How do I parse out the fields in a comma separated string using sscanf while supporting empty fields?

问题:

I have a comma separated string which might contain empty fields. For example:

1,2,,4 

Using a basic

sscanf(string,"%[^,],%[^,],%[^,],%[^,],%[^,]", &val1, &val2, &val3, &val4); 

I get all the values prior to the empty field, and unexpected results from the empty field onwards.

When I remove the expression for the empty field from the sscanf(),

sscanf(string,"%[^,],%[^,],,%[^,],%[^,]", &val1, &val2, &val3, &val4); 

everything works out fine.

Since I don't know when I'm going to get an empty field, is there a way to rewrite the expression to handle empty fields elegantly?

回答1:

If you use strtok with the comma as your separator character you'll get a list of strings one or more of which will be null/zero length.

Have a look at my answer here for more information.

回答2:

man sscanf:

[ Matches a nonempty sequence of characters from the specified set of accepted characters;

(emphasis added).

回答3:

This looks like you are currently dealing with CSV values. If you need to extend it to handle quoted strings (so that fields can contain commas, for example), you will find that the scanf-family can't handle all the complexities of the format. Thus, you will need to use code specifically designed to handle (your variant of) CSV-format.

You will find a discussion of a set CSV library implementations in 'The Practice of Programming' - in C and C++. No doubt there are many others available.

回答4:

scanf() returns the number of items assigned. Maybe you can use that info ...

char *data = "1, 2,,, 5, 6"; int a[6]; int assigned = sscanf(data, "%d,%d,%d,%d,%d,%d", a, a+1, a+2, a+3, a+4, a+5); if (assigned 

Ugh! And that's only for 1 missing value
As has been pointed out by other answers, you're much better off parsing the string in the 'usual' way: fgets() and strtok().

回答5:

Here is my version to scan comma separated int values. The code detect empty and non-integer fields.

#include   #include    int main(){   char str[] = " 1 , 2 x, , 4 ";   printf("str: '%s'\n", str );    for( char *s2 = str; s2; ){     while( *s2 == ' ' || *s2 == '\t' ) s2++;     char *s1 = strsep( &s2, "," );     if( !*s1 ){       printf("val: (empty)\n" );     }     else{       int val;       char ch;       int ret = sscanf( s1, " %i %c", &val, &ch );       if( ret != 1 ){         printf("val: (syntax error)\n" );       }       else{         printf("val: %i\n", val );       }     }   }    return 0; } 

Result:

str: ' 1 , 2 x, , 4 ' val: 1 val: (syntax error) val: (empty) val: 4 

回答6:

Put a '*' after the '%' to skip reading. In addition it is possible to read only 3 characters noting '%3s' for example.

回答7:

I arrived here looking for answers to the same question. I didn't want to leave behind the scanf funcion either. In the end, I build a zsscanf myself, where I parsed the format, sscanf'ed every data one by one and checked the return of sscanf to see if I got an empty read in any. This was somewhat my particular case: I wanted only some of the fields, some of which could be empty, and could not assume the separator.

#include  #include   int zsscanf(char *data, char *format, ...) {     va_list argp;     va_start(argp, format);     int fptr = 0, sptr = 0, iptr = 0, isptr = 0, ok, saved = 0;     char def[32];     while (1)     {         if (format[fptr] != '%')         {             ok = sscanf(&format[fptr], "%28[^%]%n", def, &iptr);             if (!ok) break;             fptr += iptr;             def[iptr] = '%';             def[iptr+1] = 'n';             def[iptr+2] = 0;             ok = sscanf(&data[sptr], def, &isptr);             if (!ok) break;             sptr += isptr;         }         else             if (format[fptr+1] == '%')             {                 if (data[sptr] == '%')                 {                     fptr += 2;                     sptr += 1;                 }                 else                 {                     ok = -1;                     break;                 }             }             else             {                 void *savehere = NULL;                 ok = sscanf(&format[fptr], "%%%28[^%]%n", &def[1], &iptr);                 if (!ok) break;                 fptr += iptr;                 def[0] = '%';                 def[iptr] = '%';                 def[iptr+1] = 'n';                 def[iptr+2] = 0;                 isptr = 0;                 if (def[1] != '*')                 {                     savehere = va_arg(argp, void*);                     ok = sscanf(&data[sptr], def, savehere, &isptr);                     if (ok == 0 && isptr == 0)                     {                         // Let's assume only char types. Won't hurt in other cases.                         ((char*)savehere)[0] = 0;                         ok = 1;                     }                     if (ok > 0)                     {                         saved++;                     }                 }                 else                 {                     ok = sscanf(&data[sptr], def, &isptr) == 0;                 }                 if (ok 

