How to print out a double value without losing first digit - c++

When I run my code, it only prints the decimal parts of the double. On another page, I took a inputted double and printed out the double the way it was inputted.
But for my following code, it only prints out the decimals. For example, when I input 1.95 it only prints out 0.95. Why is it removing the first digit? I see nothing in my code that points to this.
I have already tried it in a more simple way and it worked. And I dont see any problems that would mess with the double in my code.
#include <iostream>
using namespace std;
int main()
{
double price;
char user_input;
do
{
cout << "Enter the purchase price (xx.xx) or `q' to quit: ";
cin >> user_input;
if (user_input == 'q')
{
return 0;
}
else
{
cin >> price;
int multiple = price * 100;
if (multiple % 5 == 0)
{
break;
}
else
{
cout << "Illegal price: Must be a non-negative multiple of 5 cents.\n" << endl;
}
}
} while (user_input != 'q');
cout << price << endl;
}
When I input 1.95, I get 0.95. But the output should be 1.95.

Problem covered in other answer: Reading for the 'q' removed the first character from the stream before it could be parsed into a double.
A solution: Read the double first. If the read fails, check to see if the input is a 'q'.
#include <iostream>
#include <limits>
using namespace std;
int main()
{
double price;
while (true)
{
cout << "Enter the purchase price (xx.xx) or `q' to quit: ";
if (cin >> price)
{
// use price
}
else // reading price failed. Find out why.
{
if (!cin.eof()) // didn't hit the end of the stream
{
// clear fail flag
cin.clear();
char user_input;
if (cin >> user_input && user_input == 'q') // test for q
{
break; // note: Not return. Cannot print price if the
// program returns
}
// Not a q or not readable. clean up whatever crap is still
// in the stream
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
}
else
{
// someone closed the stream. Not much you can do here but exit
cerr << "Stream closed or broken. Cannot continue.";
return -1;
}
}
}
cout << price << endl;// Undefined behaviour if price was never set.
}
Another reasonable alternative is to read all input as std::string. If the string is not "q", attempt to convert it to a double with std::stod or an std::istringstream.

When you type 1.95 in the command line, variable user_input gets assigned '1', and price gets assigned .95.

Related

Whats a good way to get the program to end based on user input?

I did my "Hello World", I'm just getting started on my programming adventure with C++. Here is the first thing I've written, what are some ways to get it to end with user input? I'd like a yes or no option that would terminate the program. Also any feedback is welcome, thank you
#include <iostream>
using namespace std;
void Welcome();
void calculateNum();
void tryAgain();
int main() {
Welcome();
while (true) {
calculateNum();
tryAgain();
}
system("pause");
}
void calculateNum() {
float userNumber;
cin >> userNumber;
for (int i = 100; i >= 1; i--) {
float cNumber = i* userNumber;
cout << i << " >>>>> " << cNumber << endl;
}
}
void Welcome() {
cout << "Welcome \n Enter a number to see the first 100 multiples \n";
}
void tryAgain() {
cout << "Try again? Enter another number... ";
}
Here is one option:
Switch to do ... while loop, with the condition at the end.
Make your tryAgain() function return a boolean and put it in the while condition.
In tryAgain function read input from the user, and compare it to expected answers.
First, lets add a new header for string, it will make some things easier:
#include <string>
Second, lets rebuild the loop:
do {
calculateNum();
} while (tryAgain());
And finally, lets modify the function:
bool tryAgain() {
string answer;
cout << "Try again? (yes / no)\n";
cin >> answer;
if (answer == "yes") return true;
return false;
}
Now, there is a slightly shorter way to write that return, but it might be confusing for new learners:
return answer == "yes";
You don't need the if because == is an operator that returns bool type value.
You can change your calculateNum() in the following way:
Change the return value of your calculateNum() function into bool to indicate whether the program shall continue or stop
read the input into a std::string
check if the string is equal to your exit string like 'q' for quit
3.a in that case, your function returns false to indicate the caller that the program shall stop
3.b otherwise, create a stringstream with your string and read the content of the stream into your float variable and continue as you do like now
In your loop in your main function you break if calculateNum() returned false
Here is a simple solution:
#include <iostream>
// Here are two new Includes!
#include <sstream>
#include <string>
using namespace std;
void Welcome();
// Change return value of calculateNum()
bool calculateNum();
void tryAgain();
int main()
{
Welcome();
while (true)
{
if (!calculateNum())
break;
tryAgain();
}
system("pause");
}
bool calculateNum()
{
//Read input into string
string userInput;
cin >> userInput;
//Check for quit - string - here just simple q
if (userInput == "q")
return false;
//otherwise use a std::stringstream to read the string into a float as done before from cin.
float userNumber;
stringstream ss(userInput);
ss >> userNumber;
//and proces your numbers as before
for (int i = 100; i >= 1; i--)
{
float cNumber = i * userNumber;
cout << i << " >>>>> " << cNumber << endl;
}
return true;
}
void Welcome()
{
cout << "Welcome \n Enter a number to see the first 100 multiples \n";
}
void tryAgain()
{
cout << "Try again? Enter another number... ";
}
Having your users input in a string you can even do further checks like checking if the user entered a valid number, interpret localized numbers like . and , for decimal delimitters depending on your system settings and so on.

