Infinite printf()

[Nextstopearth]

This code accept an integer response from input fine, but if you give it a character it will infinitly repeat the last printf statement. If anybody can see why I'd love to hear. Thanks.

#include <stdio.h>

#define STORE_NAME "Sierra Sporting Goods"

int main(void){
int response = 0;


printf("\t%s\n\n", STORE_NAME);
printf("1 = Add a record\n");
printf("2 = Report\n");
printf("3 = Delete a record\n");
printf("4 = Change a record\n");
printf("5 = Quit\n");

while(response != 5 ){
scanf("%d", &response);
printf("\nplease enter 5 when done\n");
}

return 0;
}

[Dino]

Works for me. Perhaps you are executing a version of your program that is not this one.

[Salem]

scanf returns a result.
Use it to decide on your level of success at reading an integer.

[matsp]

scanf returns a result.
Use it to decide on your level of success at reading an integer.

And, if it fails to read what you expected, you need to "clean up" the input, which is described in the FAQ.

--
Mats

[Dino]

Oops - the character I was entering was 5!

Dino says ditto.

[dwks]

Something like this should work:

int number;

printf("Enter a number: ");
while(scanf("%d", &number) != 1) {
    printf("Invalid number, please try again: ");
    while(getchar() != '\n') {}
}

printf("Number: %d\n", number);

That doesn't handle EOF properly, though.

[Dino]

Here's my verbose solution.

#include <stdio.h>

#define STORE_NAME "Sierra Sporting Goods"

void add_report() { 
	printf("Report added\n") ; 
	return ; 
} 

void report() { 
	printf("Report run\n") ; 
	return ; 
} 

void delete_record() { 
	printf("record deleted\n") ; 
	return ; 
} 

void change_record() { 
	printf("record changed\n") ; 
	return ; 
} 


int main(void) {
	int response = 0;
	int chars_read ; 

	printf("\t&#37;s\n\n", STORE_NAME);
	printf("1 = Add a record\n");
	printf("2 = Report\n");
	printf("3 = Delete a record\n");
	printf("4 = Change a record\n");
	printf("5 = Quit\n");

	while ( response != 5 ) {
		chars_read = scanf("%d", &response);
			
		switch (chars_read) {    // outer switch 
			case EOF:
				return -1 ; 
			case 0: 
				while (getchar() != '\n') ;  /* clear buffer */ 
				break ; 
			case 1: 
				switch (response) {    // inner switch 
					case 1: 
						add_report() ; 
						break ; 
					case 2: 
						report() ; 
						break ; 
					case 3: 
						delete_record() ; 
						break ; 
					case 4: 
						change_record() ; 
						break ; 
					case 5:
						break ;   // allow the while loop to exit 
					default: 
						printf("Invalid option %d\n", response) ; 
						break ; 
				}  // inner switch  
				continue ;  // after switch / case 1, 2, 3 or 4 
			default: 
				printf("chars_read = %d, Should not occur\n", chars_read) ; 
				return -1 ; 
		}   // outer switch
			
	} // while response != 5 
	return 0;
}

[dwks]

No, you have to compare getchar() against EOF as well.

Here's a loop that does pretty much the same thing as yours.

for(;;) {
    void (*function[])(void) = {
        add_report,
        report,
        delete_record,
        change_record
    }

    while(scanf("%d", &response) != 1) {
        int c;
        while((c = getchar()) != '\n' && c != EOF);
        if(c == EOF) break;
        printf("Invalid number, try again: ");
    }

    if(response >= 0 && response < sizeof(function) / sizeof(*function)) {
        (function[x])();
    }
    else if(response == 5) break;
    else printf("Enter a number from 0 to 5\n");
}

Use a switch if you don't like arrays of function pointers . . . .

[Dino]

dwks, nice concise solution. I like the function pointers. I'm going to follow your lead and come up with a more succinct solution myself. Stay tuned.

