first of all I'm sorry for my
bad english.
I'm trying to read some numbers and write them into a vector in C++.
This should go as long as the input is a double number and the
loop should be stopped if the user writes an 'a'.
My Question is how can I check if the input is 'a'.
Breaking the loop is not the problem
while(true){
if(!(cin>>userInput)){
//here i want to know if the input is 'a' or some other stuff//
//also i want to do some other stuff like printing everything//
//what already is in the vector//
//when everything is done; break//
}
else
//the input is a valid number and i push it into my vector//
'userInput' is defined as double so the loop will stop.
My Problem is, if the user write 'q' the loop stops but it's instantly stoping the whole program. My try look like this:
while(true){ //read as long as you can
cout<<"Input a number. With 'q' you can stop: "<<endl;
if(!(cin>>userInput)){ //here the progam stops when the input is anything but a number
cout<<"How many numbers do you want to add up?"<<endl; //there are numbers in a vector that should be added up
cin>>numberOfAdditions;
break;
}
So I have a vector with some numbers the users writes down (20,50,90,...)
When the input is equal to 'q' (in this example everything but numbers )
the loop stops and I want to ask the user how many numbers should be added.
The cout-command is displayed but the input is beeing skipped.
So my program is not reading how many valued from the vector I want to add.
I hope you know what I mean and I don't want to use two questions and two variables to save the input but if it's not working without it I'll change my program.
Have a nice Day :)
Because your Input variable is of type double you have to flush the Input from cin before reading again. Otherwise there is still a newline in the buffer.
Consider the following example:
#include <iostream>
using namespace std;
int main(){
double userInput;
int numberOfAdditions;
while(true){ //read as long as you can
cout<<"Input a number. With 'q' you can stop: "<<endl;
if(!(cin>>userInput)){ //here the progam stops when the input is anything but a number
cout<<"How many numbers do you want to add up?"<<endl; //there are numbers in a vector that should be added up
cin.ignore (std::numeric_limits<std::streamsize>::max(), '\n');
cin.clear();
cin.ignore(INT_MAX,'\n');
cin >> numberOfAdditions;
break;
}
}
return 0;
}
The two Statements:
cin.clear();
cin.ignore(INT_MAX,'\n');
are flushing the Input stream until the newline is encountered.
The first answer already explains how to flush the cin stream after the user types in a char.
If you want to determine which character it was, you should define userInput as std::string. If the string is not "q" or "a" or whatever you are looking for, you have to cast the string to a double, just like this:
std::string str;
cin >> str;
if (str == "j")
// User typed in a special character
// ...some code...
else
double d = atof(str.c_str()); // Cast user input to double
Notice that the result of the cast is zero, if the user typed in any other string than the ones you especially look for.
Related
I was reading a book on c++ (c++ primer plus) and found this code.
The purpose of the program is that if a numeric input is expected from the user to read it to ,say an array->
1)Reset cin to new input
2)Get rid of the bad input
3)Prompt the user to try again (exact words of the book)
Here is the code->(exact code copied from the book)
#include <iostream>
const int Max=5;
int main()
{
using namespace std;
//get data
int golf[Max];
cout<<"please enter your golf scores.\n";
cout<<"you must enter "<<Max<<" rounds.\n";
int i;
for(i=0;i<Max;i++) {
cout<<"round #"<<i+1<<": ";
while(!(cin>>golf[i])) {
cin.clear();
while(cin.get()!='\n')
continue;
cout<<"PLease enter a number: ";
}
}
double total=0.0;
for(i=0;i<Max;i++)
total+=golf[i];
cout<<total/Max<<" = average score "<<Max<<" rounds\n";
return 0;
}
The particular part which i don't understand is:
cin.clear();
while(cin.get()!='\n')
continue;
I'm a little unclear about the function of cin.clear() , the need of continue here , and what the while test conidtion does and how does it work.
A test run->
(italic part is the user input)
Please enter your golf scores.
