Infinite loop on checking character in a do while - c++

Question

The goal is to calculate the percentage of marks over 10. Marks are between 0 and 20. I have an issue with the while loop when I click on 'N'. I get an infinite loop.
#include <iostream>
int main() {
float MARK, PERCENTAGE;
int NBR_MARK, NBR_MARK_10;
MARK = 0;
NBR_MARK = 0;
NBR_MARK_10 = 0;
PERCENTAGE = 0;
char R = 'N';
std::cout << "Enter a mark ?" << std::endl;
std::cin >> MARK;
while (MARK < 0 || MARK > 20) {
std::cout << " Please, enter a mark between 0 and 20" << std::endl;
std::cin >> MARK;
}
std::cout << "Do you want to enter a new mark ?" << std::endl;
std::cout << "Click on 'O' to continue and on 'N' to stop" << std::endl;
std::cin >> R;
if ((R != 'N') || (R != 'n'))
std::cout << "You will continue" << std::endl;
do {
std::cout << "Enter a new mark" << std::endl;
std::cin >> MARK;
std::cout << std::endl;
while ((MARK < 0) || (MARK > 20)) {
std::cout << "Please, enter a mark between 0 and 20" << std::endl;
std::cin >> MARK;
std::cout << std::endl;
}
NBR_MARK++;
if (MARK > 10) NBR_MARK_10++;
std::cout << "To stop press 'N'" << std::endl;
std::cin >> R;
}
while ((R != 'N') || (R <= 'n'));
PERCENTAGE = NBR_MARK_10 / NBR_MARK * 100;
std::cout << "Le % de notes > 10 est de: " << PERCENTAGE << " %" << std::endl;
return 0;
}

Answer 1

#include<iostream>
using namespace std;
int main() {
float MARK=0.0, PERCENTAGE=0.0;
int NBR_MARK_10;
float NBR_MARK = 0.0;
NBR_MARK_10 = 0;
char R = 'a';
while(R!='N')
{
std::cout << "Enter a mark ?" << std::endl;
std::cin >> MARK;
while (MARK < 0 || MARK > 20) {
std::cout << " Please, enter a mark between 0 and 20" << std::endl;
std::cin >> MARK;
}
NBR_MARK+=1.0;
if (MARK > 10) NBR_MARK_10++;
std::cout << "Do you want to enter a new mark ?" << std::endl;
std::cout << "Click on 'O' to continue and on 'N' to stop" << std::endl;
cin >> R;
}
PERCENTAGE = (NBR_MARK_10 / NBR_MARK) * 100;
std::cout << "Le % de notes > 10 est de: " << PERCENTAGE << " %" << std::endl;
return 0;
}
use a while loop when your checking the next input of R and make sure you make one of the counter variables to float else youll end up getting percentage as zero for few testcases.

Answer 2

You have two problems with the code. As Sharath Chander P pointed out, there is clearly a logical issue in the code flow. Even if you press 'N' (for the first time) the code block inside do..while will execute since the condition is checked only after executing all the statements inside the do..while loop.
Now to your question on the infinite loop. It is because the expression while ((R != 'N') || (R <= 'n')); will be 'true' when you press the key 'N'. When you press 'N', the value of R will be 78 (ASCII value of 'N'). Since the ASCII value of 'n' is 110, the checking R<='n' will return true. (Clearly 78 is less than 110.)
Since (R!='N') is false and R<='n' is true, the expression while ((R != 'N') || (R <= 'n')); will be evaluated as while(false || true).
Since the result will be "true", the loop will continue.
Now if you press 'n' then the expression will be evaluated as while(true || true) that too will result in "true" and the loop will continue;