Display a std::string from index i to the end of the string

Question

For example:

char s[]="abcde";
    for(int i=0;i<strlen(s);i++)
          cout<<s+i<<endl;

Output:

abcde
bcde
cde
de
e

And this doesn't ruin the char array.

What I want to ask for help is, is there another way we can do that with the std::string class?

Never use a call of `strlen` as condition in a loop while iterating on string. You do extra work, it makes complexity _O(n^2)_ instead of _O(n)_. — αλεχολυτ, Mar 09 '16 at 10:04
@alexolut: Never use _the same_ call of `strlen`. Microsoft sometimes uses a pattern where a list of strings is represented as one contiguous character array, and the list is terminated by an empty string. The loop you need there is `for(char* s = begin; strlen(s) ; s+=strlen(s)+1)` This is clearly O(n*m) because you have O(n) strings of length O(m). — MSalters, Mar 09 '16 at 11:26
@MSalters not enough clearly as I see, because of call of `strlen` twice on iteration. — αλεχολυτ, Mar 09 '16 at 11:44
@alexolut: True, but `O(n*m*2)` is identical to `O(n*m)`. That said, you can optimize it: `size_t len; for(char* s = begin; len = strlen(s) ; s+=len+1)` — MSalters, Mar 09 '16 at 11:53
@MSalters Agreed. My first sentence was about iterating on string **by symbols**. In your case - that's loop on list of strings by strings. — αλεχολυτ, Mar 09 '16 at 12:16

score 6 · Accepted Answer · edited May 23 '17 at 12:23

Cheap and nasty way, closest to what you already have: use std::string::c_str()

std::string const s{"abcde"};
char const *c = s.c_str();
for (std::size_t i=0; i<s.size(); ++i)
    std::cout << c+i << '\n';

The more string-like way is to use std::string::substr:

std::string const s{"abcde"};
for (std::size_t i=0; i<s.size(); ++i)
    std::cout << s.substr(i) << '\n';

Note that the second approach does involve allocating memory for the new string every time, but I think here the benefit of clarity (using a function specifically named and designed for what you are trying to do), and avoiding raw pointers (often a cause of confusion for beginners) both outweigh the performance (non-)issue here.

A more modern approach that combines the clarity of using specific C++ components designed for what you want, with avoiding extra allocations, is to use std::experimental::string_view, which is designed to wrap an existing string of characters:

std::string const s{"abcde"};
std::experimental::string_view sv{s};
for (std::size_t i=0; i<s.size(); ++i) {
    std::cout << sv << '\n';
    sv.remove_prefix(1);
}

std::experimental::string_view::remove_prefix does what you might expect: shrinks the view of the string it is wrapping at the front, by the number specified.

string_view is not technically part of The Standard yet, so I don't really recommend it for production use, since you won't get good cross-platform support. But it's quite fun to learn about. You might consider the very similar (not coincidentally) component, boost::string_ref to be production quality.

Also, please reconsider your use of what are often considered bad practices: using namespace std; and endl (those are links to explanations).

Display a std::string from index i to the end of the string

1 Answers1