How to check if user input a string value?

Question

So here's my code:

main()
{
        char *iochar;                                                                                   

        printf(" Enter a standard line: ");                                                             
        scanf ( "%s", &iochar);

        if (iochar != NULL)
        {
                printf(" Here's what you entered: %s ", &iochar);
        }
        else
        {
                printf(" Oops! Looks like you forgot to enter something! ");
        }
}

My problem is that I can scan the user entry and store it and if something exists it puts out the correct message. But if I just hit return to test for no input (null, empty, etc) the program neither quits nor outputs my error message. It just hangs until I input something.

I've been programming in Java for the last two semesters so C is totally lost on me right now. So any help would be much appreciated.

so you just want to validate whether input is a string or not? — Ousmane D.
– Ousmane D., Commented Feb 22, 2017 at 18:37
Don't use scanf (ever). Use fgets, or, if you have it, getline. Then "just hitting return" will produce the string "\n" and typing control-D (which forces an EOF condition) will produce the string "". Incidentally, the spaces at the ends of your second and third printf strings should be \n instead. — zwol
– zwol, Commented Feb 22, 2017 at 18:37
@OusmaneDiaw At this point, yes. My code has to do other things, but I have to be able to validate that there is in fact some sort of input. — Polly
– Polly, Commented Feb 22, 2017 at 18:38
Where to begin? No memory allocation for char *iochar;. Not checking the function return value from scanf. Using & for string arguments to scanf and printf... I suggest the man pages for functions you use, and enabling full compiler warnings. — Weather Vane
– Weather Vane, Commented Feb 22, 2017 at 18:39
The %s conversion specification for scanf() skips white space, including newlines and blanks and tabs, and doesn't start saving anything until it comes across a non-white-space character. If you care about newlines, you can't use scanf() because it doesn't care about newlines (unless you work very hard — so hard that it isn't worth doing). If you care about lines, use a line-based input function (e.g. fgets()) and then maybe sscanf() to parse the line that it reads. — Jonathan Leffler
– Jonathan Leffler, Commented Feb 22, 2017 at 19:02

chux · Accepted Answer · 2017-02-22 19:03:19Z

2

scanf ( "%s", &iochar); is not useful for detecting an empty line.

"%s" first directs scanf() to read and discard all leading white-space, including '\n' before proceeding to read non-white-space characters. So code has lost the '\n'.

Instead use fgets()

// Enough room for 80 characters + \n + \0
#define LINE_SIZE (80 + 1 + 1)
char buf[LINE_SIZE];

if (fgets(buf, sizeof buf, stdin) == NULL) {
  puts("end-of-file or input error detected");
} else if (buf[0] == '\n') {
  puts("Empty line entered");
} else {
  printf("Input line: <%s>\n", buf);
}

Input

Hello World (and <Enter>)

Output (Note the \n read in is still retained)

Input line: <Hello World
>

To be clear: In C, a string is a sequence of characters up to and including a terminating null character '\0'. Users do not enter strings. User input is usually a line of text up to and including a terminating line feed '\n'. fgets() reads a line of input and then appends a null character to the resultant buffer is a string`.

edited Feb 22, 2017 at 19:03

answered Feb 22, 2017 at 18:58

chux

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1 Comment

Polly Over a year ago

Oh my gosh thanks so much! It works like a dream now! Haha.

Ry- · Accepted Answer · 2017-02-22 19:36:57Z

0

Here's my advice: Don't use scanf. Its rules are confusing, and it's very difficult to get the behaviour you want. Instead, I recommend you use getch to manually read each character into a buffer until the user enters a newline character '\n'. Then you can zero terminate the buffer with a 0, and then use the buffer as a normal string. For it to be empty, just check the length of the string with strlen(x).

edited Feb 22, 2017 at 19:36

Ry-♦

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answered Feb 22, 2017 at 18:39

user2933559

2 Comments

Weather Vane Over a year ago

You know of fgets?

user2933559 Over a year ago

Yes, but I prefer to not use standard library functions that I can implement myself.

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