For loop and while loop comparison - c++

Question

So I did the next exercise, just with while loop:
Write a program that prompts the user for two integers.
Print each number in the range specified by those two integers.
Here is the code:
#include <iostream>
int main()
{
std::cout << "Write two numbers: " << std::endl;
int v1 = 0, v2 = 0;
std::cin >> v1 >> v2;
std::cout << "The numbers between " << v1 << " and " << v2 << " are: " << std::endl;
while (v2 < v1 && ++v2 < v1)
{
std::cout << v2 << std::endl;
}
while (v1 < v2 && ++v1 < v2)
{
std::cout << v1 << std::endl;
}
}
Now I have to do it with the for loop, which I did like this:
#include <iostream>
int main()
{
std::cout << "Write two numbers: " << std::endl;
int a, b;
std::cin >> a >> b;
std::cout << "The numbers between " << a << " and " << b << " are: " << std::endl;
for (; a < b && ++a < b; a)
{
std::cout << a << std::endl;
}
for (; b < a && ++b < a; b)
{
std::cout << b << std::endl;
}
}
It looks almost the same, but it works.
My questions is: I'm I missing something about the for loop, could I do it simpler?
PD: Just for loop, I'm not in the If chapter yet, I want to go step by step on the "C++ Primer 5th edition".

Answer 1

for is specified in terms of while, you aren't missing anything.
for (init-statement conditionopt;
iteration-expressionopt) statement
produces code equivalent to:
{ init-statement while (condition) { statement
iteration-expression; } }
Except that
Names declared by the init-statement (if init-statement is a declaration) and names declared by condition (if condition is a
declaration) are in the same scope (which is also the scope of
statement).
continue in the statement will execute iteration-expression
Empty condition is equivalent to while(true)
from cppreference
You don't need anything in iteration-expression for(;;) is equivalent to while(true)
it would be more normal to increment in the iteration-expression, and not repeat almost the same test.
#include <iostream>
int main()
{
std::cout << "Write two numbers: " << std::endl;
int a, b;
std::cin >> a >> b;
std::cout << "The numbers between " << a << " and " << b << " are: " << std::endl;
for (; a < b; ++a)
{
std::cout << a << std::endl;
}
for (; b < a; ++b)
{
std::cout << b << std::endl;
}
}

Answer 2

The for loop is intended to loop between two numbers. The use of your for loops unreadable for it's intentions. Take a look at this
int end = 10;
for (int begin = 0; begin < end; ++begin){/*do something*/}
This is the standard structure of a for loop. Now for your example you will get the following
#include <algorithm>
if (a > b) std::swap(a, b);
for (int begin = a; begin <= b; ++begin){
std::cout << begin << std::endl;
}

Answer 3

There is no need on using something like a < b && ++a < b since a < b is contained in ++a < b condition. So just using ++a < b you will get the same results.
Now about the for you should write it like this just to make your code a bit clear:
for (; a < b; ++a)
{
std::cout << a << std::endl;
}
I am not using the initialization sentence as you have already initialized your variables, however I encorage you to initialize a in the for sentence
In general terms, the for is divided in three sections:
for (<initialization sentence>; <condition sentence>; <post-execution sentence>)
The initialization only runs when the for sentence is reached, and the loop is running while the condition is met. The post-execution it is normally used to increase or change state of the variables involved in the condition criteria. The only constraint you have is that condition must be a boolean sentence.
None of those sections should be an assigment, a common comparison or a variable increment. You could use whatever fits your requirements and met the for constraints.
To sum up, a for sentence is a 'wrapped' structure of a while. You could get the same results with both. The difference is on a cleaner image of your code and a better understanding of the algorithms.

Answer 4

The code in the question is somewhat confusing, because it tries to do several things at once. I'd separate them.
Instead of writing two loops, I'd just change the limits:
if (v2 < v1)
std::swap(v2, v1);
Now it's easy:
while (++v1 < v2)
std::cout << v1 << '\n';
Same thing for the for loop. After adjusting the limits, just do it:
for ( ; ++a < b; )
std::cout << a << '\n';