Code not rejecting alphanumeric input. Should only accept alpha

MSF1981 · 02-14-2009, 06:40 PM

This program is only supposed to accept alpha characters ie ABCD. But it is accepting alphanumeric input such as ABCD9. What am I dong wrong?

Code:

void A_getOriginalString()
{
	if (!(isalpha(*originalString))) /* Why does work for 5554 but not for ABCD9? */
	{
		puts("Letters only please. Try again.\n");
		fflush(stdin);
		gets(originalString);
	}
}

MK27 · 02-14-2009, 06:49 PM

Test one character at a time.

MSF1981 · 02-14-2009, 06:55 PM

Quote:

Originally Posted by MK27

Test one character at a time.

Duh!! I Thanks!

This worked...

Code:

void A_getOriginalString()
{
	int i;
	int length = strlen(originalString);
	
	for (i = 0; i < length; ++i)
	{
		originalString[i];
	}

	if (!(isalpha(originalString[i++])))
	{
		puts("Letters only please. Try again.\n");
		fflush(stdin);
		gets(originalString);
	}
}

MSF1981 · 02-14-2009, 07:03 PM

Quote:

Originally Posted by MSF1981

Duh!! I Thanks!

This worked...

Code:

void A_getOriginalString()
{
	int i;
	int length = strlen(originalString);
	
	for (i = 0; i < length; ++i)
	{
		originalString[i];
	}

	if (!(isalpha(originalString[i++])))
	{
		puts("Letters only please. Try again.\n");
		fflush(stdin);
		gets(originalString);
	}
}

Oops! Perhaps this doesn't work. Now it's not accepting any string as valid input.

MSF1981 · 02-14-2009, 07:10 PM

Okay, now I think I got it for realz....

Code:

	int i;
	int length = strlen(originalString);
	
	for (i = 0; i < length; ++i)
	{
		originalString[i];
		
		if (!(isalpha(originalString[i])))
		{
			puts("Letters only please. Try again.\n");
			fflush(stdin);
			gets(originalString);
		}
		
	}

MSF1981 · 02-14-2009, 07:21 PM

Quote:

Originally Posted by MSF1981

Okay, now I think I got it for realz....

Code:

	int i;
	int length = strlen(originalString);
	
	for (i = 0; i < length; ++i)
	{
		originalString[i];
		
		if (!(isalpha(originalString[i])))
		{
			puts("Letters only please. Try again.\n");
			fflush(stdin);
			gets(originalString);
		}
		
	}

Okay maybe I had it for fakes. I think it should be

Code:

length - 1

MK27 · 02-14-2009, 07:29 PM

Quote:

Okay maybe I had it for fakes. I think it should be

Code:

length - 1

Nope. It's because you are pre-incrementing (as opposed to post-incrementing) i:

Quote:

Originally Posted by MSF1981

Code:

	int i;
	int length = strlen(originalString);
	
	for (i = 0; i < length; ++i)
	{
		originalString[i];
		
		if (!(isalpha(originalString[i])))
		{
			puts("Letters only please. Try again.\n");
			fflush(stdin);
			gets(originalString);
		}
		
	}

Use i++. And the line in red is meaningless/superfluous.

MSF1981 · 02-14-2009, 07:33 PM

Quote:

Originally Posted by MK27

Nope. It's because you are pre-incrementing (as opposed to post-incrementing) i:

Use i++. And the line in red is meaningless/superfluous.

Ahh... Very cool! Thanks!

**laserlight** · 02-14-2009, 11:33 PM

This results in undefined behaviour:

Code:

fflush(stdin);

This makes you vulnerable to a buffer overflow:

Code:

gets(originalString);

I suggest that you write isalpha for a string, e.g.,

Code:

int isalphaString(const char *str)
{
    for (; *str != '\0'; ++str)
    {
        if (!isalpha(*str))
        {
            return 0;
        }
    }
    return 1;
}

Now, in the loop where you request for the user to enter the string, read in the input using something safer like fgets(), then use isalphaString() to check if the input is correct. If it is, end the loop, otherwise print the error message and request for user input again.

If you do use fgets(), then note that it places the newline in the string unless the string read is of the maximum length. Consequently, it may be useful to write a function to simplify the process of removing that newline, then replace the fgets() call with it, e.g.,

Code:

char *getLine(char *str, size_t size, FILE *fp)
{
    if ((str = fgets(str, size, fp)))
    {
        char *new_line = strchr(str, '\n');
        if (new_line)
        {
            *new_line = '\0';
        }
    }
    return str;
}

02-14-2009, 06:40 PM	#1
MSF1981 Registered User Join Date: Feb 2009 Location: Seattle Posts: 39	Code not rejecting alphanumeric input. Should only accept alpha This program is only supposed to accept alpha characters ie ABCD. But it is accepting alphanumeric input such as ABCD9. What am I dong wrong? Code: void A_getOriginalString() { if (!(isalpha(originalString))) / Why does work for 5554 but not for ABCD9? */ { puts("Letters only please. Try again.\n"); fflush(stdin); gets(originalString); } }
MSF1981 is offline

02-14-2009, 06:49 PM	#2
MK27 subminimalist Join Date: Jul 2008 Location: NYC Posts: 3,944	Test one character at a time. __________________ Accuracy and integrity mean nothing if you don't make it past the censors...PYTHAGORAS
MK27 is offline

02-14-2009, 11:33 PM	#9
laserlight C++ Witch Join Date: Oct 2003 Location: Singapore Posts: 10,363	This results in undefined behaviour: Code: fflush(stdin); This makes you vulnerable to a buffer overflow: Code: gets(originalString); I suggest that you write isalpha for a string, e.g., Code: int isalphaString(const char str) { for (; str != '\0'; ++str) { if (!isalpha(str)) { return 0; } } return 1; } Now, in the loop where you request for the user to enter the string, read in the input using something safer like fgets(), then use isalphaString() to check if the input is correct. If it is, end the loop, otherwise print the error message and request for user input again. If you do use fgets(), then note that it places the newline in the string unless the string read is of the maximum length. Consequently, it may be useful to write a function to simplify the process of removing that newline, then replace the fgets() call with it, e.g., Code: char getLine(char str, size_t size, FILE fp) { if ((str = fgets(str, size, fp))) { char new_line = strchr(str, '\n'); if (new_line) { new_line = '\0'; } } return str; } __________________ C + C++ Compiler: MinGW port of GCC Build + Version Control System: SCons + Bazaar Look up a C/C++ Reference and learn How To Ask Questions The Smart Way Last edited by laserlight; 02-15-2009 at 12:24 AM.
laserlight is online now

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