sscanf() 如何拆分 C 中的字符串

Question

So basically I have a simple string and i am trying to use sscanf() to split this string and store the values in appropriate variables:

#include<stdio.h>
int main()
{

    int i=10,j;
    char ch = 'A',ch2;
    float a=3.14,b;
    char str[20]="10A3.140000";


    sscanf(str,"%d%c%f",&j,&ch2,&b);
    printf("%d %c %f\n",j,ch2,b);
    return 0;
}

Now my doubt is how exactly sscanf knows where to split the string. Here the value 10 goes in variable j . Character A goes in variable ch2 and 3.140000 goes in variable b . How exactly sscanf is parsing this string and storing values in different variables. It is appreciable if you can explain how exactly sscanf works with any string. I am having a hard time understanding it.

Answer 1

scanf consumes the input string according to the format specifiers you defined in the format string.

Let's analyze your format string:

sscanf(str,"%d%c%f",&j,&ch2,&b);

• %d parses the string searching for decimal integers . It means that only numeric characters in the range 0-9 are accepted. Furthermore a leading sign character is accepted (like in number -123 ). All characters meeting this criteria are consumed
• %c scans any character, saving its ASCII value in destination variable. A single character is consumed
• %f parses the string searching for floats . Like the %d case all decimal numbers and sign characters are accepted, but it also accepts the . character interpreting it as the floating point

So, let's look again at you input string:

char str[20]="10A3.140000";

1. scanf starts parsing from the first char. Since there are decimal numbers it consumes them until the first non-numeric character is found ( 'A' ). The integer 10 is stored in j .

   If the string was finished here, scanf would have returned 1.

2. scanf continues parsing from the letter 'A' . Since we have %c format specifier this character is consumed and the value 65 (ASCII for 'A' ) is stored in ch2 .

   If the string was finished here, scanf would have returned 2.

3. After the letter 'A' is consumed scanf continues parsing from character '3' . The %f format specifier will accept all the remaining characters (because they are all valid), and the value 3.14 is stored within variable b .

   scanf now returns 3.

---

I tried to empathize the value returned by scanf because it is checking it that you can have some control on what's going on. In your case

if( sscanf(str,"%d%c%f",&j,&ch2,&b) == 3 )
{
    printf("%d %c %f\n",j,ch2,b);
}

would prevent those scenarios in which a malformed string would have lead to undefined behavior. To understand this scenario, consider the following string:

char str[20]="10AB3.140000";

we have two non numeric characters after the first integer. Then:

• %d would still consume the value 10
• %c would still consume the letter 'A'
• but %f would not consume the following string, because the substring "B3.140000" can't be parsed as a float : that leading 'B' forces scanf to interrupt parsing and the value 2 would be returned.

So, without a check on the return value, the uninitialized variable b would be accessed.

Answer 2

This explanation is little over-simplified and there is more than this going on inside the sscanf as the source code is itself more than 3,000 lines of code.

The format specifier mentioned by you in the example is listed below with their corresponding match pattern
%d : [0-9] , [-,+]
%f : [0-9] , [-,+,.]
%c : [az] , [AZ] , Other ASCII Characters

Parsing

1. First, for %d , it will first search for [+,-] signs, and if not found then it will look for [0-9] single digits in sequence. As soon as it sees a non-single digit (A = 65) in the string, it will end its search for %d and save the value of the int to the corresponding address passed to sscanf .
2. Second, for %c , now it will take the next format specifier passed ie %c and begin again from the location where it previously stopped. This time it just has to read a single byte from the string and save it to the corresponding address passed to sscanf
3. Third, for %f , it will first search for [+,-] signs, and if not found it will then look for . and the single digits [0-9] . This continues until it gets something which is not part of the %f , which in this case will be \0 . And save it to the corresponding address passed to sscanf

Finally, once everything is completed and the string is split, it returns the total number of arguments parsed. So, it a good programming practice to compare the number of the passed argument list with the returned value.

Reference:

1. sscanf source code
2. vfscanf source code