Output:

    2: |TVC-CCTV-0002|ELECTRICAL_TopoLine_823|     2: |TVC-CCTV-0000|| 

Be warned, it's not fully tested and has severe limitations (the most obvious ones: accepts only %...s, %...c, %...[...] and requires separators as %...[...]; otherwise I'd really hade to care about the format string, this way I only care about %).

回答8:

I had to modify this code a bit to work properly:

//rm token_pure;gcc -Wall -O3 -o token_pure token_pure.c; ./token_pure  #include  #include   int main () {     char str[] = " 1 , 2 x, , 4 ";     char *s1;     char *s2;     s2=(void*)&str; //this is here to avoid warning of assignment from incompatible pointer type          do {             while( *s2 == ' ' || *s2 == '\t' )  s2++;             s1 = strsep( &s2, "," );             if( !*s1 ){                 printf("val: (empty)\n" );             }             else{                 int val;                 char ch;                 int ret = sscanf( s1, " %i %c", &val, &ch );                 if( ret != 1 ){                     printf("val: (syntax error)\n" );                 }                 else{                     printf("val: %i\n", val );                 }             }         } while (s2!=0 );         return 0;     } 

and the output:

val: 1 val: (syntax error) val: (empty) val: 4 

回答9:

I made a modification for tab delimited TSV files, hopefully it may help:

//rm token_tab;gcc -Wall -O3 -o token_tab token_tab.c; ./token_tab  #include  #include   int main () { //  char str[] = " 1     2 x         text   4 ";     char str[] = " 1\t 2 x\t\t text\t4 ";     char *s1;     char *s2;     s2=(void*)&str; //this is here to avoid warning of assignment from incompatible pointer type          do {             while( *s2 == ' ')  s2++;             s1 = strsep( &s2, "\t" );             if( !*s1 ){                 printf("val: (empty)\n" );             }             else{                 int val;                 char ch;                 int ret = sscanf( s1, " %i %c", &val, &ch );                 if( ret != 1 ){                     printf("val: (syntax error or string)=%s\n", s1 );                 }                 else{                     printf("val: %i\n", val );                 }             }         } while (s2!=0 );         return 0;     } 

And the ouput:

val: 1 val: (syntax error or string)=2 x val: (empty) val: (syntax error or string)=text val: 4 

回答10:

There are some problems with strtok() listed here: http://benpfaff.org/writings/clc/strtok.html

Hence, it is better to avoid strtok.

Now, consider a string containing a empty field as follows:

char myCSVString[101] = "-1.4,2.6,,-0.24,1.26"; // specify input here 

You can use simple function to be able convert String in CSV format to read them to a float Array:

int strCSV2Float(float *strFloatArray , char *myCSVStringing); 

Please find the Usage below:

#include  #include     int strCSV2Float(float *strFloatArray , char *myCSVStringing);    void main()  {      char myCSVString[101] = "-1.4,2.6,,-0.24,1.26"; // specify input here     float floatArr[10]; // specify size here      int totalValues = 0;      printf("myCSVString == %s \n",&myCSVString[0]);      totalValues = strCSV2Float(&floatArr[0] , &myCSVString[0]); // call the function here       int floatValueCount = 0;      for (floatValueCount = 0 ; floatValueCount  0 )         {           int aIter =0;           wordLength = (wordEndChar - wordStartChar);           char word[55] = "";           for (aIter = 0;  aIter 

Output is as follows :

myCSVString == -1.4,2.6,,-0.24,1.26  floatArr[0] = -1.400000 floatArr[1] = 2.600000 floatArr[2] = 0.000000 floatArr[3] = -0.240000 floatArr[4] = 1.260000 

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