How do I get this to register -1 as an exit & show the numbered list as starting from 1?

Question

Okay so the prompt was:

"Write a simple program (to demonstrate use of an array) that asks a user to type in numbers and keep typing in numbers until the user enters -1 or the total number of numbers entered reaches 20. Your program will stop asking for new input based on either condition above. Once one of the above conditions are met, the program will output all numbers entered as follows: 1. 888 2. 999 3. 4 …..and so on. Lastly, the program needs to display the sum of all values entered, excluding the -1 used to terminate the user input loop. The array should be used to store the user's inputs, and display them back before the program terminates."

Now, I have most of the prompt completed. It stops at 20 entered variables and displays them and the sum correctly. However, no matter how hard I try, I can't get it to register -1 as an exit. Nor can I get the bulleted list to start from 1. and end at 20. Everything else works just fine.

#include <stdio.h>
#define NUMBER_INPUTS 20 

int main(void){
    double userInput[NUMBER_INPUTS] = { 0 };
    int i = 0;

    while (i < NUMBER_INPUTS){
        printf("Enter number: \n", i);
        scanf("%lf", &userInput[i]);
        i++;        
    }

    for(i=0; i<NUMBER_INPUTS; i++)
        printf("%i. %.1lf \n", i, userInput[i]);    

    double total=0;
    for(i=0; i<NUMBER_INPUTS; i++)
        total+=userInput[i];
    printf("Total: %.1lf \n", total);

    return 0;
}

This is the code that works just fine. I've tried different do-whiles in different places to register the -1 but it messes up the output list AND doesn't stop the run.

#include <stdio.h>
#define NUMBER_INPUTS 20    

int main(void){
    double userInput[NUMBER_INPUTS] = { 0 };
    int i = 0;

do{
    while (i < NUMBER_INPUTS){
        printf("Enter number: \n", i);
        scanf("%lf", &userInput[i]);
        i++;        
    }
}while (userInput[i] != -1);

    for(i=0; i<NUMBER_INPUTS; i++)
        printf("%i. %.1lf \n", i, userInput[i]);    

    double total=0;
    for(i=0; i<NUMBER_INPUTS; i++)
        total+=userInput[i];
    printf("Total: %.1lf \n", total);

    return 0;
}

Like this? Just stops at the 20th variable entered. Doesn't matter if I've entered a -1 it just keeps going.

And for the list I attempted to use

i=i+1

to get the list from 1. to 20. to try and bump the i variable up one but for some reason that just shorted the output list to only show 20. and the total? I don't know what I'm doing wrong and would appreciate some input. Thank you.

Answer 1

Be very careful in comparison on floating-point numbers with exact values. (the point of the two links). While double can represent -1 exactly, don't fall into the habit of believing that floating-point numbers can represent any value exactly -- they can't. See: Is floating point math broken? and Why Are Floating Point Numbers Inaccurate?

To stop reading at the input of -1, you need to preserve the value of i (the number of elements) that have a valid double stored when you leave the read loop (that is the number of array elements filled). int n = i; will work (or just use the separate counter n and save i for use as a loop variable). Then in your print loop. for (i = 0; i < n; i++). As you have it, you are attempting to print NUMBER_INPUTS every time. If the user enters -1 after 3 inputs -- that's not going to work.

In order to use any input function correctly, you must validate the return. (this is especially true with scanf with contains many pitfalls for the new C programmer) Otherwise on a matching-failure, your array element will be left indeterminate, character extraction from stdin will cease leaving the offending characters in stdin unread just waiting to bite you again on your next attempted input. Instead -- validate, validate, validate. Lest ye violate Commandment No. 6. Example:

        if (scanf ("%lf", &userInput[n]) != 1) {    /* validate input */
            fputs ("error: invalid double input.\n", stderr);
            return 1;
        }

Putting the rest together, and giving an example of checking a floating-point value within a tolerance of +/- 1E-5, you could do:

#include <stdio.h>

#define NUMBER_INPUTS 20    

int main (void) {

    double userInput[NUMBER_INPUTS] = { 0 }, total = 0;
    int i = 0, n = 0;

    while (n < NUMBER_INPUTS){
        fputs ("Enter number: ", stdout);
        if (scanf ("%lf", &userInput[n]) != 1) {    /* validate input */
            fputs ("error: invalid double input.\n", stderr);
            return 1;
        }
        /* check for -1 (with +/- 1E-5 tolerance) */
        if (userInput[n] > -1.00001 && userInput[n] < -0.99998)
            break;
        total += userInput[n];
        n++;
    }

    putchar ('\n');
    for(i = 0; i < n; i++)
        printf("%d. %.1lf\n", i + 1, userInput[i]);    

    printf ("\nTotal: %.1lf \n", total);

    return 0;
}

Example Use/Output

$ ./bin/total
Enter number: 10
Enter number: 20
Enter number: 30
Enter number: 40
Enter number: 50
Enter number: -1

1. 10.0
2. 20.0
3. 30.0
4. 40.0
5. 50.0

Total: 150.0

Look things over and let me know if you have further questions. You were quite close.

Edit - updated output element indication to i + 1 to meet your requirement of Showing The Numbered List Starting From 1.