提高效率-从字符串C ++解析整数

Question

I have a string:

12:56:72

I need to get the 3 numbers ( 12 , 56 , and 72 ) individually.

I am doing:

int i=0,j=0,k=0,n, p[3]; // p[i] stores each ith value
char x[], b[];
x="12:34:56";
n=strlen(x);
while(i<strlen){
        b[k]=x[i];
        ++i;
        ++k;
        if(x[i==]':'){
            p[j]=int(b);
            ++j;
            ++i;
            k=0;
            char temp[] = b;
            b=new char[];
            delete temp;
            }
        }

Can this be done more efficiently?

Answer 1

To be "more efficient", you will have to profile.

Here is another solution:

const std::string test_data("01:23:45");
unsigned int hours;
unsigned int minutes;
unsigned int seconds;
char separator;
std::istringstream input(test_data);
// Here's the parsing part
input >> hours >> separator >> minutes >> separator >> seconds;

Whether this is "more efficient" or not, must be measured.
It looks simpler and safer.

Edit 1: Method 2
Processors don't like loops or branches, so we can try to minimize. This optimization assumes perfect input as a string.

static const char test_data[] = "01:23:45";
unsigned int hours;
unsigned int minutes;
unsigned int seconds;
char c;
unsigned int index = 0;
hours = test_data[index++] - '0';
if (test_data[index] != ':')
{
  hours = hours * 10 + test_data[index++] - '0';
}
++index; // Skip ':'
minutes = test_data[index++] - '0';
if (test_data[index] != ':')
{
  minutes = minutes * 10 + test_data[index++] - '0';
}
++index; // Skip ':'
seconds = test_data[index++] - '0';
if (test_data[index] != ':')
{
  seconds = seconds * 10 + test_data[index++] - '0';
}

For highest optimizations, you have to make some assumptions. Another assumption is that the character encoding is UTF8 or ASCII, eg '1' - '0' == 1.

Answer 2

Since you are reading the data in from a file stream you can use the operator >> of that stream to your advantage. When it inputs data into a int it will keep reading the file until it hits a character that would be invalid for an int . This meas it will read the integer part and leave the colon in the file. Because of this behavior we can use

ifstream fin("filename.ext");
int a, b, c;
char colon;
while(fin >> a >> colon >> b >> colon >> c)
{
    // do stuff with a, b and c
}

And this will read file line by line as long as the file has lines in the format of 12:56:72 .

Answer 3

It looks like a clock, but still, you can scan strings with sscanf:

#include <iostream>

using namespace std;

int main()
{
   char myclock[] = "10:11:12";
   int hours, minutes, seconds;
   sscanf(myclock, "%d:%d:%d", &hours, &minutes, &seconds);
   cout << hours << endl;
   cout << minutes << endl;
   cout << seconds << endl;
   return 0;
}

Answer 4

You could consider using C++ functions.

#include <string>
#include  <iostream>
using namespace std;

int main() {
    size_t i=0,j=0,k=0, p[3]; // p[i] stores each ith value
    string x("12:34:56");
    for (j=0; j<3; j++) {
        p[j] = stoi(x.substr(k), &i);
        k += i+1;
    }

    for (j =0; j < 3; j++)
        cout << p[j] << endl;
}