Todd

[dwks]

If you wanted to be really fancy, you could use something like this.

struct {
    const char *description;
    void (*function)();
} item[] = {
    {"Add a record", add_record},
    /* ... */
};

Then you could print the descriptions like this:

for(size_t x = 0; x < sizeof(item) / sizeof(*item); x ++) {
    printf("&#37;d = %s\n", (int)x, item[x].description);
}

[Dino]

That's exactly what I'm working on - still working on the implementation...

{ 
{ 1, "Add a Report",	add_report	} , 
{ 2, "Report",			report	} , 
{ 3, "Delete a record", delete_record	} , 
{ 4, "Change a record", change_record	} , 
{ 5, "Quit",			quit	}  
}

[dwks]

But just think -- if you use the index in the structure/whatever as the menu item number, then menu items are independent from the menu. The numbers are assigned automatically. You can add an item just by adding a line like this.

{"Delete a report", delete_report},

I think it's probably just overkill. I can do it in codeform if I want to, but we're trying to help out someone who's a bit alien to C here.

Nextstopearth

[Nextstopearth]

Hey thanks for the replies everyone, that's really awesome.

So help me to understand this-

When scanf() fails to read the specified form of input, it leaves the nonconforming input in place to be read the next time. (<-- straight out of "c primer plus"). So the next time it trys to read it will start with the offending input again. Is there a way to get scanf() to discard that character instead of placing it back? Is that what matsp was refering to as "cleaning up the input"?

Thanks alot guys, just trying to find out exactly what's going on under the hood here.

[Dino]

Hey thanks for the replies everyone, that's really awesome.

So help me to understand this-

When scanf() fails to read the specified form of input, it leaves the nonconforming input in place to be read the next time. (<-- straight out of "c primer plus").

Yes.

So the next time it trys to read it will start with the offending input again.

Yes.

Is there a way to get scanf() to discard that character instead of placing it back?

scanf()? No.

Is that what matsp was refering to as "cleaning up the input"?

Yes, but you can't via scanf(). See the getchar() code above for how to do that.

[Dino]

Here's my succinct version. Thanks for the tips dwks.

#include <stdio.h>

#define STORE_NAME "Sierra Sporting Goods"

void add_report() { 
	printf("Report added\n") ; 
	return ; 
} 

void report() { 
	printf("Report run\n") ; 
	return ; 
} 

void delete_record() { 
	printf("record deleted\n") ; 
	return ; 
} 

void change_record() { 
	printf("record changed\n") ; 
	return ; 
} 

int scanf_ok(int chars_read, int max, int * choice ) { 
	if (chars_read == EOF || chars_read == 0 ) return 0 ; 
	if (*choice < 1 || *choice > max) return 0 ; 
	return 1 ; 
} 


int main(void) {
	int response = 0;
	int chars_read ; 
	int i ; 
	struct {
		const char *text ;
		void (*function)() ;
	} item[] = {
		{ "Add a Report"   ,add_report		} , 
		{ "Report"         ,report		} , 
		{ "Delete a record",delete_record	} , 
		{ "Change a record",change_record	} , 
		{ "Quit"           ,0			}  
	} ;
			
	while ( 1 ) {

		// Present the menu of options.  
		printf("\t%s\n\n", STORE_NAME) ;
		for (i = 0 ; i < ( sizeof(item) / sizeof(*item))  ; i++) { 
			printf("%d = %s\n", i+1, item[i].text ) ; 
		}

		// Read the response and validate it and process it 
		if ( scanf_ok( scanf("%d", &response), sizeof(item) / sizeof(*item) , &response ) ) {
			if (item[response-1].function == 0) break ; 
			else item[response-1].function() ; 
		}
		// else, tell user to try again.
		else {
			printf("Invalid or Non numeric value entered.  Try again.\n") ; 
			while ( ( chars_read=getchar() ) != '\n' && chars_read != EOF) ;  /* clear buffer */ 
		}

	} // while 1  
	return 0;
}

Any tips for improvement?