Stop a for loop when user is finished entering input in c

Question

First of all, thank you for the assist!

I\'m new to the C language (and programming in general) and I\'m trying to write a program wherein the user inputs data point

Answer 1

You have received a number of good answers to your question, and there are several more ways to take input of doubles and stop on "done". Since you are learning C, always, ALWAYS (in case it wasn't clear), check the return of scanf to validate the number of conversions you expected actually took place.[1] (this also provides your way to end input on "done" (or any non-double entered causing scanf to return less than 1)

As noted in the comment, arrays are zero based in C. When you are taking input, you will want to use count as your array-index, rather than i (in this case if you exit the read on each failure -- it doesn't matter, but you could just as easily prompt again for additional input and increment count only on a successful return from scanf) Back to your question. If you set up your read loop to continually loop until there is a scanf failure, you can make use of a temporary variable to initially capture the input value, and only assign the value to your array and increment your index on success. e.g. (with a constant MAXD = 1048)

for (;;) {  /* loop until scanf input fails (with 'done') */

    double tmp;  /* block scope declarations are fine */

    printf (" data[%4d]: ", count);

    if (count < MAXD && scanf(" %lf", &tmp) == 1)
        data[count++] = tmp;
    else
        break;
} 

(you can even move a copy of the prompt above the loop, and move the one above after the if (....) {...} to eliminate the prompt when the array limit (MAXD) is reached -- that's left as an exercise)

In the example above you have 2 conditions you enforce before storing a value. (1) you limit the number of values your user can store to MAXD, and (2) you only store a value if a valid conversion to double takes place in scanf. You leave the loop if either of the conditions fails (which if you enter "done" as a double-value, it will).

Putting the pieces together and dropping a few additional tips in the comments, you could test with something like the following:

#include 

enum { MAXD = 1048 };   /* declare constants instead of using magic numbers */

int main (void) {

    double data[MAXD] = {0};    /* in ISO C declarations come before code */
    int i, count = 0;           /* initializing variable saves debug time */

    printf ("\n Welcome! \n\n Please enter each data point. "
            "Enter 'done' when finished.\n\n");

    for (;;) {  /* loop until scanf input fails (with 'done') */

        double tmp;  /* block scope declarations are fine */

        printf (" data[%4d]: ", count);

        if (count < MAXD && scanf(" %lf", &tmp) == 1)
            data[count++] = tmp;
        else
            break;
    } 

    printf ("\n %d values entered:\n\n", count);

    for (i = 0; i < count; i++)
        printf ("  data[%4d] : %.2lf\n", i, data[i]);

    return 0;   /* main() is type 'int' and returns a value */
}

Example Use/Output

$ ./bin/scanfdoubles

 Welcome!

 Please enter each data point. Enter 'done' when finished.

 data[   0]: 1.1
 data[   1]: 1.2
 data[   2]: 1.3
 data[   3]: 1.4
 data[   4]: 1.5
 data[   5]: 1.6
 data[   6]: done

 6 values entered:

  data[   0] : 1.10
  data[   1] : 1.20
  data[   2] : 1.30
  data[   3] : 1.40
  data[   4] : 1.50
  data[   5] : 1.60

Look things over and let me know if you have any questions.

footnotes:

1. while you can use scanf to take user-input in C, you are better off using a line-oriented function (like fgets) and then parsing the complete line (with, e.g. sscanf). The allows you to both (1) validate the read (e.g. the return of fgets) and then (2) separately validate the value entered by the user. This decoupling of your read, and your parsing has many advantages.