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comp.lang.c

Second instance of scnaf not working

raseelbhagat@gmail.com

8/9/2008 12:29:00 PM

Hi,
I am writing a simple program in which I am using scnaf() twice. The
pseudo code of the program is as follows :

....
printf("Enter lesson no.:");
scanf("%d",lesson);
...
fp = fopen("questions.txt","r");
while(fgets(buf, sizeof(buf), fp) {
fputs(buf, stdout);
}
fclose(fp);
printf("Choose answer\n");
scanf("%c", &c);
if (c == 'A')
printf("Right Answer\n");
else
pritnf("Wrong answer\n");
.....

In the above code, if I don't use the first scanf(), everything works
properly.
Otherwise, after entering an int for the first scanf, the program just
"falls through" without waiting for user input for the second scanf()
and printing "Wrong Answer".

Am I missing out on something here ?
5 Answers

raseelbhagat@gmail.com

8/9/2008 12:44:00 PM

0

On Aug 9, 5:28 pm, "raseelbha...@gmail.com" <raseelbha...@gmail.com>
wrote:
> Hi,
> I am writing a simple program in which I am using scnaf() twice. The
> pseudo code of the program is as follows :
>
> ...
> printf("Enter lesson no.:");
> scanf("%d",lesson);
> ..
> fp = fopen("questions.txt","r");
> while(fgets(buf, sizeof(buf), fp) {
>    fputs(buf, stdout);}
>
> fclose(fp);
> printf("Choose answer\n");
> scanf("%c", &c);
> if (c == 'A')
>     printf("Right Answer\n");
> else
>     pritnf("Wrong answer\n");
> ....
>
> In the above code, if I don't use the first scanf(), everything works
> properly.
> Otherwise, after entering an int for the first scanf, the program just
> "falls through" without waiting for user input for the second scanf()
> and printing "Wrong Answer".
>
> Am I missing out on something here ?


An additional progress to my above conundrum is : Substituting "%s"
with "%c" in the second scanf() solves the problem.
But I don't understand, shouldn't "%c" have worked too ?

vippstar

8/9/2008 12:46:00 PM

0

On Aug 9, 3:28 pm, "raseelbha...@gmail.com" <raseelbha...@gmail.com>
wrote:
> Hi,
> I am writing a simple program in which I am using scnaf() twice. The
> pseudo code of the program is as follows :
>
> ...
> printf("Enter lesson no.:");

add fflush(stdout); after printf here.

> scanf("%d",lesson);

change this to %4d and check the return value of scanf
if((rc = scanf("%4d", lesson)) != 1)

by the way, is 'lesson' really a pointer to int, or plain int? Latter
case, change it to &lesson.

> ..
> fp = fopen("questions.txt","r");

if(fp == NULL) /* handle error */

> while(fgets(buf, sizeof(buf), fp) {
> fputs(buf, stdout);
> }
>
> fclose(fp);
> printf("Choose answer\n");
> scanf("%c", &c);

Most probably what you have entered before in the first scanf is this:

42<RET>

The <RET> leaves a newline in the stream. the first scanf sees this
and stops reading the integer, leaving the newline in the stream.
Then the second scanf reads the newline instead of waiting for input.
To fix this, before the second scanf, add these two lines:

scanf("%*[^\n]");
getchar();

<snip rest code and questions>

vippstar

8/9/2008 12:52:00 PM

0

On Aug 9, 3:44 pm, "raseelbha...@gmail.com" <raseelbha...@gmail.com>
wrote:
<snip>

> An additional progress to my above conundrum is : Substituting "%s"
> with "%c" in the second scanf() solves the problem.
> But I don't understand, shouldn't "%c" have worked too ?

See my other reply. Before the second scanf, the stream contains a
newline.
's' ignores all isspace bytes. (\n, ' ', \t ...)

pete

8/9/2008 1:11:00 PM

0

raseelbhagat@gmail.com wrote:
> Hi,

> pseudo code

That's a poor choice to post to the C language newsgroup.

> ...
> printf("Enter lesson no.:");
> scanf("%d",lesson);
> ..
> fp = fopen("questions.txt","r");
> while(fgets(buf, sizeof(buf), fp) {
> fputs(buf, stdout);
> }
> fclose(fp);
> printf("Choose answer\n");
> scanf("%c", &c);
> if (c == 'A')
> printf("Right Answer\n");
> else
> pritnf("Wrong answer\n");
> ....
>
> In the above code, if I don't use the first scanf(), everything works
> properly.
> Otherwise, after entering an int for the first scanf, the program just
> "falls through" without waiting for user input for the second scanf()
> and printing "Wrong Answer".
>
> Am I missing out on something here ?

/* BEGIN new.c output */

Enter lesson no.:7
Choose answer
A
Right Answer

/* END new.c output */



/* BEGIN new.c */

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

int main(void)
{
int lesson[1];
char c;

puts("/* BEGIN new.c output */\n");
printf("Enter lesson no.:");
fflush(stdout);
if (scanf("%d", lesson) != 1) {
puts("Adios Amiga!");
exit(EXIT_FAILURE);
}
getchar();
puts("Choose answer");
if (scanf("%c", &c) != 1) {
puts("Adios Amoeba!");
exit(EXIT_FAILURE);
}
getchar();
if (c == 'A') {
puts("Right Answer");
} else {
puts("Wrong answer");
}
puts("\n/* END new.c output */");
return 0;
}

/* END new.c */


--
pete

CBFalconer

8/9/2008 3:56:00 PM

0

"raseelbhagat@gmail.com" wrote:
>
> I am writing a simple program in which I am using scnaf() twice.
> The pseudo code of the program is as follows :
>
> ...
> printf("Enter lesson no.:");
> scanf("%d",lesson);
> ..
> fp = fopen("questions.txt","r");
> while(fgets(buf, sizeof(buf), fp) {
> fputs(buf, stdout);
> }
.... snip ...
>
> In the above code, if I don't use the first scanf(), everything
> works properly. Otherwise, after entering an int for the first
> scanf, the program just "falls through" without waiting for user
> input for the second scanf() and printing "Wrong Answer".
>
> Am I missing out on something here ?

Yes. First, each scanf should have the result checked. Also, each
should be followed by absorbing the remainder of the input line.
Change the first section to:

printf("Enter lesson no.:"); fflush(stdout);
if (1 != scanf("%d",lesson))
badinputerrorhandle();
else {
flushln(stdin);
(* success, continue *)

and the very useful flushln function is:

/* flush input through the next following '\n' inclusive */
int flushln(FILE *f) {
int ch;

while (('\n' != (ch = getc(f)) && (EOF != ch)) continue;
return ch;
}

This treatment should be used on any interactive use of scanf.

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home.a...
Try the download section.