Validation design

Code:

#include <stdio.h>
#include <stdlib.h>


int main()
{
	int stop, ch, valid = 0;


	do
	{
		// display title
		printf("Individual Salesman Commission\n");
		printf("==============================\n");


        // prompt and read salesman id
        printf("Salesman ID (NO.): ");
        scanf("%d", &stop);


		// Only give user to pass if it is number
		while ( (ch = getchar()) != '\n')
		{
			// if it is not valid input
			if(ch > 1)
			{
				while ( (ch = getchar()) != '\n');


				printf("Invalid input. Please input again...\n");
				getchar();
				system("cls");


				// display title
				printf("Individual Salesman Commission\n");
				printf("==============================\n");


				// prompt and read salesman id
				printf("Salesman ID (NO.): ");
				scanf("%d", &stop);
			} // end if
		} // end while


        valid = 1; // end loop process
		
		// clear screen display  
		system("cls");
	} while (valid == 0); // loop if the input is invalid


	if(stop <= 0 )
	{
		// Display error message
		printf("No record found.\n");
	} // end else


	printf("Press Enter to exit program...\n");
	getchar();


} // end function main

I can't understand why input '-' or '+' jump to display no record found.

Don't post again until you have read and fully understood this link.

You are assuming that people will understand what problem you're having, despite giving minimal (and, as it happens, useless) relevant information.

You should initialise 'stop' and check the return value of scanf. Perhaps that will solve your issue.

Code:

#include <stdio.h>
#include <stdlib.h>


int main()
{
	int stop, ch, valid = 0;


	do
	{
		// display title
		printf("Individual Salesman Commission\n");
		printf("==============================\n");


        // prompt and read salesman id
        printf("Salesman ID (NO.): ");
        scanf("%d", &stop);


		// Only give user to pass if it is number
		while ( (ch = getchar()) != '\n'|| stop < 1)
		{
			// if it is not valid input
			if(ch > 1)
			{
				while ( (ch = getchar()) != '\n');


				printf("Invalid input. Please input again...\n");
				getchar();
				system("cls");


				// display title
				printf("Individual Salesman Commission\n");
				printf("==============================\n");


				// prompt and read salesman id
				printf("Salesman ID (NO.): ");
				scanf("%d", &stop);
			} // end if
		} // end while


        valid = 1; // end loop process
		
		// clear screen display  
		system("cls");
	} while (valid == 0); // loop if the input is invalid


	if(stop <= 0)
	{
		// Display error message
		printf("No record found.\n");
	} // end else


	printf("Press Enter to exit program...\n");
	getchar();


} // end function main

There is a issue that after I input '-' or '+' symbol, you enter to enter 2 times to continue when reaching "Invalid input. Please input again..." statement. It is due to the scanf("%d", &stop) grabs '-' or '+' symbol and then the newline grabs the newline. You can test with the following code:

Code:

#include <stdio.h>
#include <stdlib.h>


int main()
{
	int stop, ch, valid = 0;


	do
	{
		// display title
		printf("Individual Salesman Commission\n");
		printf("==============================\n");


        // prompt and read salesman id
        printf("Salesman ID (NO.): ");
        scanf("%d", &stop);


		printf("value: %d\n",stop);
		system("pause");
		// Only give user to pass if it is number
		while ( (ch = getchar()) != '\n'|| stop < 1)
		{
			printf("ch: %d\n",ch);
			system("pause");
			// if it is not valid input
			if(ch > 1)
			{
				while ( (ch = getchar()) != '\n');


				printf("Invalid input. Please input again...\n");
				getchar();
				system("cls");


				// display title
				printf("Individual Salesman Commission\n");
				printf("==============================\n");


				// prompt and read salesman id
				printf("Salesman ID (NO.): ");
				scanf("%d", &stop);
			} // end if
		} // end while


        valid = 1; // end loop process
		
		// clear screen display  
		system("cls");
	} while (valid == 0); // loop if the input is invalid


	if(stop <= 0)
	{
		// Display error message
		printf("No record found.\n");
	} // end else


	printf("Press Enter to exit program...\n");
	getchar();


} // end function main

So, how should I modify the code to solve this issue?

Have you ever heard of the KISS principle?

A simple do-while-loop is all you need:

Code:

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int nr = 0, ch;

    do
    {
        printf("Enter number: ");
        scanf("%d", &nr);
        while ((ch = getchar()) != '\n' && ch != EOF)
            ;
        if (nr < 1)
            puts("\nInvalid number, try again");
    } while (nr < 1);
    
    return 0;
}

Whenever you want to flush the input stream after a scanf()-call do it immediately.

Bye, Andreas

But Andi, that doesn't really work when you enter something like 1a, 1- or 2abcd.. ( basically any numbers followed by a character or string)

And when user input ' - ', it goes straight into getchar()? If true, is it because of scanf() itself only reads the format specified by the coder, other than that it will be discarded? Well this is what I thought of after setting up breakpoints and monitor the values in each variable.

If I'm wrong, hope you can explain it step by step. Thanks!

Oh and I modified your code to be able to fix the problem I mentioned. Is this okay?
*ps : it doesn't work after I enter 111ddd , and +

Code:

#include <stdio.h>
#include <stdlib.h>


int main()
{
    int nr = 0, ch;


    printf("Enter number: ");
    do
    {
        scanf("%d", &nr);
        while ( (ch = getchar()) != '\n' && ch != EOF){
            if(ch > 1){
                printf("\nError : ");
                while ( (ch = getchar()) != '\n' && ch != EOF);
                scanf("%d",&nr);
            }
        }
        if(nr < 1){
            printf("\nError : ");
        }
    } while (nr < 1);


    return 0;
}

Originally Posted by xeon321

But Andi, that doesn't really work when you enter something like 1a, 1- or 2abcd.. ( basically any numbers followed by a character or string)

My code is tolerant and accepts a number if the input starts with it and discards any following characters.
If you only want "clean" numbers (nothing before and after) I suggest a combination of fgets() and sscanf() because mixing scanf() and getchar() is always tricky.

