Removing starting characters from string using std::string::erase - c++

Question

I am trying to truncate beginning zeros from a string, so I used sequence erase function
string& erase (size_t pos = 0, size_t len = npos);
This is my implementaion:
string str="000010557";
for(char c:str){
cout<<c<<" "<<str<<" "<<"\n";// for debug purpose
if(c=='0')
str.erase(0,1);
else
break;
}
cout<<str;
the output string that I got is 0010557 instead of10557 and debug statements prints:
0 000010557
0 00010557
1 0010557
I read the documentation of erase and this post thinking might there be some iterator invalidation but implementing the code snippet recommended in the accepted answer also gave the same output, please help me understand where is the problem.
I am new to using stl library functions, so please forgive any negligence of mine,Thanks.

Answer 1

Your for loop is incrementing the position from which c is extracted, even if you erase a leading zero. Thus, after two runs of the loop, you have erased the first and third of the leading zeros, then the c value will be the first 1.
Here's an attempt at following what happens in your code:
Start of first loop:
"000010557"
^
c is '0', so erase is called, making the string:
"00010557"
At the end of this first loop, the position is incremented, so...
Start of second loop:
"00010557"
^ (Note that we've skipped a zero!)
c is '0', so erase is called, making the string:
"0010557"
End of loop, position increment, and we skip another zero, so...
Start of third loop:
"0010557"
^
c is not '0', so we break out of the loop.
Instead, you should use a while loop, testing only the first character:
int main()
{
string str = "000010557";
char c;
while ((c = str.at(0)) == '0') {
cout << c << " " << str << " " << "\n";// for debug purpose
str.erase(0, 1);
}
cout << str;
}
Output:
0 000010557
0 00010557
0 0010557
0 010557
10557
Of course, you only need the c variable for your 'debugging' line so, without that, you can just have:
int main()
{
string str = "000010557";
while (str.at(0) == '0') str.erase(0, 1);
cout << str;
}

Answer 2

Even if you get this code to work, it's not a good solution. Removing a single character from the front of a string means moving all of the subsequent characters down one position, and the code does that for every leading zero. Instead, count the leading zeros and remove them all at once:
std::string::size_type non_zero_pos = 0;
while (non_zero_pos < str.size() && str[non_zero_pos] == '0')
++non_zero_pos;
str.erase(0, non_zero_pos);
That way, the (expensive) erase operation is only done once.
Or use iterators:
auto non_zero_it = std::find_first_not_of(std::begin(str), std::end(str), "0");
str.erase(std::begin(str), non_zero_it);
Edit: fixed search for non-0 iterator.