Confusion with the OR logical Operator in a Loop - c++

A bit earlier on an IRC channel, someone asked a question about his code - essentially, his program was running on an infinite loop:
#include <iostream>
using namespace std;
int main()
{
cout << "This program adds one to your input." << endl;
char choice = 'n';
int num = 0;
while (choice != 'y' || choice != 'Y')
{
cout << "Enter number: ";
cin >> num;
num++;
cout << "Your number plus 1 is: " << num << endl;
cout << endl << "Continue/Quit? Type 'Y' to quit, any other key to continue: ";
cin >> choice;
}
cout << "By choosing 'Y', you exit the loop." << endl;
return 0;
}
Paying attention to the loop header, it seems that the loop should be working perfectly fine, but whenever it comes time for me to enter Y or y in order to exit, the loop keeps running. Considering that the while loop won't evaluate the expression to the right if the one on the left is true, this makes it especially confusing. But in any case, even if I input Y or y the loop keeps running! I'd like a somewhat in-depth explanation of why this happens, I've been trying to reason it out, look back in my textbook etc. but I can't seem to understand why... I've changed the OR into an AND, but what makes OR so bad and causes it to malfunction?
Thanks all.

Condition (choice != 'y' || choice != 'Y') is always true, so loop will run indefinetely.
If choice == 'y', then you get (false || true) == true;
if choice == 'Y', then you get (true || false) == true.
You need to use while(choice != 'y' && choice != 'Y') instead, in this case you exit loop only if you get 'y' or 'Y', otherwise you get (true && true) and continue.

The OR operator between many statements returns true if at least 1 of the statements is true, no matter if the others are false or true. In your case, choice != 'y' || choice != 'Y' will be evaluated like this:
First statement is true: Execute while loop.
First statement is false: Check if the second statement is true.
Second statement is true: Execute while loop.
Second statement is false: don't execute while loop.
Specifically, when the compiler arrives at choice != 'y', if choice == 'Y', then it will still execute, because choice != 'y' is true, in fact choice is equals to Y, so it's different from y.
If, on the other hand, choice is equals to y, then it will check if the second statement is true, but we know that choice is equals to y, so choice is different from Y, and so on...

You make a mistake,you should change "||" to "&&"

Related

While Loop when to use OR or AND? [duplicate]

This question already has answers here:
While loop with multiple conditions in C++
(4 answers)
Closed 2 years ago.
I need help with this simple question.
I'm starting to learn more about while loops and I'm not sure what I'm doing wrong.
Here's a snippet of the code that I'm working on, so when I'm using or in while loop the loop executes indefinitely. But when I use AND the loop stops can somebody please explain? Shouldn't OR be used instead as per definition?
void displayMenu() {
char option;
while (option != 'Q' or 'q') { //And or OR?
cout << "P - Print numbers" << endl;
cout << "A - Add a number" << endl;
cout << "M - Display mean of the numbers" << endl;
cout << "S - Display the smallest number" << endl;
cout << "L - Display the largest number" << endl;
cout << "Q- Quit" << endl;
cout << "Choose Your Option-" << endl;
cin >> option;
if (option == 'P' or 'p')
Print();
}
}
You have phrased your condition (option != 'Q' or 'q') like you speak it.
"option not upperQ or lowerQ".
That is understood by the compiler as "option is not upperQ; or q", where "q" is a non zero thing. Nonzeros are interpreted as "true". So the whole thing always evaluates to "true", always, even for option having a values like 'Y' or '2'.
You have to rephrase much less coloqually as
"option is not 'Q' and it is not 'q' "
and you have to use the appropriate operators. Using additional () to make sure that what you mean is understood does not hurt.
That is done as
( (option != 'Q') && (option != 'q') )
Using or, or in this case and instead of the logical && is possible in certain situations, and the additional () are not required either. The main issue is however with your choice of "or" instead of "and" and using the () is a safe way as long as you are not very familiar with the operators and their order of evaluation in expressions like this one.
Why exactly you need to use && ("and") instead of or or || is a question of what happens with the two interesting letters. Lets take 'Q' it obviously is one of the two you are looking for. But it is not 'q'. So the second part of the condition with is true. With || that is sufficient and the whole thing is evaluated to "true" and the loop continues - obviously not what you want. The same is for 'q', it is not 'Q' and there for || continues.
What you actually want is "neither" and that is the same as "not this and not that". Hence you need "not and not", !a && !b or more bluntly explicit (option != 'Q') && (option != 'q').
That can be reprhased with implicit knowledge of operator precedence (look it up and try to memorize) to option != 'Q' && option != 'q'.