Code:

#include <stdio.h>
#include <errno.h>
#include <string.h>
 
int main(void)
{
    int nr, items;
    char ch;
 
    while (1)
    {
        char buf[20];

        printf("Enter number: ");
        fgets(buf, sizeof(buf), stdin);

        errno = 0;
        items = sscanf(buf, "%d%c", &nr, &ch);
        if (items == 2 && ch == '\n')
            if (errno == ERANGE)
                puts("Number out of range, try again");
            else if (nr > 0)
                break;
            else
                puts("Number must be greater 0, try again");
        else
            puts("Not a number, try again");

        if (buf[strlen(buf) - 1] != '\n')
            while ((ch = getchar()) != '\n' && ch != EOF)
                ;
    }
    printf("The number is %d\n", nr);
     
    return 0;
}

On line 10 we start an infinite loop.
On line 15 we read the input into our buffer.
On line 17 we set "errno" to 0 so that we can be sure that any error we get is from sscanf() on the following line.
On line 18 we try to extract a number from the input and the character following the number. We also store the return value from sscanf().
On line 19 we check if we've got two items from sscanf() (the number and the following character) and if the character is the newline (thus there was nothing after the number).
If both conditions are true we check if the number is a valid integer. If the number is out of range (either smaller than INT_MIN or bigger than INT_MAX) than "errno" will be set to ERANGE (a macro defined in <errno.h>).
Then we check for an optional valid range for our program (in this case we only want positive numbers starting with 1). If the number is ok we break out of the loop else we print an error message.
If the condition on line 19 wasn't true, we didn't get a number from the user and print an error message.

Finally we check on line 29 if fgets() has read the newline character (the user has inputted less characters than our buffer is able to hold). If the last character in the input isn't the newline, there are still characters left in the input stream which we will consume. Thus we will start again with an empty input stream.

Bye, Andreas

bug.

Originally Posted by grumpy

Right 98% of the time, and don't care about the other 3%.

There you go

ulti-killer: AndiPersti's code works fine for me for the same input (eleven 9's). Can you post your current code?

I just compile the code of AndiPersti

The '1215752191' is because you're in a 32-bit architecture (and it worked for christop because he's probably on a 64-bit architecture). Eleven 9's is too big to be represented in a 32-bit integer, so it wraps around. If you look at only the least significant 32-bits, the number '99999999999' is actually '1215752191'. When you hit 2^32 your arithmetic wraps around and starts from zeros (when you're using signed numbers, it will wrap around into negative numbers because of two's complement).

If you want to work with numbers that are arbitrarily large, the GMP library does exactly that (and will use function calls to do each operation and break it down into steps that will fit on your processor). For most purposes, the int type is sufficient.

On line 19 we check if we've got two items from sscanf() (the number and the following character) and if the character is the newline (thus there was nothing after the number).
If both conditions are true we check if the number is a valid integer. If the number is out of range (either smaller than INT_MIN or bigger than INT_MAX) than "errno" will be set to ERANGE (a macro defined in <errno.h>).
Then we check for an optional valid range for our program (in this case we only want positive numbers starting with 1). If the number is ok we break out of the loop else we print an error message.

Well the problem here is that sscanf does NOT set errno on any kind of numeric overflow.
It just keeps accumulating digits (and silently overflowing) until there are no more digits.

If you want ERANGE type detection, use strtol() or strtoul() (or strtod for floats/doubles)

Originally Posted by Salem

Well the problem here is that sscanf does NOT set errno on any kind of numeric overflow.
It just keeps accumulating digits (and silently overflowing) until there are no more digits.

You're right. I haven't read the man-page carefully enough:

CONFORMING TO
The functions fscanf(), scanf(), and sscanf() conform to C89 and C99 and POSIX.1-2001. These standards do not specify the ERANGE error.

So ERANGE for sscanf() only works with the GNU C Library.

Bye, Andreas

Heh, and I hadn't realised that gcc had this sneaky addition over the standard.

Code:

#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <limits.h>
 
int main(void)
{
    int nr, items;
    char ch;
    printf("%d\n",INT_MAX);
    printf("%lld\n",LLONG_MAX);
 
    while (1)
    {
        char buf[30];

        printf("Enter number: ");
        fgets(buf, sizeof(buf), stdin);

        errno = 0;
        items = sscanf(buf, "%d%c", &nr, &ch);
        if (items == 2 && ch == '\n')
            if (errno != 0)
                perror("Number out of range, try again");
            else
                break;
        else
            puts("Not a number, try again");

        if (buf[strlen(buf) - 1] != '\n')
            while ((ch = getchar()) != '\n' && ch != EOF)
                ;
    }
    printf("The number is %d\n", nr);
     
    return 0;
}

However, it seems to be almost completely useless.

Code:

$ ./a.out 
2147483647
9223372036854775807
Enter number: 2147483648
The number is -2147483648

$ ./a.out 
2147483647
9223372036854775807
Enter number: 9223372036854775808
Number out of range, try again: Numerical result out of range
Enter number: 9223372036854775807
The number is -1

It only seems to generate ERANGE if the number input exceeds the limits of long long int, regardless of the width of the datatype being converted at the moment.

If you have to read your data into the longest data type, then manually check the range to see if will fit in a smaller int, then you may as well use strtol() from the outset. It's the same PITA either way.