abnormal behavior of scanf

Question

I have code as below which scans a matrix, w - width of matrix, h - heigth of matrix.

I am using Visual Studio 2010.

Every time I get a char it increases j by 2 (I put the break point and come to know this behavior).

int w = 0, h = 0;
char map[21][21];
int i,j;

scanf("%d%d", &w, &h);
for(i = 1; i <= h; ++i){
    for(j = 1; j <= w; ++j){
        //fflush(stdin);
        scanf("%c",&map[i][j]);
        //fflush(stdin);
    }
}

What can be the reason behind this?

at the time of scan i am giving value, w = 7, h = 5.

I don't see any error in my code....Please help me.

what input are you using? If I give it something like `7 5abcdefghijklmnopqrstuvwxyzABCDEFGHI` (which is what it appears to expect) it works as expected. Perhaps you are trying to read data in some different format? — Chris Dodd, Oct 12 '13 at 18:15
thanks for fflush(stdin) behavior, I didn't know it. But still facing the issue. — user2322888, Oct 12 '13 at 18:20
@ChrisDodd...No no my format is correct. I am giving input one by one. — user2322888, Oct 12 '13 at 18:23
@P0W: If you run on POSIX-derived systems, `fflush(stdin)` has undefined behaviour. If you run on Windows-derived systems, the behaviour is documented by Microsoft; it discards any data already in the input buffer. — Jonathan Leffler, Oct 12 '13 at 19:24

haccks · Answer 1 · 2013-10-12T19:40:39.773

5

The problem is due to '\n' characters (on pressing Enter ) left behind by scanf.
One way to eat up these newline character is place a ' ' before %c in scanf;

 scanf(" %c",&map[i][j]);  
        ^
        |
      space

Another way is to use a loop to eat up all the \n by getchar()

  int ch;
  while((ch=getchar())!='\n' && ch != EOF );

edited Oct 12 '13 at 19:40

answered Oct 12 '13 at 19:10

haccks

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It will work if the user presses enter several times; the blank in the format eats up all the spaces, tabs, newlines before the next non-blank character is read by `%c`. – Jonathan Leffler Oct 12 '13 at 19:22
@JonathanLeffler; Yes you are right. I thought about reading by `getchar`. That's why I mistaken. Edited now. – haccks Oct 12 '13 at 19:27

score 2 · Answer 2 · answered Oct 12 '13 at 17:53

2

You need to skip trailing newline from previous scanf

Don't use fflush(stdin)

Use:

int c;

while((c=getchar())!='\n' && c != EOF ); //eats newline came from scanf

after scanf call

answered Oct 12 '13 at 17:53

P0W

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score 0 · Answer 3 · answered Sep 27 '17 at 17:49

Here is an answer to vuppala srikar. His question "fscanf issue while reading input from text file [duplicate]" does not appear to me to be an exact duplicate :

In his question, the scanf is done on an opened text file (fptr).

Instead of :

while((fscanf(fptr,"%c %d",&c,&val))==2)
{
    printf("%c %d\n",c,val);
}

which reads only the first line of the file, I suggest :

char line [50] ;
while (fgets( line,sizeof( line ),fptr ))
    if (sscanf(line,"%c %d",&c,&val)==2)
        {
        printf("%c %d\n",c,val);
        }

I hope my answer arrives not too late...

abnormal behavior of scanf

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