c++ : Defining array within an if-else condition erases after it exits

I have a script that fills in an array, either by using a function, or not.

It takes in an argument readmode, it decides whether to use the function or not; and an argument len_arr, which decides the length of the array.

I simplified the code so that every iteration the array length increases every iteration, but in actual situation, every iteration i load audio files so that are of different length, hence i use malloc to init, an then i free it once i am done post-processing it.

Main Script

// imports
  #include <iostream>     /* cout */
  #include <stdio.h>      /* printf */
  #include <stdlib.h>     /* atoi */
  using namespace std;

  void function_1(int len_arr, double * output_buffer){
    for (int j=0; j<len_arr; ++j){
      output_buffer[j] = 1.0*j;
    }
  }


int main (int argc, char ** argv) {
  int readmode = atoi(argv[1]);

  // double *buffer;
  // cout << "1 : Memory Address of buffer="<<buffer << endl;

  for (int len_arr=1; len_arr<=5; ++len_arr){

    if (readmode==1){
      double *buffer = (double *)malloc(len_arr * sizeof(double));
      function_1(len_arr, (double*)buffer);
      cout << "\n2 : Memory Address of buffer="<<buffer << endl;
      printf("readmode=1 : buffer[:5]=[%f %f %f %f %f]\n", buffer[0],buffer[1],buffer[2],buffer[3],buffer[4]);

    } else if (readmode==2){
      double *buffer = (double *)malloc(len_arr * sizeof(double));
      for (int j=0; j<len_arr; ++j){
        buffer[j] = 1.0*j;
      }
      cout << "\n3 : Memory Address of buffer="<<buffer << endl;
      printf("readmode=2 : buffer[:5]=[%f %f %f %f %f]\n", buffer[0],buffer[1],buffer[2],buffer[3],buffer[4]);
      
    }

  /* Do my post-processing with buffer */

  // verbose
    cout << "\n4 : Memory Address of buffer="<<buffer << endl;
    printf("readmode=%i : buffer[:5]=[%f %f %f %f %f]\n", readmode, buffer[0],buffer[1],buffer[2],buffer[3],buffer[4]);
    free(buffer);

  }
  return 0 ;
}

I compiled it like this

g++ -o ./debug_ifelse.out -Ofast -Wall -Wextra -std=c++11 "./debug_ifelse.cpp"

Here are the problems i faced:

1. If i dont initialise double *buffer prior to entering the if-else, i get compilation error that it was not declared, even though i did define it within each of the condition.

./debug_ifelse.cpp: In function ‘int main(int, char**)’:
./debug_ifelse.cpp:41:48: error: ‘buffer’ was not declared in this scope
     cout << "\n4 : Memory Address of buffer="<<buffer << endl;
                                                ^
./debug_ifelse.cpp: At global scope:
./debug_ifelse.cpp:14:15: warning: unused parameter ‘argc’ [-Wunused-parameter]
 int main (int argc, char ** argv) {
               ^

2. When i do initialise it (though i dont see the meaning to), the variable double *buffer outside and within the if-else seems to be in different memory address, and i cant access the one that was defined within the if-else.

1 : Memory Address of buffer=0

2 : Memory Address of buffer=0x24f3030
readmode=1 : buffer[:5]=[0.000000 0.000000 0.000000 0.000000 0.000000]

4 : Memory Address of buffer=0
Segmentation fault