Second instance of scnaf not working

[raseelbhagat]

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 ?

 

[raseelbhagat]

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]

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]

On Aug 9, 3:44 pm, "(e-mail address removed)" <[email protected]>
wrote:

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 ...)

 

[CBFalconer]

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.