Why is this pointer messed up when accessed? [duplicate] - c++

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This question already has answers here:
Can a local variable's memory be accessed outside its scope?
(20 answers)
Closed 9 years ago.
Consider the following code:
int* solve(int input[], int len) {
//Processing and building the calc array. Can be ignored.
int calc[3*(len-1)];
calc[0] = input[0];
calc[1] = 1;
calc[2] = 1;
for (int b = 1; b < len - 1; b++) {
calc[3*b] = 0;
calc[3*b + 1] = 0;
calc[3*b + 2] = 0;
}
if (input[0] < input[1]) {
calc[3] = input[1];
calc[4] = 0;
calc[5] = 1;
} else {
calc[3] = input[0];
calc[4] = 1;
calc[5] = 0;
}
for (int i = 2; i < len - 1; i++) {
for (int j = 0; j < i; j++) {
if ((i - j > 1 || calc[3*j + 2] == 0) && calc[3*j] + input[i] > calc[3*i]) {
calc[3*i] = calc[3*j] + input[i];
calc[3*i + 1] = calc[3*j + 1];
calc[3*i + 2] = 1;
} else if (input[i] > input[j] && calc[3*i] < calc[3*j] - input[j] + input[i]) {
calc[3*i] = calc[3*j] - input[j] + input[i];
calc[3*i + 1] = calc[3*j + 1];
calc[3*i + 2] = 1;
} else if (calc[3*i] < calc[3*j]) {
calc[3*i] = calc[3*j];
calc[3*i + 1] = calc[3*j + 1];
calc[3*i + 2] = 0;
}
}
}
//Printing the array
cout<<"Calc array: ";
for (int a = 0; a < len - 1; a++) {
cout<<"("<<calc[3*a]<<" "<<calc[3*a + 1]<<" "<<calc[3*a+2]<<") ";
}
cout<<endl;
//Returning a pointer to the array
int *pointer = calc;
return pointer;
}
int main() {
//Taking input. Can be ignored.
int len;
cin>>len;
int input[len];
for (int i = 0; i < len; i++) {
cin>>input[i];
}
//Assigning another pointer to the array that the solve() function returns.
int *a = solve(input, len);
int *b;
//Printing the array that the pointer points to.
cout<<"A Array: ";
for (int x = 0; x < len - 1; x++) {
cout<<"("<<a[3*x]<<" "<<a[3*x + 1]<<" "<<a[3*x+2]<<") ";
}
cout<<endl;
//Ignore code from here.
int c;
if (a[3*(len - 2) + 1] == 1) {
input[0] = -10*10*10*10;
b = solve(input, len);
if (b[3*(len - 2) + 2] == 1) {
if (input[len-1] > input[len-2]) {
c = b[3*(len - 2)] - input[len-2] + input[len - 1];
cout<<c<<endl;
} else {
c = b[3*(len - 2)];
}
} else {
c = b[3*(len - 2)] + input[len-1];
}
if (c < a[3*(len - 2)]) {
cout<<a[3*(len - 2)];
} else {
cout<<c<<endl;
cout<<a[3*(len - 2)]<<" "<<a[3*(len - 2) + 1]<<" "<<a[3*(len - 2) + 2];
cout<<"This route"<<endl;
}
} else {
input[1] = -10*10*10*10;
b = solve(input, len);
if (a[3*(len - 2) + 2] == 1) {
if (input[len-1] > input[len-2]) {
c = a[3*(len - 2)] - input[len-2] + input[len - 1];
} else {
c = a[3*(len - 2)];
}
} else {
c = a[3*(len - 2)] + input[len-1];
}
if (c > b[3*(len - 2)]) {
cout<<b[3*(len - 2)];
} else {
cout<<c;
}
}
}
Now the problem here is, that when I print the calc array inside the solve() function the first time it prints perfectly and gives the following desired output:
Calc array: (10 1 1) (10 1 0) (12 1 1) (15 1 1) (19 1 1)
But when I print it again inside the main() function, I get the following output:
A Array: (135712 0 1259266624) (2045 1 0) (4792936 0 32) (15 4792936 0) (2357952 0 4792936)
I have just migrated from Python to C++, and I find it extremely cumbersome, at times such as these. I have tried all sorts of modifications to the code but I still can't figure out the problem. Any help would be appreciated.

Answer 1

calc is a local variable whose lifetime starts at it's definition and ends when the function exits.
Since you're returning a pointer to it when exiting the function, the dereferencing of said pointer will be undefined behaviour (since the "object" behind it no longer exists).
If you want a variable to survive function return, you'll need to do something like allocating it dynamically, changing:
int calc[3*(len-1)];
to:
int *calc = new int [(3 * (len - 1)];
and then ensuring you delete[] it in the caller when you're done with it.