Reputation: 10499
I need to parse several C-style strings (around 500k) containing 4 floating point numbers separated by a single space character. Following is an example of a single string:
"90292 5879 89042.2576 5879"
I need to store these numbers in two structures representing two points. Considering that the string can be modified while parsed, and that 99.99% of the times the numbers are just unsigned integers, what's the fastest way to do it?
Following is my current implementation:
#include <iostream>
#include <cassert>
#include <chrono>
#include <algorithm>
#include <vector>
#include <string>
using namespace std;
using namespace chrono;
struct PointF
{
float x;
float y;
};
void parse_points(char* points, PointF& p1, PointF& p2)
{
auto start = points;
const auto end = start + strlen(points);
// p1.x
start = std::find(start, end, ' ');
assert(start < end);
*start = '\0';
p1.x = static_cast<float>(atof(points));
points = start + 1;
// p1.y
start = std::find(start, end, ' ');
assert(start < end);
*start = '\0';
p1.y = static_cast<float>(atof(points));
points = start + 1;
// p2.x
start = std::find(start, end, ' ');
assert(start < end);
*start = '\0';
p2.x = static_cast<float>(atof(points));
points = start + 1;
// p2.y
start = std::find(start, end, ' ');
assert(start == end);
p2.y = static_cast<float>(atof(points));
}
int main()
{
const auto n = 500000;
char points_str[] = "90292 5879 89042.2576 5879";
PointF p1, p2;
vector<string> data(n);
for (auto& s : data)
s.assign(points_str);
const auto t0 = system_clock::now();
for (auto i = 0; i < n; i++)
parse_points(const_cast<char*>(data[i].c_str()), p1, p2);
const auto t1 = system_clock::now();
const auto elapsed = duration_cast<milliseconds>(t1 - t0).count();
cout << "Elapsed: " << elapsed << " ms" << endl;
cin.get();
return 0;
}
Upvotes: 1
Views: 367
Reputation: 1357
Here is my version without strlen
but with use of strtok_s
.
On my machine it takes 1.1sec
instead of 1.5sec
.
void parse_points(char* points, PointF& p1, PointF& p2)
{
char *next_token1 = nullptr;
// p1.x
points = strtok_s(points, " ", &next_token1);
p1.x = points ? static_cast<float>(atof(points)) : 0.0f;
// p1.y
points = strtok_s(nullptr, " ", &next_token1);
p1.y = points ? static_cast<float>(atof(points)) : 0.0f;
// p2.x
points = strtok_s(nullptr, " ", &next_token1);
p2.x = points ? static_cast<float>(atof(points)) : 0.0f;
// p2.y
points = strtok_s(nullptr, " ", &next_token1);
p2.y = points ? static_cast<float>(atof(points)) : 0.0f;
}
int main()
{
const auto n = 500000;
char points_str[] = "90292 5879 89042.2576 5879";
PointF p1, p2;
vector<string> data(n);
for (auto& s : data)
s.assign(points_str);
const auto t0 = system_clock::now();
for (auto i = 0; i < n; i++)
parse_points(const_cast<char*>(data[i].c_str()), p1, p2);
const auto t1 = system_clock::now();
const auto elapsed = duration_cast<milliseconds>(t1 - t0).count();
cout << "Elapsed: " << elapsed << " ms" << endl;
//cin.get();
return 0;
}
Upvotes: 0
Reputation: 10880
I see multiple issues with the code (and it's actually good that you've asked):
I'd suggest this: (note that std::experimental::optional<>
is equivalent here to boost::optional<>
)
#include <iostream>
#include <cstring>
#include <utility>
#include <experimental/optional>
struct PointF
{
float x;
float y;
};
std::experimental::optional<std::pair<PointF, PointF>> parse_points(char* pch)
{
pch = strtok (pch, " ");
if (pch != NULL)
{
float x0 = atof(pch);
pch = strtok (NULL, " ");
if (pch != NULL)
{
float y0 = atof(pch);
pch = strtok (NULL, " ");
if (pch != NULL)
{
float x1 = atof(pch);
pch = strtok (NULL, " ");
if (pch != NULL)
{
float y1 = atof(pch);
PointF p0{x0, y0}, p1{x1, y1};
return std::make_pair(p0, p1);
}
}
}
}
return std::experimental::nullopt;
}
int main() {
const char str[] ="90292 5879 89042.2576 5879";
char* pch0 = new char[sizeof(str)], *pch = pch0;
memcpy(pch0, str, sizeof(str));
std::experimental::optional<std::pair<PointF, PointF>> pOpt( parse_points(pch0) );
if(pOpt)
std::cout << pOpt->first.x << " " << pOpt->first.y << " "
<< pOpt->second.x << " " << pOpt->second.y << " " << std::endl;
delete pch;
}
Upvotes: 0
Reputation: 647
You can implement atof
that return the pos of space
. In this way, you need traverse each string only once.
eg.
char *atof(char *point, float &num) {
num = 0;
bool neg = false, dot = false;
float decimal = 0, mul = 0.1;
if (*point == '-') {
neg = true;
point++;
} else if (*point == '+') {
point++;
}
while (*point != ' ' && *point) {
if (*point == '.') {
dot = true;
} else {
if (dot) {
decimal += (*point - '0') * mul;
mul *= 0.1;
} else {
num = num * 10 + *point - '0';
}
}
point++;
}
if (dot) {
num += decimal;
}
if (neg) {
num = -num;
}
return point;
}
Upvotes: -1
Reputation: 57678
Given a string with floating point values, space separated:
const std::string example_input = "90292 5879 89042.2576 5879";
You should profile to see what is faster, reading as floating point:
std::istringstream text_stream(example_input);
std::vector<double> container;
double value;
while (text_stream >> value)
{
container.push_back(value);
}
Or reading as integers, taking a performance hit if there is an indication of floating point:
std::istringstream text_stream(example_input);
std::vector<double> container;
double value;
signed int int_value;
std::streampos position_before_read = text_stream.tellg();
while (text_stream >> int_value)
{
// check the next character for possible floating point differences.
char c;
text_stream >> c;
switch (c)
{
case '.':
case 'E': case 'e':
// Rewind to before the number and read as floating point
text_stream.seekg(position_before_read);
text_stream >> value;
break;
default:
value = 1.0 * int_value;
break;
}
container.push_back(value);
position_before_read = text_stream.tellg();
}
My guess is that the standard libraries are optimized to read floating point, a lot better than the above example and account for all variances of floating point format.
Note: alternatively, you could read the decimals and exponents as integers, if present, then build a floating point value with all three pieces.
Upvotes: 0