Undefined behaviour when entering `char` for integer variable with `std::cin`

I have this while loop, just to check if the entered number is 2. If the user entered by accident a letter instead of a number the loop goes to infinity even though I've added isdigit, but didn't fix the loop from going crazy if the input is a character. This is code:
int num1;
bool cond {false};
while(!cond){
cout<<"enter 2:";
cin>>num1;
if (!isdigit(num1)){
cout<<"not a digit:";
cin>>num1;
}
//
if(num1 == 2)
cond = true;
}
I would suggest trying something a little more straightforward instead of your current code:
#include <iostream>
#include <limits>
using namespace std;
int main()
{
int num1;
cout << "Please enter the number 2:";
cin >> num1;
while (num1 != 2)
{
cin.clear(); //Clears the error flag on cin.
cin.ignore(numeric_limits<streamsize>::max(), '\n');
cout << "You did not enter the number 2, Please try again:";
cin >> num1;
}
return 0;
}
Now, cin.ignore(numeric_limits<streamsize>::max(), '\n'); is when it ignores up until '\n' or EOF \n is the delimiter meaning that, that is the character at which cin will stop ignoring.
Furthermore, numeric_limits<streamsize>::max() is basically saying there is no limit to the number of characters to ignore.
You need to use the header file #include<limits> to use this.
I recommend separating the reading of input data from converting it to a number. A good method for this is to use stringstream. Here's a working example:
#include <iostream>
#include <string>
#include <sstream>
using namespace std;
int main()
{
int num1;
string input;
bool cond{ false };
cout << "enter 2:";
while (!cond) {
getline(cin, input);
stringstream ss(input);
ss >> num1;
if( ss.fail() ){
cout << "not a digit:";
continue;
}
//
if (num1 == 2)
cond = true;
else
cout << "enter 2:";
}
return 0;
}
int num1;
bool cond {false};
do{
cout<<"enter 2:";
cin>>num1;
if (cin.good()) {
cond = true;
}else {
cin.clear();
cin.ignore();
cout << "Invalid, please enter 2." << endl;
}
}while(!cond);
While false, execute statements. Also, if you want the user to re-enter a number, cin must be flushed.
Try declaring the variable num1 as char because isdigit(ch) works for char and not for int.
I hope this solves your problem
Why does the loop iterate infinitely?
Let's take this example
int a;
std::cin >> a;
std::cout << "Entered: " << a;
Now let's test it with different inputs
with int
5
Entered: 5
10
Entered: 10
Yes just as we would expect, but what happens when you enter a char?
with char
r
Entered: 0
f
Entered: 0
Why does this happen?
When you declare the variable int, and then do std::cin >> , you are telling the input method that the user will enter an integer, but when it doesn't get what it expected, it will fail. C++ will not implicitly convert the value of char into int. Hence, you get strange results.
How to solve this?
As I have mentioned earlier, it fails. When this fails you can catch it this way
if (!(std::cin >> a))
{
std::cout << "Invalid input ! \n";
}
We're saying, if the input fails, show the message.
let's test this new code.
int a;
if (!(std::cin >> a))
{
std::cout << "Invalid input !\n";
}
else
{
std::cout << "Entered: " << a;
}
5
Entered: 5
r
Invalid input !
How to print char value of the int?
If you want to just print the ASCII value of the entered number, you need to cast the value into char.
You can do
int num = 48;
char character_value = num;
Here C++ will implicitly convert char into int.
But if you need a safer type of conversion, prefer using static_cast<>.
The syntax looks like this
int a = 5;
char character_value = static_cast<char>(a);
Static cast in C++
Type casting in C++
Dealing with invalid input in C++

Detect blank input on integer type variable?

