How to read string separated by / with scanf

问题:

I`ve been trying to this for quite a while now and after some research I had no success, so my last resort was asking a question. My input looks like this:

1.0.0.0/255.0.0.0/127.0.0.1/1112 
1.2.0.0/255.255.0.0/2.4.6.9/1112
1.2.3.0/255.255.255.0/1.2.3.1/111

I need to extract 4 strings from each line, so for exemple the first line would give me

s1 = 1.0.0.0
s2 = 255.0.0.0
s3 = 127.0.0.1
s4 = 1112

Here is what I have tried:

scanf("%s/%s/%s/%s", str1, str2, str3, str4); // This doesn't consume the "/"
scanf("%[^/]s%[^/]s%[^/]s%[^/]s", str1, str2, str3, str4); // This only gets the first string
scanf(""%[^\n]s%*c%s%*c%s%*c%s", str1, str2, str3, str4); // Hera %*c was supposed to consume the "/" and do not store it, it doen't happen tho

How can I get the 4 strings from each input line using a single scanf inside a while (!feof(fileIn)) ? Thank you.

回答1:

There are a few issues with the posted code. The scanset directive is %[]; there is no s in this. The format strings using %[^/]s are attempting to match a literal s in the input. But this will always fail because %[^/] matches any character except for /. When a / is encountered, the match fails and the / character is left in the input stream. It is this character which must be consumed before continuing on to the next input field.

Also, note that while(!feof(file)){} is always wrong. Instead, try fetching input by lines using fgets(), and parsing with sscanf(). The fgets() function returns a null pointer when end-of-file is reached.

Further, you should always specify a maximum width when reading strings with scanf() family functions to avoid buffer overflow.

Here is an example program:

#include <stdio.h>

int main(void)
{
    char input[4096];
    char str1[100];
    char str2[100];
    char str3[100];
    char str4[100];

    while (fgets(input, sizeof input, stdin)) {
        sscanf(input, " %99[^/]/ %99[^/]/ %99[^/]/ %99[^/]",
               str1, str2, str3, str4);

        puts(str1);
        puts(str2);
        puts(str3);
        puts(str4);
    }

    return 0;
}

Sample interaction using sample input from the question:

λ> ./a.out < readstring_test.txt 
1.0.0.0
255.0.0.0
127.0.0.1
1112 

1.2.0.0
255.255.0.0
2.4.6.9
1112

1.2.3.0
255.255.255.0
1.2.3.1
111

回答2:

You already got quite close: you missed to consume the delimiter in your second approach:

scanf("%[^/]/%[^/]/%[^/]/%[^/]", str1, str2, str3, str4);

should do the job.