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I'm parsing a string (a char*) and I'm using sscanf to parse numbers from the string into doubles, like so:
// char* expression; double value = 0; sscanf(expression, "%lf", &value); This works great, but I would then like to continue parsing the string through conventional means. I need to know how many characters have been parsed by sscanf so that I may resume my manual parsing from the new offset.
Obviously, the easiest way would be to somehow calculate the number of characters that sscanf parses, but if there's no simple way to do that, I am open to alternative double parsing options. However, I'm currently using sscanf because it's fast, simple, and readable. Either way, I just need a way to evaluate the double and continue parsing after it.
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The sscanf() function returns the number of fields that were successfully converted and assigned. The return value does not include fields that were read but not assigned. The return value is EOF when the end of the string is encountered before anything is converted.
The sscanf() function reads data from buffer into the locations that are given by argument-list . Each argument must be a pointer to a variable with a type that corresponds to a type specifier in the format-string . The sscanf() function returns the number of fields that were successfully converted and assigned.
The scanf function reads data from standard input stream stdin into the locations given by each entry in the argument list. The argument list, if it exists, follows the format string. The sscanf function reads data from buffer into the locations given by argument list.
If you use fgets() + sscanf() , you must enter both values on the same line, whereas fscanf() on stdin (or equivalently, scanf() ) will read them off different lines if it doesn't find the second value on the first line you entered.
You can use the format specifier %n and provide an additional int * argument to sscanf():
int pos; sscanf(expression, "%lf%n", &value, &pos); Description for format specifier n from the C99 standard:
No input is consumed. The corresponding argument shall be a pointer to signed integer into which is to be written the number of characters read from the input stream so far by this call to the
fscanffunction. Execution of a%ndirective does not increment the assignment count returned at the completion of execution of thefscanffunction. No argument is converted, but one is consumed. If the conversion specification includes an assignment suppressing character or a field width, the behavior is undefined.
Always check the return value of sscanf() to ensure that assignments were made, and subsequent code does not mistakenly process variables whose values were unchanged:
/* Number of assignments made is returned, which in this case must be 1. */ if (1 == sscanf(expression, "%lf%n", &value, &pos)) { /* Use 'value' and 'pos'. */ } 
int i, j, k; char s[20]; if (sscanf(somevar, "%d %19s %d%n", &i, s, &j, &k) != 3) ...something went wrong... The variable k contains the character count up to the point where the end of the integer stored in j was scanned.
Note that the %n is not counted in the successful conversions. You can use %n several times in the format string if you need to.
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