I am currently working on a text based adventure game as a project for class. I have mostly everything started and working fine. The only problem is when I ask the user which room they want to change to, if they enter a blank input, then a message should output saying "You must choose a room." For the life of me I cannot figure it out. Any help is much appreciated.
Code:
#include <iostream>
#include <cstdlib>
#include <ctime>
using namespace std;
int main() {
bool game_play = true;
bool game_start = true;
int room_change;
int room_current = 0;
while (game_play == true) {
if (game_start == true) {
srand((unsigned int)time(NULL));
room_change = rand() % 2 + 1;
game_start = false;
}
else {
for (bool check = false; check == false;) { // Check if input is invalid
cin >> room_change;
if (cin.fail()) {
cout << "Choose an existing room.";
cin.clear();
cin.ignore();
}
else if (room_change == room_current) {
cout << "You're already in that room.";
}
else {
check = true;
}
}
}
switch (room_change) {
case 1:
cout << "You are in room 1.";
room_current = 1;
break;
case 2:
cout << "You are in room 2.";
room_current = 2;
break;
case 3:
game_play = false;
break;
default:
cout << "That room doesn't exist.";
}
}
return 0;
}
I just ran your code and when you hit enter, it will keep waiting until you enter a number or something invalid such as a character or a string. I did find that if you change your code from
cin >> room_change;
to
cin >> noskipws >> room_change;
when the user inputs a blank, it will cause the cin.fail() to return true and then proceed to print "Choose an existing room."
In your situation, the while loop will keep getting called until we have valid input. The "Choose an existing room" does get repeated because room_change is an integer, so when we hit enter, the '\n' will be left in the buffer. The while loop on the next iteration then reads that '\n' and executes the cin.fail() before letting you input something else. One solution I found is to use more cin.ignore() statements.
for (bool check = false; check == false;) { // Check if input is invalid
cin >> noskipws >> room_change;
if (cin.fail()) {
cout << "Choose an existing room.";
cin.clear();
cin.ignore();
} else if (room_change == room_current) {
cout << "You're already in that room.";
cin.ignore();
} else {
check = true;
cin.ignore();
}
}
The reason is because we want to get rid of that '\n' so that the cin.fail() does not execute. However, I did find that when you input a character, it will print "Choose an existing room" twice. It will print the first time because a character is not an integer, and a second time because of that '\n'.
The only problem is when I ask the user which room they want to change to, if they enter a blank input, then a message should output saying "You must choose a room."
Using std::getline and then extracting the number from the line using a std::istringstream is a better strategy for that.
std::string line;
std::cout << "Choose an existing room. ";
while ( std::getline(std::cin, line) )
{
// Try to get the room_change using istringstream.
std::istringstream str(line);
if ( str >> room_change )
{
// Successfully read the room.
break;
}
// Problem reading room_change.
// Try again.
std::cout << "Choose an existing room. ";
}
#include <iostream>
using namespace std;
int main(){
int room_change=200;
cout<<"Enter Blank";
cin>>room_change;
if(room_change==NULL){
cout<<"There is NO-THING"<<endl;
}
if(room_change!=NULL){
cout<<"There is something and that is :"<<room_change<<endl;
}
return 0;
}
But a much simpler approach to this would be to use Strings. If this is a Homework of sort and you are limited to Integer variable only. Its much more complicated if you want to detect if an Buffer is empty or not. Regardless of homework limitation, the OS layer input is String based. How can I use cin.get() to detect an empty user input?

C++ - cin to only take one integer

This code works fine if I enter something that isn't a number in, e.g. F: it will print the error message. However, if I enter e.g. 2F2 or , it will take the 2 and pass the check, continue in my code and on the next cin >> statement it will put the F in, and then it loops back and puts the 2 in.
How do I make it so it only accepts a single number e.g. 2 and not e.g. 2F2 or 2.2?
int bet = 0;
// User input for bet
cout << " Place your bet: ";
cin >> bet;
cout <<
// Check if the bet is a number
if (!cin.good())
{
cin.clear();
cin.ignore();
cout << endl << "Please enter a valid number" << endl;
return;
}
bool Checknum(std::string line) {
bool isnum = true;
int decimalpoint = 0;
for (unsigned int i = 0; i < line.length(); ++i) {
if (isdigit(line[i]) == false) {
if (line[i] == '.') {
++decimalpoint; // Checks if the input has a decimal point that is causing the error.
}
else {
isnum = false;
break;
}
}
}
if (decimalpoint > 1) // If it has more than one decimal point.
isnum = false;
return isnum;
}
If you take a string from the user, this should work. You can convert the string to an integer or a float(stoi or stof, respectively). It may not be the best solution there is, but this is what I have. Excuse the indentation.
Do getline to read one whole line of input from cin.
Create a stringstream to parse the string you got.
In this parser, read the number; if it fails - error
Read whitespace; if it doesn't arrive to the end of string - error
#include <sstream>
...
int bet = 0;
std::cout << " Place your bet: ";
while (true)
{
std::string temp_str;
std::getline(cin, temp_str);
std::stringstream parser(temp_str);
if (parser >> bet && (parser >> std::ws).eof())
break; // success
cout << endl << "Please enter a valid number" << endl;
}
This code keeps printing the error message until it receives valid input. Not sure this is exactly what you want, but it's pretty customary UI.
Here >> ws means "read all the whitespace". And eof ("end of file") means "end of the input string".

