将2D数组和字符串向量作为参数传递给函数

Question

I have written two function in which I am passing vector of string to a particular function (PrintStringVector) just to print content and in second function, passing the array of pointers to print the content.The first function is working fine but second one is giving error which is below the my code.

#include <cmath>
#include <stdlib.h>
#include <cstdio>
#include <string>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
int n;

void PrintStringVector(vector<string> & v){


    for(int i=0;i<v.size();i++){ cout<<v[i]<<endl;}

}


void PrintStringArray(const char *arr[n]){

    for(int i=0;i<n;i++){ cout<<arr[i]<<endl;}



}

int main() {

    vector<string>  vec;                    
    cin>>n;
    const char *arr[n];

    for(int i=0;i<n;i++){
        string str;
        cin>>str; 
        vec.push_back(str);
        arr[i]=str.c_str();
    }

    PrintStringVector(vec);
    PrintStringArray(arr);


    return 0;
}

ERRORS:

        vinod@vinod-Inspiron-3537:~/Documents/hackerrank$ g++       passing_vector_of_string_or_passing_2d_array.cpp 
        passing_vector_of_string_or_passing_2d_array.cpp:17:35: error: expected ‘,’ or ‘...’ before ‘arr’
        void PrintStringArray(const char* arr[n]){
                                           ^
        passing_vector_of_string_or_passing_2d_array.cpp: In function ‘void PrintStringArray(const char*)’:
        passing_vector_of_string_or_passing_2d_array.cpp:19:33: error: ‘arr’ was not declared in this scope
        for(int i=0;i<n;i++){ cout<<arr[i]<<endl;}
                                         ^
        passing_vector_of_string_or_passing_2d_array.cpp: In function ‘int main()’:
        passing_vector_of_string_or_passing_2d_array.cpp:40:25: error: cannot convert ‘const char**’ to ‘const char*’ for argument ‘1’ to ‘void PrintStringArray(const char*)’
        PrintStringArray(arr);

Answer 1

In C++ you cannot have VLA (variable length arrays). Since the size of the array arr is only known at runtime you cannot use it as a size of a fixed size array. You should use new to allocate it. Eg

const char **arr = new const char*[n];

Also modify the function signature like this

void PrintStringArray(const char *arr[]){

or like this

void PrintStringArray(const char **arr){

Finally remember to delete arr once you are finished with it.

delete[] arr;

Answer 2

const char *arr[n]

This is not a valid declaration (unless n is a constant expression). C++ has no variable length arrays.

"But it works for me inside main !'

That's because g++ implements an extension to C++. This extension does not seem compatible with C variable-length arrays, and is generally buggy. Don't use it.

"But how can I have comparable functionality?"

Use std::vector . Do not use pointers, new[] and delete[] unless you know extremely well why you need these low-level primitives.