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:

0x0D 0x0A separate strings.

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

Define 'string array' in greater detail. That phrase could mean multiple things. — john
– john, Commented Feb 23, 2021 at 13:37
The fastest way would probably be to create an array of pointers to your existing char array. — john
– john, Commented Feb 23, 2021 at 13:38
using std::fstream and std::getline would do all this in a couple of lines of code rather than using c functions. — Alan Birtles
– Alan Birtles, Commented Feb 23, 2021 at 13:40
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? — KamilCuk
– KamilCuk, Commented Feb 23, 2021 at 13:55
@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 — Deim
– Deim, Commented Feb 23, 2021 at 14:12

john · Accepted Answer · 2021-02-23 15:23:51Z

4

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.

answered Feb 23, 2021 at 15:23

john

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Alan Birtles · Accepted Answer · 2021-02-23 13:59:54Z

3

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;
}

edited Feb 23, 2021 at 13:59

answered Feb 23, 2021 at 13:45

Alan Birtles

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3 Comments

Deim Over a year ago

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 Over a year ago

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 Over a year ago

Up to 50Gb. I planed to read big files by parts (~100Mb each part)

KeyC0de · Accepted Answer · 2021-02-23 13:48:14Z

0

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.

answered Feb 23, 2021 at 13:48

KeyC0de

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