Why is this code giving me segmentation fault?-CodePudding

I have written a code for the rainwater trapping problem https://www.geeksforgeeks.org/trapping-rain-water/ , but am getting segmentation fault in this code.

I have passed the given array into the two methods and as far as I have checked, am not trying to access any invalid memory.

#include<bits/stdc  .h>
using namespace std;

vector<int> maxl(int arr[],int n){
    vector<int> v1;
    v1[0]=arr[0];
    for(int i=1;i<n;i  ){
        v1[i]=max(v1[i-1],arr[i]);
    }
    return v1;
}
 
vector<int> maxr(int arr[],int n){
    vector<int> v2;
    v2[n-1]=arr[n-1];
    for(int i=n-2;i>=0;i--){
        v2[i]=max(v2[i 1],arr[i]);
    }
    return v2;
}

int main(){
    int a[]={3,0,0,2,0,4};
    vector<int> vl,vr;
    vl=maxl(a,6);
    vr=maxr(a,6);

    int lvl[6], unit[6];
    int s=0;
    for(int i=0;i<6;i  ){
        lvl[i]=min(vl[i],vr[i]);
        cout<<lvl[i]<<" ";
        unit[i]=lvl[i]-a[i];
        s =unit[i];
    }
    cout<<s;
    return 0;
}

Please lemme know my mistake :) Thanks

CodePudding user response：

Your code tries to access objects that don't exist. For example:

vector<int> maxl(int arr[],int n){
    vector<int> v1; // v1 is an empty vector
    v1[0]=arr[0]; // you try to modify the first element

An empty vector has no first element, so you can't modify its first element.

CodePudding user response：

Use gdb to check what is happening.

Starting program: /home/user/a.out 

Breakpoint 1, main () at test.cpp:22
22  int main(){
(gdb) n
23      int a[]={3,0,0,2,0,4};
(gdb) 
24      vector<int> vl,vr;
(gdb) 
25      vl=maxl(a,6);
(gdb) s
maxl (arr=0x0, n=21845) at test.cpp:4
4   vector<int> maxl(int arr[],int n){
(gdb) n
5       vector<int> v1;
(gdb) 
6       v1[0]=arr[0];
(gdb) 

Program received signal SIGSEGV, Segmentation fault.
maxl (arr=0x7fffffffdd80, n=6) at test.cpp:6
6       v1[0]=arr[0];

You can see the problem is in assigning v1[0]=arr[0].