CODEWITHSUNDEEP
ctype-conversion
Mohamed Yacout
Mohamed Yacout

Reputation: 133

String to double saving time conversion in c

I'm working on a benchmarking task for an operation (in C language under Linux) and in this operation we use a conversion from string to double a lot (using atof or sscanf functions) but the problem here is that they gave a relatively large time (145, 270 ns respectively) which is not suitable for the operation. So, Do you know any other conversion mechanism that save time?

I tried to use casting but it gave zero

double d;

char ch[] = "123.154";

d = *((double *) ch);

printf ("%lf\n", d);         // result 0 (compiled using gcc)

Regards

Upvotes: 1

Views: 178

Answers (2)

Aki Suihkonen
Aki Suihkonen

Reputation: 20037

atof() and sscanf() are generic methods that accept a huge variety of formats. If you know that the floating point values follow a certain pattern, you can try to optimize the conversion for the expected format. ie. no need to support (+-), no need to support Inf, Nan, or sci-notation (1.3e43) etc.

One can make a look up table that converts 3 characters at once from strings: table[a*256+b*16+c] = a*100+b*10+c; where one simply concatenates the 4 LSB bits of e.g. string "432"; the hex value of the index would be then 0x432 and the content would be 432.

Casting means changing the interpretation of some binary data. Doubles or floats and integers are not binary compatible (except for the value of (+) 0). However the following cast works to check, if three first characters in a strings are numbers:

 char num[]="123.123";
 if ((*((int*)num) & 0x00f0f0f0) == 0x00303030) // can use faster conversion

 // this means interpreting the pointer to string as a pointer to integer
 // and then referencing the contents of the memory _as_ integer (+ some bitmasking)

Further, if the set of floating points is relatively small, or some particular value is very frequent, one might trade space for speed and opt for a hash table. If hash table has a match, one can compare the strings 4 or 8 bytes in parallel to verify (or skip that part, if the input is known to be valid). One can also combine these techniques by hashing the first 4 characters for an initial guess and continue from that next 4 characters at a time.

Upvotes: 2

Ramy Al Zuhouri
Ramy Al Zuhouri

Reputation: 21986

double d;
char ch[] = "123.154";
d = atof(ch);
printf ("%f\n", d);

Or: 

printf ("%s\n", ch);

Upvotes: 0

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