Unable to figure out runtime error in below program - c++

Question

I was solving ArithmaticII.I am getting correct output for the below input
Input:
4
1 + 1 * 2 =
29 / 5 =
103 * 103 * 5 =
50 * 40 * 250 + 791 =
Output:
4
5
53045
500791
I am getting correct output ,But when i am submitting my solution to spoj , I
am getting runtime error as SIGABRT ,Due to the overflow issue I am using stoll,When i tried to debug my program everything was seemed to be running fine .
Note->It may also contain spaces to improve readability.
This line looks suspicious to me because my programs stops(runtime error) when i doesn't provide
space in the input (1 * 1+2=)
terminate called after throwing an instance of 'std::invalid_argument'
what(): stoll
Please help where I am doing wrong?
#include<iostream>
#include<string>
#include<sstream>
using namespace std;
int main() {
int t;
string str;
cin >> t;
while (t--) {
///using cin.ignore() as input as preceded by a single line
cin.ignore();
getline(cin, str, '\n');
stringstream split(str);
///now use getline with specified delimeter to split string stream
string intermediate;
int flag = 0;
long long int ans=1;
while (getline(split, intermediate, ' ')) {
if (intermediate == "=") {
cout << ans<<"\n";
break;
}
if (intermediate == "*") {
flag = 1;
continue;
}
else if (intermediate == "/") {
flag = 2;
continue;
}
else if (intermediate == "+") {
flag = 3;
continue;
}
else if(intermediate == "-"){
flag = 4;
continue;
}
if (flag == 1) {
ans *= stoll(intermediate);
}
else if (flag == 2) {
ans /= stoll(intermediate);
}
else if (flag == 3) {
ans += stoll(intermediate);
}
else if (flag == 4) {
ans -= stoll(intermediate);
}
else if (flag == 0) {
ans = stoll(intermediate);
}
}
}
}

Answer 1

Using exactly the input you posted above:
4
1 + 1 * 2 =
29 / 5 =
103 * 103 * 5 =
50 * 40 * 250 + 791 =
I'm able to reproduce your error:
terminate called after throwing an instance of 'std::invalid_argument'
what(): stoll
Aborted
I'll bet you removed the newlines in the sample input to simplify things--that's what it looks like from your code anyways. You're doing a getline and pulling exactly as many lines as you're told in the first input. That means you'll pull an empty line (intermediate will be an empty string) and you'll try to process it.
And what happens when you try to call stoll("")? The error you got!
You could try to check if you get an empty line and then continue, something like:
getline(cin, str, '\n');
if(str.empty()) {
t++; //we didn't actually do anything, so increment t
continue;
}
stringstream split(str);
...
Or, you could use the fact that the >> operator already stops when it hits whitespace (and eats all the whitespace it can to get to the next edible character). So instead of reading a line at a time, you could continue reading until you hit a = character.

Answer 2

cin >> t; does not eat the newline character after reading the number 4 from the input. As the program enters the loop cin.ignore() finishes the first line of the input and then getline(...) reads a blank line. You get an empty string which cannot be converted to an int.
Putting one more cin.ignore() after reading t fixes the runtime error.
You still have to handle the case when there are no spaces between the tokens. This requires rethinking the inner while loop:
long long int ans;
char op;
split >> ans;
while (split >> skipws >> op) {
if (op == '=') {
cout << ans << '\n';
break;
}
long long int n;
split >> n;
if (op == '*')
ans *= n;
else if (op == '/')
ans /= n;
else if (op == '+')
ans += n;
else if(op == '-')
ans -= n;
}