C++ remove function moving characters to the left [duplicate] - c++

Question

This question already has answers here:
Difference between erase and remove
(7 answers)
Closed 2 years ago.
I recently tried a code mentioned somewhere on this site to remove blank spaces in a string. The answer suggested the function remove from the algorithm library (amazingly explained here: https://www.geeksforgeeks.org/stdremove-stdremove_if-c/) but it gives an unexpected output. It replaces all the blank spaces with some random numbers. Here is the code.
#include <iostream>
#include <algorithm>
using namespace std;
int main()
{
string a;
int b;
getline(cin, a);
remove(a.begin(), a.end(), ' ');
b = stoi(a);
cout << b << endl;
return 0;
}
If I input 14 546 32 for example it outputs 145463232. Oddly enough if I input 1 2 3 4 5 it outputs the correct thing: 12345.
Expected output, input:
I input any number with blank spaces in between some numbers.
It outputs the number without spaces.
I tried compiling it online with this compiler: https://www.onlinegdb.com/. It has the exact same output. Can anybody figure out what is wrong with the code. And also i need to turn the string into an integer to do some mathematical operations with the integer afterwards (that is why I use the stoi function). Thanks.

Answer 1

Interesting thing about std::remove is that it does not actually remove anything. =)
You will need to erase the character by yourself like this:
#include <iostream>
#include <algorithm>
using namespace std;
int main()
{
string a;
int b;
getline(cin, a);
a.erase(remove(a.begin(), a.end(), ' '), a.end());
b = stoi(a);
cout << b << endl;
return 0;
}
You can read more about Erase–remove here:
https://en.wikipedia.org/wiki/Erase%E2%80%93remove_idiom

Answer 2

std::remove removes the elements that compare equal to its argument by shifting the remaining elements using move-assignment. It doesn't resize the container after shifting those elements. In your case, since the objects are of type char, they are simply copied to the beginning of the array. So the end of your string stays the same.
When you remove spaces from the first number, and shift everything to left, you get this:
old string: 14 546 32
new string: 145463232
When you do the same in the second case:
old string: 1 2 3 4 5
new string: 12345 4 5
The only reason you get the correct result in the second one is the space in-between. First number in the string (12345) is converted to int and returned.
Note that std::remove also returns an iterator to the new end of the list. You could use that to resize the string.
auto end = remove(a.begin(), a.end(), ' ');
a.resize(end - a.begin());