15.      printf("List of data values\n");
16.      for  (i = 0; i < in_use; ++i)
17.          printf("%13.3f\n", arr[i]);
18.
 19.      return (0);
20. }

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Yes, dbl_sizep is a pointer to in_use in main.
The two i's in the different functions are different variables.
Don't let that confuse you.
You could change the name of i in fill_to_sentinel to x and the program would be the same.

In while loop will final value of i be 8 or 7? If I type in 8 values then value of i will be 7 because for the first element i will be equal to 0. How will *dbl_sizep be equal to i (*dbl_sizep=i) if the value of i is by 1 less than number of elements stored in array? If I type in sentinel value the value of i shouldn't be incremented because it breaks the condition.

Quote:

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If I type in 8 values then value of i will be 7

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No, it will be 8. Although the first value of i will be 0, it will be incremented right after it is used as an index. The same thing happens on each iteration: the old value is used to index the array, then i is incremented.

Alright, now it's all much clearer to me! And what about for (i=0; i<in_use ; ++i)? Is it incremented at the same time it is used as index as this part lists the array values arr[0]-arr[7]?

A for loop like this:

Code:

---

for (i = 0; i < in_use; ++i) {
    printf("%13.3f\n", arr[i]);
}

---

Is essentially "syntactic sugar" for a while loop like this:

Code:

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i = 0;
while (i < in_use) {
    printf("%13.3f\n", arr[i]);
    ++i;
}

---

As you can see, i is incremented after it is used as an index. So it is used as 0 then incremented to 1, used as 1 then incremented to 2, and as soon as it reaches in_use the loop stops, so it's never used as an index with that value. I.e., if in_use is 8, then i is used as an index from 0 to 7, just as it should be.

Thank you so much for choosing to spend part of your time in clearing these things up for me. I think I finally got it. :)