条件异常

Question

i am self taught programmer i face a problem here that if condition to enter only ( H OR T OR . ) doesn't get the string . go to the ( printf ("Enter valid characters please \\n"); )

below is my code :

int i = 0 ;
int L ;

printf ("Please enter the lenght of report \n"); 
scanf ("%d" , &L);

if ( L>=1 && L<=500 )
{
        printf ("Please enter the Reoprt \n");
        string P = get_string ();
        if (P[i] !='H' && P[i] != 'T' && P[i] != '.')
        {
                printf ("Enter valid characters please  \n");
        }
        else 
        {
                printf ("GOOD3 \n");
        }
}
else 
{
        printf ("Please enter valid Length \n"); 
}

Answer 1

The reason you're having this problem is because scanf("%d", &L) will fetch one or more digits from the input and nothing else . As a result, it will leave behind the newline character that you entered after typing in your number.

When you call get_string() (I assume this is the same as GetString() in the CS50 library?), the first character it sees is this residual newline character, so all you're getting back is an empty string.

You can fix this easily enough. Just replace scanf ("%d" , &L); with L = GetInt(); .

Or, alternatively, replace scanf ("%d" , &L); with scanf ("%d%c" , &L, &newline); , with newline declared as a char variable at the top of your main() function. This will consume the line break that follows the number, so that GetString() won't treat it as an empty string. For more details on how scanf() works, type man scanf at the command line.