2 while loops vs if else statement in 1 while loop - c++

Question

First a little introduction:
I'm a novice C++ programmer (I'm new to programming) writing a little multiplication tables practising program. The project started as a small program to teach myself the basics of programming and I keep adding new features as I learn more and more about programming. At first it just had basics like ask for input, loops and if-else statements. But now it uses vectors, read and writes to files, creates a directory etc.
You can see the code here: Project on Bitbucket
My program now is going to have 2 modes: practise a single multiplication table that the user can choose himself or practise all multiplication tables mixed. Now both modes work quite different internally. And I developed the mixed mode as a separate program, as would ease the development, I could just focus on writing the code itself instead of also bothering how I will integrate it in the existing code.
Below the code of the currently separate mixed mode program:
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <algorithm>
#include <time.h>
using namespace std;
using std::string;
int newquestion(vector<int> remaining_multiplication_tables, vector<int> multiplication_tables, int table_selecter){
cout << remaining_multiplication_tables[table_selecter] << " * " << multiplication_tables[remaining_multiplication_tables[table_selecter]-1]<< " =" << "\n";
return remaining_multiplication_tables[table_selecter] * multiplication_tables[remaining_multiplication_tables[table_selecter]-1];
}
int main(){
int usersanswer_int;
int cpu_answer;
int table_selecter;
string usersanswer;
vector<int> remaining_multiplication_tables = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
vector<int> multiplication_tables(10, 1);//fill vector with 10 elements that contain the value '1'. This vector will store the "progress" of each multiplication_table.
srand(time(0));
table_selecter = rand() % remaining_multiplication_tables.size();
cpu_answer = newquestion(remaining_multiplication_tables, multiplication_tables, table_selecter);
while(remaining_multiplication_tables.size() != 0){
getline(cin, usersanswer);
stringstream usersanswer_stream(usersanswer);
usersanswer_stream >> usersanswer_int;
if(usersanswer_int == cpu_answer){
cout << "Your answer is correct! :)" << "\n";
if(multiplication_tables[remaining_multiplication_tables[table_selecter]-1] == 10){
remaining_multiplication_tables.erase(remaining_multiplication_tables.begin() + table_selecter);
}
else{
multiplication_tables[remaining_multiplication_tables[table_selecter]-1] +=1;
}
if (remaining_multiplication_tables.size() != 0){
table_selecter = rand() % remaining_multiplication_tables.size();
cpu_answer = newquestion(remaining_multiplication_tables, multiplication_tables, table_selecter);
}
}
else{
cout << "Unfortunately your answer isn't correct! :(" << "\n";
}
}
return 0;
}
As you can see the newquestion function for the mixed mode is quite different. Also the while loop includes other mixed mode specific code.
Now if I want to integrate the mixed multiplication tables mode into the existing main program I have 2 choices:
-I can clutter up the while loop with if-else statements to check each time the loop runs whether mode == 10 (single multiplication table mode) or mode == 100 (mixed multiplication tables mode). And also place a if-else statement in the newquestion() function to check if mode == 10 or mode == 100
-I can let the program check on startup whether the user chose single multiplication table or mixed multiplication tables mode and create 2 while loops and 2 newquestion() functions. That would look like this:
int newquestion_mixed(){
//newquestion function for mixed mode
}
int newquestion_single(){
//newquestion function for single mode
}
//initialization
if mode == 10
//create necessary variables for single mode
while(){
//single mode loop
}
else{
//create necessary variables for mixed mode
while(){
//mixed mode loop
}
}
Now why would I bother creating 2 separate loops and functions? Well isn't it inefficient if the program checks each time the loop runs (each time the user is asked a new question, for example: '5 * 3 =') which mode the user chose? I'm worried about the performance with this option. Now I hear you think: but why would you bother about performance for such a simple, little non-performance critical application with the extremely powerful processors today and the huge amounts of RAM? Well, as I said earlier this program is mainly about teaching myself a good coding style and learning how to program etc. So I want to teach myself the good habits from the beginning.
The 2 while loops and functions option is much more efficient will use less CPU, but more space and includes duplicating code. I don't know if this is a good style either.
So basically I'm asking the experts what's the best style/way to handle this kind of things. Also if you spot something bad in my code/bad style please tell me, I'm very open to feedback because I'm still a novice. ;)

Answer 1

First, a fundamental rule of programming is that of "don't prematurely optimize the code" - that is, don't fiddle around with little details, before you have the code working correctly, and write code that expresses what you want done as clearly as possible. This is good coding style. To obsess over the details of "which is faster" (in a loop that spends most of it's time waiting for the user to input some number) is not good coding style.
Once it's working correcetly, analyse (using for example a profiler tool) where the code is spending it's time (assuming performance is a major factor in the first place). Once you have located the major "hotspot", then try to make that better in some way - how you go about that depends very much on what that particular hot-spot code does.
As to which performs best will highly depend on details the code and the compiler (and which compiler optimizations are chosen). It is quite likely that having an if inside a while-loop will run slower, but modern compilers are quite clever, and I have certainly seen cases where the compiler hoists such a choice out of the loop, in cases where the conditions don't change. Having two while-loops is harder for the compiler to "make better", because it most likely won't see that you are doing the same thing in both loops [because the compiler works from the bottom of the parse-tree up, and it will optimize the inside of the while-loop first, then go out to the if-else side, and at that point it's "lost track" of what's going on inside each loop].
Which is clearer, to have one while loop with an if inside, or an if with two while-loops, that's another good question.
Of course, the object oriented solution is to have two classes - one for mixed, another for single - and just run one loop, that calls the relevant (virtual) member function of the object created based on an if-else statement before the loop.

Answer 2

Modern CPU branch predictors are so good that if during the loop the condition never changes, it will probably be as fast as having two while loops in each branch.