Question

The program asks for a number. The program should loop the scanf() function if the user types anything else other than numbers.

The code is,

 do{
     printf("Enter row number\n");
     scanf("%d",&row);
  }while(row>='a' && row<='z');

But the above code doesn’t work. I keep getting an error when typing in a letter. I tried manipulating around it and the whole thing loops infinitely.

Any suggestions?

Also how can I tell the C Compiler not to break the loop unless the input is an INTEGER?

Answer 1

You are ignoring the return from scanf() which tells you whether the typed information was accurate for the conversion (%d) or not. If it was inaccurate, you have to do error recovery, which is not particularly easy with scanf(). Most people go for the approach of ‘read a line of input and then parse it’, where the error recovery is simpler.

---

I understand that return values as essential in error checking, but how do I scan if its numbers or letters? Can I say if (input!=(integers)) or anything similar?

This is why people don’t use scanf(). If you get the line of data into a buffer (character array), then you can check the contents of the array as often as you like. If you use scanf(), you don’t get a reliable chance to process the data until after scanf() decides it has an error.

The functions (usually also available as macros) in <ctype.h> allow you to classify characters. The functions in <stdlib.h> provide reliable conversions from strings to integers of various sorts.

So, you can think about doing something like:

char buffer[256];

while (fgets(buffer, sizeof(buffer), stdin))
{
    ...check contents of buffer using isdigit() etc...
    ...or just plough ahead with...
    long value;
    char *end;
    errno = 0;
    value = strtol(buffer, &end, 0);
    if (errno != 0 || (*end != '\0' && !isspace(*end))
        ...diagnose problems...
}

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This code is a bit out of my league at the moment .. is there a simpler way?

Well, I suppose you can use atoi() instead of strtol(), which simplifies the error handling (because it is less precise):

char buffer[256];

while (fgets(buffer, sizeof(buffer), stdin))
{
    int value = atoi(buffer);
    if (value == 0)
    {
        puts("zero read - exiting loop");
        break;
    }
}

It doesn’t get much simpler than this. I don’t know which part of the previous solution you felt was beyond you. The alternatives, it seems to me, are much fiddlier, involving reading one character at a time and saving the digits and rejecting the non-digits:

char buffer[256];
char *dst = buffer;
int c;
int value;

while ((c = getchar()) != EOF)
{
    if (isdigit(c))
    {
        *dst++ = c;  /* Ignoring buffer overflow - bad! */
    }
    else if (isspace(c))
    {
        *dst = '\0';
        value = atoi(buffer);
        break;
    }
    else
    {
        printf("Unexpected character '%c'!\n", c);
    }
}

Etcetera. There are various issues to resolve in that code – like resetting the pointer after an erroneous character, and avoiding buffer overflow, and dealing with signs on the numbers, and … well, all sorts of stuff that I’d rather leave to routines like fgets() and strtol().