insert string into vector character by character - c++

Question

I'm trying to insert the characters of the string into a char vector but place the letters in reverse order . can anyone tell me why this doesn't work
int main()
{
string a = "Hello";
vector<char> arr(5);
for(int i = 4 ; i == 0 ; i--)
{
arr.push_back(a[i]);
cout << arr[i];
}
return 0;
}
im trying to push back the character in reverse order 1 by 1

Answer 1

Their are several problems with your code:
you are creating a vector whose size is initially 5, and then you are attempting to push 5 additional chars into it, for a total of 10 chars. You need to either:
initialize it's capacity instead of its size.
initialize the size as you are, but use arr[4-i] instead of arr.push_back() inside your loop.
your loop is never entered at all, since i == 0 is never true.
Try something more like this instead:
#include <iostream>
#include <string>
#include <vector>
using namespace std;
int main() {
string a = "Hello";
size_t len = a.size();
vector<char> arr;
arr.reserve(len);
for(int i = len-1; i >= 0; i--) {
arr.push_back(a[i]);
}
for(size_t i = 0; i < len; ++i) {
cout << arr[i];
}
return 0;
}
Alternatively:
#include <iostream>
#include <string>
#include <vector>
using namespace std;
int main() {
string a = "Hello";
size_t len = a.size();
vector<char> arr(len);
for(int i = len-1; i >= 0; i--) {
arr[len-1-i] = a[i];
}
for(size_t i = 0; i < len; ++i) {
cout << arr[i];
}
return 0;
}
Another way to deal with this in a more C++-ish way is to use iterators instead:
#include <iostream>
#include <string>
#include <vector>
using namespace std;
int main() {
string a = "Hello";
vector<char> arr;
arr.reserve(a.size());
for(auto iter = a.rbegin(); iter != a.rend(); ++iter) {
arr.push_back(*iter);
}
for(auto ch : arr) {
cout << ch;
}
return 0;
}
Alternatively:
#include <iostream>
#include <string>
#include <vector>
using namespace std;
int main() {
string a = "Hello";
vector<char> arr(a.size());
auto arr_iter = arr.begin();
for(auto a_iter = a.rbegin(); a_iter != a.rend(); ++a_iter, ++arr_iter) {
*arr_iter = *a_iter;
}
for(auto ch : arr) {
cout << ch;
}
return 0;
}
And then you can get rid of the manual loops altogether:
#include <iostream>
#include <string>
#include <vector>
#include <iterator>
#include <algorithm>
using namespace std;
int main() {
string a = "Hello";
vector<char> arr;
arr.reserve(a.size());
copy(a.rbegin(), a.rend(), back_inserter(arr));
cout.write(arr.data(), arr.size());
return 0;
}
Alternatively:
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;
int main() {
string a = "Hello";
vector<char> arr(a.size());
copy(a.rbegin(), a.rend(), arr.begin());
cout.write(arr.data(), arr.size());
return 0;
}
Alternatively:
#include <iostream>
#include <string>
#include <vector>
using namespace std;
int main() {
string a = "Hello";
vector<char> arr(a.rbegin(), a.rend());
cout.write(arr.data(), arr.size());
return 0;
}

Answer 2

I'm not sure why you're complicating matters when a vector is perfectly capable of taking an iterator in the constructor? The vast majority of your code can therefore be replaced with a simple:
vector<char> arr(a.rbegin(), a.rend());
The complete program below shows this in action:
#include <iostream>
#include <string>
#include <vector>
using std::cout; using std::string; using std::vector;
int main() {
string a = ")-: yug emosdnah a si xaP";
vector<char> arr(a.rbegin(), a.rend());
for (auto ch: arr) cout << ch;
cout << '\n';
}

Answer 3

for(int i = 4 ; i == 0 ; i--)
That means to keep going while i is equal to zero - but i starts out at four, so the for loop terminates immediately.
You probably meant
for(int i = 4 ; i >= 0 ; i--)