C++ Convert char array to string array

Question

I have next C++ code in VS2019 under Windows 10:

char const* const fileName = "random_StringArray_10000000";
FILE* infile;
long fileSize;
char* buffer;
size_t readBytes; 
infile = fopen(fileName, "rb");
if (infile == NULL)
{
 fputs("File error", stderr); exit(1);
}
fseek(infile, 0, SEEK_END);
fileSize = ftell(infile);
rewind(infile);
buffer = (char*)malloc(sizeof(char) * fileSize);
if (buffer == NULL)
{
 fputs("Memory error", stderr); exit(2);
}
auto start = chrono::steady_clock::now();
readBytes = fread(buffer, 1, fileSize, infile); 
auto end = chrono::steady_clock::now();
if (readBytes != fileSize)
{
 fputs("Reading error", stderr); exit(3);
}
fclose(infile);
free(buffer);
auto elapsed_ms = chrono::duration_cast<chrono::milliseconds>(end - start);
cout << "Elapsed ms: " << elapsed_ms.count() << endl;
cout << "String count: " << stringCount << endl;
system("pause");
return 0;

This method used because it is fastest way to read file from disk under VS2019. Now i need to convert char array to the string array. random_StringArray_10000000 - UTF8 text file. Strings lenght 8 - 120 symbols. Hex view of this file: enter image description here

0x0D 0x0A separate strings.

Which fastest way to convert char array (buffer) to the C++ string array?

• john: Define 'string array' in greater detail. That phrase could mean multiple things.
• john: The fastest way would probably be to create an array of pointers to your existing char array.
• Alan Birtles: using std::fstream and std::getline would do all this in a couple of lines of code rather than using c functions.
• KamilCuk: You sound like you want to split a string on newline characters. it is fastest way to read file Are you sure? Have you measured it?
• Deim: @KamilCuk yes am sure about it. I run difficult configuration tests (ru.stackoverflow.com/questions/1246550/…) and on this way is the fastest for me

Answer 1

There seems to be a regularity to your data, all strings are eight characters long and separated by the same two characters. With that in mind the following seems fairly fast.

size_t arraySize = readBytes/10;
std::string* array = new std::string[arraySize];
for (size_t i = 0; i < arraySize; ++i)
 array[i].assign(buffer + 10*i, 8);

Of course timing is necessary to be sure what is fastest.

Answer 2

Reading lines of text from a file is much simpler if you use the classes from the c++ standard library.

This should be all of the code you need:

#include <fstream>
#include <vector>
#include <string>
#include <iostream>
int main()
{
 char const* const fileName = "random_StringArray_10000000";
 std::ifstream in(fileName);
 if (!in)
 {
 std::cout << "File error\n";
 return 1;
 }
 std::vector<std::string> lines;
 std::string line;
 while (std::getline(in, line))
 {
 lines.push_back(std::move(line));
 }
 return 0;
}

• Deim: getline give me in debug mode only 2-4 Mb/sec and 10 Mb/sec in Release -O2. fread - 240 Mb/sec in debug mode and up to 355 Mb/sec in Release mode
• Alan Birtles: benchmarking in debug is meaningless, how big is your file? is it really that critical to read it faster? In a lot of cases its better to write simple, correct code than prematurely optimise into something complex with hidden bugs
• Deim: Up to 50Gb. I planed to read big files by parts (~100Mb each part)

Answer 3

char as[ ] = "a char array"; : is a char array

char const* const fileName = "random_StringArray_10000000"; : is a c string

This is also a c string:

char* cs = const_cast<char*>( fileName );

If you want std::string use:

std::string s(as);

I wasn't sure which string conversion you wanted so I just added what I could off the top of my head. But here's a compilable example too.