Input not being read properly by if condition statement

one would think this is easy, but for some odd reason, my conditional statement is ignoring user input.
If I input a character 'N' or 'n' it still executes the 'Y' portion of the conditional statement, have a look:
while (i < 10) {
cout << "Would you like "<< nameOfDish[i] << "? Please enter Y or N.\n";
cin >> userResponse;
if (userResponse == 'y' || 'Y')
{
cout << "How many orders of " << nameOfDish[i] << " would you like?\n";
cin >> quantityOfDish[i];
if (quantityOfDish[i] == 0) {
cout << "I suppose you're entitled to change your mind.\n";
}
else if (quantityOfDish[i] < 0) {
cout << "Your generosity is appreciated but I must decline!\n";
quantityOfDish[i] = 0;
}
i++;
}
else if (userResponse == 'n' || 'N')
{
i++;
}
else
{
cout << "I think you mumbled NO, so I'll just go on.\n";
i++;
}
}
Is there any particular reason why despite inputting 'n' it still goes into the 'Y' if conditional block?
I have stepped through the code in the debugger, and I noticed that the userResponse variable is being read in properly. Yet, the if condition does not seem to be working properly. Thanks!
This statement (and your other if statement) is not doing what you think it does:
if (userResponse == 'n' || 'N')
Try this instead:
if (userResponse == 'n' || userResponse =='N')
You need to define each logical operation individually in a condition check. You will have to compare userResponse with n and N separately.
if (userResponse == 'y' || userResponse == 'Y')
{
cout << "How many orders of " << nameOfDish[i] << " would you like?\n";
cin >> quantityOfDish[i];
if (quantityOfDish[i] == 0) {
cout << "I suppose you're entitled to change your mind.\n";
}
else if (quantityOfDish[i] < 0) {
cout << "Your generosity is appreciated but I must decline!\n";
quantityOfDish[i] = 0;
}
i++;
}
It's been awhile since I worked in C++, but I'm fairly sure I know what's going on.
The || operator does not work on a single conditional, there must be two complete conditionals, one on each side. Try replacing your if statement with this line:
if (userResponse == 'y' || userResponse == 'Y')
Maybe you are used to SQL? You need to repeat the userResponse
if userResponse == 'n' || userResponse == 'N'
Otherwise you are actually testing
if userResponse is 'n' or the char'N' exists
The error in this code is, as others have pointed out, the if statement. However, I feel this may need some clarification. Every C++ expression returns a value. For example.
userResponse == 'y'
returns the value 1 if userResponse is 'y' and 0 if it is anything else. The operator || returns 1 if either the left or the right expression is non-zero.
Finally, the if statement checks to see whether or not the expression is zero or non-zero. So,
if (5)
cout << "X";
else
cout << "Y";
will print X and
if (0)
cout << "A";
else
cout << "B";
will print B.
Now, we can begin to understand why your code compiled successfully, but didn't do what you wanted it to.
if (userResponse == 'y' || 'Y')
In this example, the || operator will always return 1 because the expression on the right, 'Y', will always be non-zero (specifically, it will be 89, since C++ characters are just aliases for their ASCII corresponding number). And of course,
if (userResponse == 'y' || userResponse == 'Y')
work as intended. But there is a much better solution and that would be the switch statement, whose purpose is to handle situations like this. Here it is in action:
switch (userResponse) {
case 'y':
case 'Y':
//The user answered yes, handle that situation here.
break;
case 'n':
case 'N':
//The user answered no, handle that situation here.
break;
default:
// The user did not enter a valid answer,
// handle that situation here.
break;
}