Why an Infinite Loop

I've been learning C++, and this chunk of code is from a simple grading program. But when I try to get the user input, there's a problem.
If I enter a number, whether it be less than 0 or more than 100, or anything in between, my loop works fine.
But if I type in any letter, or any non-alphanumeric character (ex: +, (, %, etc.) I get an infinite loop with "Please enter a grade value between 0 and 100" printed forever.
What am I doing wrong?
Thanks.
int _tmain(int argc, _TCHAR* argv[])
{
using namespace std;
int grade = -1; // grade will hold grade value; initialized to -1
do {
cout << "Please enter a grade value between 0 and 100." << "\n";
cin >> grade;
} while (grade < 0 || grade > 100);
cout << grade << "\n";
printGrade(grade);
return 0;
}
if cin>>grade fails (aka can't parse as int) it does not consume the stream. You can try:
int main()
{ using namespace std;
int grade = -1; // grade will hold grade value; initialized to -1
do {
cout << "Please enter a grade value between 0 and 100." << "\n";
if (!(cin >> grade))
{
cin.clear();
}
} while (grade < 0 || grade > 100);
cout << grade << "\n";
return 0;
}
But this is only part of the problem. Really, you should use std::getline and parse the grade as a full line for correct input.
If cin does not receive valid input for the data type (int), the variable grade is not changed and remains at -1. You can test whether the input was successful like so
bool success = (cin >> grade);
if (! success)
{
cin.clear();
cout << "bad input\n";
break;
}
You can also use this as a shortcut if (! (cin >> grade))
Note that you need to clear the error state of cin before you use it again.
I'm pretty sure the cin failed so you may need to reset its fail flag or something like that.
Add this to your loop:
if (cin.fail())
{
cout << "failed";
cin.clear();
}
Correctly and safely reading until you get valid input is far trickier than you'd think. If there's invalid input, like a letter, the stream is set in a "failure" state, and refuses to read any more characters until you clear the state. But even if you clear the state, that input is still waiting there, in the way. So you have to ignore those characters. The easiest thing to do is simply ignore everything until the next enter key, and then try the input again.
But it gets even more complicated, because if the stream has an error, it gets set in a "bad" state, or if it reaches the end of the stream, it gets set in a "eof" state. Neither of these two are recoverable, so you must detect them and quit the program to avoid an infinite loop. Even more irritating, istreams have a .fail() function, but that checks if it's in fail or bad, which makes it nearly useless in my opinion. So I wrote a little invalid_input which checks if the stream can continue.
Note that get_grade sets the fail flag manually if the input is out-of-range.
#include <iostream>
#include <stdlib.h>
#include <limits>
bool invalid_input(std::istream& in)
{return in.rdstate() == std::ios::failbit;}
std::istream& get_single_grade(std::istream& in, int& grade) {
std::cout << "Please enter a grade value between 0 and 100." << "\n";
if (in>>grade && (grade<0 || grade>100))
in.setstate(std::ios::failbit);
return in;
}
bool get_grade(std::istream& in, int &grade) {
while(invalid_input(get_single_grade(in, grade))) { //while we failed to get data
in.clear(); //clear the failure flag
//ignore the line that the user entered, try to read the next line instead
in.ignore(std::numeric_limits<std::streamsize>::max(),'\n');
}
return in.good();
}
int main(int argc, char* argv[]) {
int grade = -1; // grade will hold grade value; initialized to -1
if (get_grade(std::cin, grade) == false) {
std::cerr << "unxpected EOF or stream error!\n";
return false;
}
std::cout << grade << "\n";
return EXIT_SUCCESS;
}
As you can see here, this doesn't get into an infinite loop when given out of bounds numbers, end of file, a stream failure, or invalid characters.