What is the purpose of "input.length();"? - c++

Question

I am taking a college level C++ course, and quite frankly nothing is really ever explained. I was given code to write, and my program works as it should. I would just like to know the purpose of certain lines.
Such as:
int i = 0;
I know I am declaring an int variable that = 0. Here my question is why the letter i? Could that be any variable name I choose?
int length = input.length();
I know I am declaring an int variable named length... but what purpose does it serve in my code?
i++
I think this ends my loop?
I have added my code for perusal. Any assistance would be greatly appreciated!
// 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++;
}
}

Answer 1

int length = input.length();
I know I am declaring an int variable named length... but what purpose
does it serve in my code?
None.
It would serve some purpose if input's length would change later on and you'd need to remember the old length for some reason.
Since this is not the case here, your professor may think that this is some kind of "optimisation" on the grounds that repeatedly calling length() may be too slow. But this is nonsense; your computer is too fast for such micro-optimisations to have an observable effect, especially with modern compilers being much better at optimising programs than the programmers themselves.
Just remove the length variable to make the code shorter.

Answer 2

Here
string input;
std::string has a method called length() which returns the length of the string, in terms of bytes. Hence you are using like
int length = input.length(); /* use variable name as other than predefined method to avoid confusion */
| |
this is just this is a method of string
a int variable

Answer 3

int i = 0;
I know I am declaring an int variable that = 0. Here my question is why the letter i? Could that be any variable name I choose?
Yes. Variable names are arbitrary, name them however you want (within the restrictions of the language syntax, of course). Just make sure you use names that are meaningful within the context in which they are being used. Readability matters when maintaining code over time.
int length = input.length();
I know I am declaring an int variable named length... but what purpose does it serve in my code?
To make a local cached copy of the character count of the input string so your loop does not have to keep calling the string's length() method over and over. Using a few bytes of local stack space can save time and overhead of retrieving the string's length, which does not change while the loop runs.
i++
I think this ends my loop?
It increments the value of the i variable, nothing more. The loop ends when the while statement evaluates as false (when i catches up to length).