How to copy multi-dimensional char array into char *

Question

Hi I'm looking for copying multidimensional char array into char *string I'm able to copy multidimensional char array into std::string result.

string result;

for (int i=0; i < key; i++)
    for (int j=0; j < text.length(); j++)
        if (rail[i][j]!='\n')
            result.push_back(rail[i][j]);`

Same I want to copy the 2D char array data into char *string

It is unclear what you are asking. Please show us the declaration of your variables. — Paul Ogilvie, Aug 30 '18 at 09:01
1) Please provide [mcve]. 2) What is wrong with your current attempt? — Algirdas Preidžius, Aug 30 '18 at 09:03
@paul This is old implementation , in this I'm copying Array data into `std::string result` Now I want copy the same Array into char *string — stack_, Aug 30 '18 at 09:04
the syntax of question is very confusing. Also I wonder how this would work on platform where eol isn't a single character... — Swift - Friday Pie, Aug 30 '18 at 09:06
@Algirdas The existing code doens't have any problem , I just want to copy the same `Array[i][j]` into ` char *string` — stack_, Aug 30 '18 at 09:07
Possible duplicate of [How to convert string to char array in C++?](https://stackoverflow.com/questions/13294067/how-to-convert-string-to-char-array-in-c) — moooeeeep, Aug 30 '18 at 09:09
@stack_ 1) If the code doesn't have any problems, then what the question is about? 2) Why doesn't the `result.c_str ()` suit your needs, if you need to get `char*` from `std::string`? — Algirdas Preidžius, Aug 30 '18 at 09:12
@Algirdas Thank You ! I will try for the same.I was asking because to avoid conversions . — stack_, Aug 30 '18 at 09:16
@stack_ "_I was asking because to avoid conversions._" What conversions are you talking about? — Algirdas Preidžius, Aug 30 '18 at 09:18
@Algirdas , At the end I want data into char* so, Directly i will take data into char* instead of string. — stack_, Aug 30 '18 at 09:29

score 0 · Answer 1 · answered Aug 30 '18 at 09:04

0

you can get the char* string from the C++ string object. There is a function c_str() which returns the C string from the C++ string.

answered Aug 30 '18 at 09:04

vinodsaluja

339
2
14

Martin Cook · Answer 2 · 2018-08-30T09:27:06.120

If a char *string means a c-style string (i.e. an array of char characters), then you need to keep track of the the latest position (index in the code below), and set each character in turn. This is if you want to avoid copying. Otherwise, construct the string first, then use c_str(), as other answers have stated.

#include <vector>
#include <iostream>
using std::vector;

#include <string>
using std::string;

int main() {
    const int key = 2;
    string text = "ab";
    string rail[2] = {text, text};

    char result[key * 100];
    int index = 0;
    for (int i=0; i < key; i++)
        for (int j=0; j < text.length(); j++)
            if (rail[i][j]!='\n')
                result[index++] = rail[i][j];

    for (int i = 0; i < index; i++) {
        std::cout << result[i] << " " << std::endl;
    }
}

score 0 · Accepted Answer · answered Aug 30 '18 at 09:20

Let assume that the actions are done consequently, i.e. first you already done this:

std::string result;
for (int i=0; i < 2; i++)
    for (int j=0; j < len; j++)
            result.push_back(Array[i][j]);

then you have already a char* array stored inside std::string.

char  *string = new char[result.size()];
string = strdup(test1.c_str());

Now if you skip the std::string step (why, platform doesn't support it?), you rather get something like that:

char  *string = new char[string_size];   // you have to either measure it beforehand or calculate from array size

int counter = 0;
for (int i=0; i < 2; i++)
    for (int j=0; j < len; j++)
            string[counter++] = Array[i][j];

If that's C code, replace new expression with malloc call. And this code got flaws like you either should allocate more memory than needed or you should count length of buffer before allocating it. Extreme case is to reallocate buffer when its capacity reached. std::string does that automatically.

I don't see why this can be a problem to devise, unless you're failing to tell us something important about what you need.

In general C++ code never looks like that unless you're designing your own containers. On ISO-compatible platform you'll have std::string, on most embedded systems, e.g. Arduino, you'll have access to String class.

How to copy multi-dimensional char array into char *

3 Answers3