"input.length();"的目的是什么？

我正在参加一门大学级别的C++课程，坦率地说，没有什么真正的解释。我得到了要编写的代码，我的程序也能正常工作。我只是想知道某些行的用途。

例如：

int i = 0;

我知道我正在声明一个=0的int变量。这里我的问题是为什么字母i？这可以是我选择的任何变量名吗？

int length = input.length();  

我知道我正在声明一个名为length的int变量。。。但是它在我的代码中有什么作用呢？

i++

我想这结束了我的循环？

我添加了我的代码以供阅读。如有任何协助，我们将不胜感激！

// Program takes user entered letter and matches it with the corresponding ICAO word.
//Program has been modified to use void and string methods
#include "stdafx.h"
#include "iostream"
#include "string"
using namespace std;
//Function Heading
void convert(string);
//Main Function
int main()
{
string input;
cout << " Enter a letter or word: ";   // Ask the user to enter a letter or word.
cin >> input;                          //get input
cout << "Phonetic Version : ";         //Display "Phonetic Version"
convert (input);                        
cout << endl;
system("pause");
}//End Main
//Function Definition
void convert(string input)
{
int i = 0;      //input variable
char letters;   //character variable 
int length = input.length(); 
while (i < length)      //While loop initialized
{
letters = input.at(i);
if (letters == 'a' || letters == 'A')
cout << "Alpha ";
else if (letters == 'b' || letters == 'B')
cout << "Bravo ";
else if (letters == 'c' || letters == 'C')
cout << "Charlie ";
else if (letters == 'd' || letters == 'D')
cout << "Delta ";
else if (letters == 'e' || letters == 'E')
cout << "Echo ";
else if (letters == 'f' || letters == 'F')
cout << "Foxtrot ";
else if (letters == 'g' || letters == 'G')
cout << "Golf ";
else if (letters == 'h' || letters == 'H')
cout << "Hotel ";
else if (letters == 'i' || letters == 'I')
cout << "India ";
else if (letters == 'j' || letters == 'J')
cout << "Juliet ";
else if (letters == 'k' || letters == 'K')
cout << "Kilo ";
else if (letters == 'l' || letters == 'L')
cout << "Lima ";
else if (letters == 'm' || letters == 'M')
cout << "Mike ";
else if (letters == 'n' || letters == 'N')
cout << "November ";
else if (letters == 'o' || letters == 'O')
cout << "Oscar ";
else if (letters == 'p' || letters == 'P')
cout << "Papa ";
else if (letters == 'q' || letters == 'Q')
cout << "Quebec ";
else if (letters == 'r' || letters == 'R')
cout << "Romeo ";
else if (letters == 's' || letters == 'S')
cout << "Sierra ";
else if (letters == 't' || letters == 'T')
cout << "Tango ";
else if (letters == 'u' || letters == 'U')
cout << "Uniform ";
else if (letters == 'v' || letters == 'V')
cout << "Victor ";
else if (letters == 'w' || letters == 'W')
cout << "Whiskey ";
else if (letters == 'x' || letters == 'X')
cout << "X-ray ";
else if (letters == 'y' || letters == 'Y')
cout << "Yankee ";
else if (letters == 'z' || letters == 'Z')
cout << "Zulu ";
i++;
}
}