C - scan user inputs into an array to be sorted

Question

Forgive me, I'm a C programming rookie. What I need to do is take values from standard input and store them in an array which is to be sorted later on down the line.

The method of entry for the user is one number on one line at a time (i.e. enter a number, press enter, enter number, press enter, etc..). When the user is done entering numbers, they press ENTER without providing a number.

My code for accepting the values and storing them is as follows. You'll probably see the issue immediately, but I'm not seeing it.

#include <stdio.h>
#define MAX 100

int main()
{
    int n, i, array[MAX];

    printf("Enter a list of integers\n");

    for(i = 0; i <= MAX; ++i){
        printf("> ");
        if (scanf("%d", &n) == -1)
            break;
        else
            scanf("%d", &n);
            array[i] = n;
    }

    printf("The array is %d", *array);
    return 0;
}

The picture below is how the program should run. I have the sort code already, and it seems to work quite well. Your help is greatly appreciated.

enter image description here

You have a “goto fail”-style bug in your code. Only the scanf line after the else is executed conditionally. The array[i] = n; always happens. You should enclose the lines after else in {curly braces}. — Zev Eisenberg
– Zev Eisenberg, Commented May 29, 2014 at 2:53
http://stackoverflow.com/questions/19794268/scanf-reading-enter-key and http://stackoverflow.com/questions/14099473/how-to-scanf-only-integer-and-repeat-reading-if-the-user-enter-non-numeric-chara for reference — Robin
– Robin, Commented May 29, 2014 at 2:53
I would remove the scanf in the else as well, that way you don't skip every other number. — Retired Ninja
– Retired Ninja, Commented May 29, 2014 at 2:54
Oh, does scanf process if it's inside an if condition? I didn't know that. These are all very useful comments so far. Thank you all. — Rich Scriven
– Rich Scriven, Commented May 29, 2014 at 3:01
Any code inside a condition part of an if statement is first executed, and the return value is evaluated (typically, 0 is considered false, other values are considered true). — Zev Eisenberg
– Zev Eisenberg, Commented May 29, 2014 at 3:07

David C. Rankin · Accepted Answer · 2014-05-29 03:08:19Z

2

You have it doing what you want, you just need a few tweaks. First, enter doesn't return -1, to keep it simple you need to enter ctrl+d to stop input. After your final input, just hit ctrl+d. Take a look:

#include <stdio.h>
#define MAX 100

int main()
{
    int n, i, array[MAX];

    printf("Enter a list of integers [ctrl+d] to end\n");

    for(i = 0; i <= MAX; ++i){
        printf("> ");
        if (scanf("%d", &n) == -1)
            break;
        array[i] = n;
    }
    puts ("");

    int z;
    for (z = 0; z < i; z++)
        printf("The array is %d\n", array[z]);

    return 0;
}

output:

Enter a list of integers [ctrl+d] to end
> 1
> 2
> 3
> 4
> 5
>
The array is 1
The array is 2
The array is 3
The array is 4
The array is 5

answered May 29, 2014 at 3:08

David C. Rankin

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7 Comments

Zev Eisenberg Over a year ago

How would you detect the Enter key (i.e. entering an empty line)? I was trying to write an answer for this, but I got stuck on how to detect that.

Rich Scriven Over a year ago

Thanks for the answer. Couple of questions. 1. What is puts("") doing there? 2. Can I make the array print on one line, like The array is 1, 2, 3, 4, 5? Also, the enter thing is pretty important. Thanks again.

David C. Rankin Over a year ago

It get's a little more complicated. You need to look at kbhit() which is a general name for an input routine using struct termios to put the keyboard in raw-unbufferd mode to allow you to accept each keystroke without a return. You can then test for the enter key being pressed directly (unsigned int 10).

David C. Rankin Over a year ago

puts("") was just a hack to force a newline without vargs parsing of printf. It was not the relevant part of the response. I didn't attempt to match the output format shown. It appeared the OP was more concerned about getting the array fill working.

David C. Rankin Over a year ago

@RichardScriven if you are interested in an example of kbhit, here is an example source called keypress2.c

|

Jonathan Leffler · Accepted Answer · 2023-02-22 17:39:05Z

1

Here is the updated previous answer to exit on enter.

#include <stdio.h>
#define MAX 100

int main()
{
    int n, i, array[MAX];
    char num[MAX];
    int res;

    printf("Enter a list of integers [ctrl+d] to end\n");

    for(i = 0; i <= MAX; ++i){
        printf("> ");
        fgets(num, sizeof(num), stdin);
        res = sscanf(num, "%d", &n);
        if(res != 1)
            break;
        n = atoi(num);
        array[i] = n;
    }
    puts ("");

    int z;
    for (z = 0; z < i; z++)
        printf("The array is %d\n", array[z]);

    return 0;
}

edited Feb 22, 2023 at 17:39

Jonathan Leffler

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answered May 29, 2014 at 5:39

user207064

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1 Comment

Jonathan Leffler Over a year ago

Note that the code should check the return value from fgets() because fgets() won't overwrite the array if it reads EOF, and that loop could run for a long time (until the program crashes — which it probably will do) unless you type a non-number to terminate the input.

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