Getting SIGESV error in following cpp code [duplicate] - c++

Question

This question already has answers here:
Segmentation Fault (SIGSEGV) when getting data as input in the vector of struct
(2 answers)
Closed 2 years ago.
Getting SIGESV error in following cpp code, kindly check.
#include <bits/stdc++.h>
using namespace std;
int main() {
int t;
cin >> t;
while (t--) {
int n, m;
int sa = 0, sb = 0, c = 0;
cin >> n >> m;
vector<int> a;
for (int i = 0; i < n; i++) {
cin >> a[i];
}
vector<int> b;
for (int i = 0; i < m; i++) {
cin >> b[i];
}
int k = min(n, m);
while (k--) {
sa = accumulate(a.begin(), a.end(), 0);
sb = accumulate(b.begin(), b.end(), 0);
if (sb >= sa) {
swap(*min_element(a.begin(), a.end()),
*max_element(b.begin(), b.end()));
c++;
} else {
break;
}
}
if (sb >= sa) {
cout << "-1" << endl;
} else {
cout << c << endl;
}
}
return 0;
}
The code asks for two arrays and swaps only if array a has a summation less than array b.
Getting error as SIGESV, also can take 'b' array as an input, what to do?

Answer 1

When you're doing cin >> a[i], a is still empty, so attempting to access its i-th element produces a segfault. Resize it before with a.resize(n) or initialise it with the proper size: vector<int> a(n);

Answer 2

The size and capacity of vector a is 0 and never changes.
This code is broken:
vector<int> a;
for(int i=0;i<n;i++){
cin>>a[i];
}
operator[]() does not grow the vector. You are assigning values to memory that does not belong to a.
You want a.push_back(i) at a minimum. What you probably really want is:
vector<int> a;
a.reserve(n);
for (int i = 0; i < n; ++i) {
int tmp;
cin >> tmp;
a.push_back(tmp);
}
Reserving memory stops the vector from re-allocating. Not doing it in a constructor also stops default initialization of all elements.
The same goes for the b vector.
You also never change the values of sa or sb, but that shouldn't crash anything, it's just a logic error from what I can see.
The file as a whole is also rife with bad practices. "Competitive" coding sites actively make you a worse programmer.

Answer 3

When you know size you need while declaring vector you should declare vector like this
vector<int>arr(n);
vector<int>arr2(m);
this will create an vector of size n will value being zero at all indexes in vector at initialization.
Don't use <bits/stdc++.h> and trying coming up for better name for variables it will really help you as well as us. I know this is compeitive programming question but still.