While loop not terminating C++

I would like to keep inputting integers to the P1 vector until a break point in this case 'q' or 'Q' is entered. The program when ran goes crazy into an endless loop once the break condition is met. Any ideas on a work around, all I can see is that because the 'q' or 'Q' is a character the integer vector is taking this as an input when the while loop runs at which point it endless loops?
#include "stdafx.h"
#include <iostream>
#include <vector>
using namespace std;
int main()
{
//Declaring Polynomial 1 and 2
vector<int> P1;
vector<int> P2;
int x = 0;
int y = 0;
while (x != 'q'||x != 'Q') {
cout << "Please enter in the first polynomial one value at a time (Press Q when done)...";
cin >> x;
P1.push_back(x);
}
//Also tested with a do while same problem
/*do
{
cout << "Please enter in the first polynomial one value at a time (Press Q when done)...";
cin >> x;
P1.push_back(x);
} while (x != 'q');*/
//Ignore this is for next part of program
vector<int> Res((P1.size() + P2.size()) + 1);
cout << P1.size() << "," << P2.size() << "," << Res.size();
return 0;
}
(x != 'q'||x != 'Q') <---- here is an error, it is obviously always true: when x==q -> true, because (x! = 'Q') == true, and the other way around. change || to &&.
The condition:
(x != 'q' || x != 'Q')
is always true resulting in an endless loop. Why that is in more details:
The x integer variable is initialized to 0. Then you check if x is different than 'q' which represents an integer value of 111. It is not equal to that value so the expression of x != 'q' is true. Then you check if x is not equal to 'Q' character which represents an integer value of 85. It is not equal to that value so the expression of x != 'Q' is also true. We end up with a condition of (true || true) which is always true.
Integral values are implicitly convertible to boolean values where 0 represents false and any other number represent true. Try something like this instead:
char c = 'y';
while (std::cin && (c == 'y' || c == 'Y')) {
// do work
std::cout << "Do you want to repeat the input? y / n?";
std::cin >> c;
}
That being said you don't need the "stdafx.h" header.
First I think you should not use int x
Second just like Ron said change || to &&
Anyway, Ron gave an example and it works.

C++ equality check on char from cin against another char never equates to true??? (No compiler errors)

I'm stuck as to why the condition below isn't triggering when either an 'n' or a 'y' is entered at the console. When executed you can't get out the the if statement, but i know for sure that
!(cin >> again)
isn't the culprit, as that was previously the only condition in the if statement and I was able to skip/enter the if block if a character/numeral was entered, which was as expected. Here is the code:
char again;
while (1) {
cout << endl;
cout << "I see another one, care to shoot again? (y/n): ";
if (!(cin >> again) || (again != 'n') || (again != 'y')) {
// Error checking for numberals & non 'y' or 'n' characters
cout << "Please enter 'y' or 'n' only." << endl;
cin.clear();
cin.ignore(1000, '\n');
continue;
}
break;
}
I'm stumped on this so any help would be hugely appreciated!
if(...|| (again != 'n') || (again != 'y')) {
is faulty logic. What you say is
if "again" is not n or it's not y, then do the following...
now, since "again" can't be n and y at the same time, this always evaluates to true; most probably, even your compiler notices that and just jumps right into your if's content.
What you want is something like
if(!(cin>>again) || ( again != 'n' && again != 'y') {
Because that reads
if cin>>again didn't work or again is neither n nor y then,...

while loop chars C++

I'm working on a Programming assignment and I can't figure out why this while loop is looping infinitely. There has been other input before this loop occurs, but I have cin.ignore() to clear any junk out before this loop. I'm scratching my brain, help?
//Prompt for readiness
char ready = 'x';
while (ready != 'Y' || ready != 'y') {
cout << "READY TO START YOUR TURN " << playerName << "? ";
ready = cin.get();
}
You need to change || (OR) to && (AND). Also, reset the stream after you read with cin.get(), i.e. put the following line after:
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
std::cin.clear();
(you need to #include <limits>)
Otherwise, cin.get() returns a char and leaves an extra \n (newline) in the stream, that gets read once more and you basically end up with the loop executing twice for each non-yes response.
think of boolean algebra. OR gate results in true if one or both sides are true. so, if you input 'y' or 'Y', your statement will result in returning true, since of the sides would still be true.
if you change || to && (AND gate), which returns true only if both of the sides are true, then your statement makes sense. user inputs 'y' or 'Y' (he's ready, so get him out of that loop!) and one of the sides is equal to false, hence loop terminates.
while (ready != 'Y' && ready != 'y') {
or
while (ready == 'Y' || ready == 'y') {
are statements that you meant.
change the code to this -
char ready = 'x';
while (ready != 'Y' && ready != 'y') {
cout << "READY TO START YOUR TURN " << playerName << "? ";
ready = cin.get();
}
It will resolve your issue.