std::vector.push_back() C++ - c++

Question

I want to initialize a vector with alphabetical letters using push_back function. Is it the right way to do it?
vector<char> v;
char letter = 'A';
for(int i=0; i<26; i++)
{
v.push_back(letter+i);
}
It works. I am just wondering whether I should use a type cast letter to an int before adding i to it?
Or is there a more efficient way?

Answer 1

Note that your code relies on a character encoding scheme that encodes the letters contiguously, like e.g. ASCII.
If that assumption holds, you could create the vector using the correct size initially, and use std::iota to initialize all elements:
std::vector<char> v(26); // Create a vector of 26 (default-initialized) elements
std::iota(begin(v), end(v), 'A'); // Assign a letter to each element in the vector
If you want your code to be portable to systems where letters aren't contiguously encoded (like a system which uses EBCDIC) then you're better of to create a string using the letters explicitly:
std::string alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"; // Thanks Nathan Oliver :)
And if you have a string with all the letters, then perhaps you won't need the vector even.

Answer 2

Looks pretty good!
I guess maybe std::array() would be an option too, as compared to std::vector() for similar tasks:
#include <iostream>
#include <array>
#include <vector>
#include <chrono>
void function1() {
std::vector<char> alphabets;
for (unsigned int index = 0; index < 26; ++index) {
alphabets.push_back(index + 'A');
// std::cout << alphabets[index] << "\t";
}
// std::cout << "\n\n";
}
void function2() {
std::vector<char> alphabets;
for (unsigned int index = 0; index < 26; ++index) {
alphabets.emplace_back(index + 'A');
// std::cout << alphabets[index] << "\t";
}
// std::cout << "\n\n";
}
void function3() {
std::array<char, 26> alphabets;
for (unsigned int index = 0; index < 26; ++index) {
alphabets[index] = index + 'A';
// std::cout << alphabets[index] << "\t";
}
// std::cout << "\n\n";
}
int main() {
const auto t1 = std::chrono::high_resolution_clock::now();
for (std::size_t i = 0; i < 1000000; ++i) {
function1();
}
const auto t2 = std::chrono::high_resolution_clock::now();
const auto duration = std::chrono::duration_cast<std::chrono::microseconds>( t2 - t1 ).count();
std::cout << duration <<
" is the rough runtime of std::vector function with push_back\t💙💙💙\t😳\n\n";
const auto t3 = std::chrono::high_resolution_clock::now();
for (std::size_t i = 0; i < 1000000; ++i) {
function2();
}
const auto t4 = std::chrono::high_resolution_clock::now();
const auto duration2 = std::chrono::duration_cast<std::chrono::microseconds>( t4 - t3 ).count();
std::cout << duration2 <<
" is the rough runtime of std::vector function with emplace_back\t💙💙💙\t😳\n\n";
const auto t5 = std::chrono::high_resolution_clock::now();
for (std::size_t i = 0; i < 1000000; ++i) {
function3();
}
const auto t6 = std::chrono::high_resolution_clock::now();
const auto duration3 = std::chrono::duration_cast<std::chrono::microseconds>( t6 - t5 ).count();
std::cout << duration3 << " is the rough runtime of std::array function\t💙💙💙\t😳\n\n";
return 0;
};