How to read empty string in C

I 开发者_开发百科have a problem with reading empty string in C. I want to read string from the following -

1. ass
2. ball
3. (empty)
4. cat

but when I use gets() it does not treat (empty) as string[2]. It reads 'cat' as string[2]. So how can I solve this problem?

char str1[15002][12];
char str2[15002][12];
char s[25];
map<string,int> Map;

int main()
{
    int ncase, i, j, n1, n2, count, Case;

    freopen("input.txt","r",stdin);
    freopen("output.txt","w",stdout);

    scanf("%d",&ncase);

    Case = 1;
    while(ncase > 0)
    {
        Map.clear();

        //this is the necessery part
        scanf("%d %d\n",&n1,&n2);
        count = 0;

        printf("n1=%d n2=%d\n",n1,n2);
        for(i = 0; i < n1; i++)
        {
          gets(str1[i]);
        }

        for(i = 0; i < n2; i++)
        {
            gets(str2[i]);
        }

        //end of reading input

        for(i = 0; i < n1; i++)
        {
            for(j = 0; j < n2; j++)
            {
                strcpy(s,str1[i]);
                strcat(s,str2[j]);

                if(Map[s] == 0){
                    count += 1;
                    Map[s] = 1;
                }
            }
        }

        printf("Case %d: %d\n", Case, count);
        Case++;
        ncase--;
    }
    return 0;
}

and input can look like

I have given the code here. The input may be like

line1>1
line2>3 3
line3>(empty line)
line4>a
line5>b
line6>c
line7>(empty)
line8>b

And I expect

str1[0]=(empty).
str1[1]=a;
str1[2]=b;

and

str2[0]=c;
str2[1]=(empty);
str2[2]=b;

OK, at last I found the problem. It is the line

printf("n1=%d n2=%d\n",n1,n2);

which creates problem in taking input by gets(). Instead of taking newline with the integer n1, n2, then I take newline as a ("%c",&ch) and then everything is okay.

Thanks to everyone who answered me.

Chances are, the string contains \r\n\0 (or \n\r\0 - never remember which comes first). \r\n is newline on Windows and \0 is the terminating character of the string.

In general, if the first character of the string is \r or\n, you read an empty string. FWIW this should work on all platforms:

char* string;
// initialize string and read something into it
if (strlen(string) == 0 || string[0] == `\r` || string[0] == `\n`)
  // string is empty

Update: you mention that you use gets, and read from a file. However, for the latter you need fgets, so there is some confusion here. Note that fgets includes the trailing newline character in the string returned, while gets does not.

Update3: The way you read from the file is indeed fishy. You reopen the standard input to read from the file - why??? The standard practice is to fopen the file, then read from it with fscanf and fgets.

Update2: stupid us (and clever @Salil :-). You say

Since C arrays are indexed from 0, string[3] contains the 4th line read! The third line is stored in string[2] - I bet that will contain the empty string you are looking for.

Output of this code:

#include <cstdio>

int main ()
{
    int i = 0;

    char string [256];
    while (gets(string)) {
        ++i;
    }
    printf("%d\n", i);

    return 0;
}

For this input

a
b

d

Is

4

Which means, gets() reads all lines correctly, which in turn means your code must be screwed up. Post it here.

First and foremost, do not use gets!!!!! It is a buffer overflow vulnerability, since you cannot specify the size of the destination buffer, and so gets() can easily overrun your buffer. Instead, use fgets() or getchar().

Since you are using map<string,int>, it is clear that you are actually using C++ code. In that case, an even better approach is to use the C++ iostreams libraries for your input and output.

Now that I've done with my rant, the problem is this... gets -- which, again, you should never ever use -- according to the spec, will read up until a newline, and "any <newline> shall be discarded". The function fgets() will copy the newline into the destination buffer, giving you the desired behavior.

If there is no string, how do you expect to read it?

Please give us a piece of code :)

==Later edit ==

OK:

"gets() reads a line from stdin into the buffer pointed to by s until either a terminating newline or EOF, which it replaces with '\0'. "

So basically, if you have:

char x[3];

gets(x);

Then this function will fill in x[0] with '\0'

If you read the manpage you'll see that gets is not recommended. Use fgets instead