You must enter 5 rounds.
round #1: 88
round #2: 87
round #3: duh
Please enter a number: 103
round #4: 94
round #5: 86
91.6 = average score 5 rounds
while(!(cin>>golf[i]))
{
cin.clear();
while(cin.get()!='\n')
continue;
cout<<"PLease enter a number: ";
}
First let's example the while condition. It seems to check if the value of the expression (cin >> golf[i]) is false. The falue of cin >> golf[i] is cin itself. So how can it be false? If a >> operation fails for the input stream, a flag named fail is set. That's what operator ! checks. So the condition means while after trying to read a number the object cin has fail set, do...
Now, once the fail flag is set, there isn't much you can do with an input stream. You won't be able to continue reading data from the stream unless you reset the fail flag. That's what cin.clear() does.
Then it munches the rest of the bad input line (which was NOT consumed since >> failed) by reading character by character until reaching the end of line.
And finally it prompts the user for new input.
Since golf[i] is an int, cin>>golf[i] will return true (or a value interpreted as true in earlier versions of C++ Standard Library) when the input is an int. Hence, while(!(cin>>golf[i])) will repeat until true is returned.
Inside the while loop you find code that ignores all input until a \n with the loop that stops only after receiving '\n' input:
while(cin.get()!='\n')
continue;
It also clears the flag indicating that cin is in a failure state by calling clear().
Note: This code makes an assumption that cin never ends, which makes it unsafe. If the input is taken from a file with invalid data, this program will be stuck in an infinite loop.
I am writing a code that tally when an integer data type or character is entered.
int numero,
countInteger = 0;
countCharacter = 0;
while ( 1 ) {
try {
cin >> numero;
cout << numero;
throw numero;
} catch (...) {
countCharacter++;
cout << "Error";
}
}
If I entered Integer, counter in "countInteger" (but not show it in the code). If I enter a character, it is aa exception and recorded in "countCharacter".
But when I run the code generates an infinite loop and does not allow me to re-enter again. They could help me please. Guide me, you may have a bad concept.
When you try to read an integer, and you give something that's not an integer as input, there are two things happening: The first is that the stream gets its failbit set, the second things that happens is that the input is not extracted. So next iteration you read the same input again, and again and again...
I suggest another tactic: Read as a character, then see if it is a digit, an alphabetic character, or something else completely. Optionally, if you need the actual full number, read as a string, and try to convert to an integer.
A clarification: Input using std::cin is buffered. When you use the input operator >> then std::cin extracts characters from the buffer. If you try to read a number, but the first character in the buffer is not a digit, then the input operator will fail, and leave the character in the buffer.
Simple (hopefully) example:
Lets say you have this code
int number;
std::cin >> number;
std::cin >> number;
std::cin >> number;
As input for that part of the code, you enter
123abc
The first input will read 123 from the input, and stop at the letter, leaving the input as
abc
Now we come to the second input, and the code will see that the first character is not a digit, so it will set the failbit in the stream and leave the input as is:
abc
Then with the third input, the exact same thing as in the second happen.
Now imagine this was in a loop instead, the input operator >> will iteration after iteration see the non-digit input an promptly return, effectively giving you an infinite loop.
Now for a clarification of my suggestion... Depending on the goals and requirements of the program, you can instead read into a character and use the character classification functions to see what types you have.
Something like
int countDigit = 0;
int countCharacter = 0;
char ch;
while (std::cin >> ch)
{
if (std::isdigit(ch))
++countDigit;
else if (std::isalpha(ch))
++countCharacter;
else
{
// Not a digit or an alphabetic character
// I.e. newlines, spaces, control characters, etc.
}
}
Finally a note about using exceptions for this: Most people would consider it bad. Exceptions are for exceptions, exceptional cases, not as part of the normal flow of the program. Throwing an exception is expensive and disrupts the normal flow. Only use them for exceptional things, like errors.
I have to loops to gather input, the first gathers input into a vector of doubles...
double input;
while (cin >> input)
{
list.push_back(input);
}
and the second gathers input into a vector of ints...
int input;
while (cin >> input)
{
list.push_back(input);
}
The second loop keeps auto-exiting and so I added the following two lines...
cin.clear();
cin.ignore(INT_MAX,'\n'); // I've also tried cin.ignore()
However this has occurred in my output being discarded in the second loop. How can I get both of these to work the way they need to?
The first loop reads until the stream goes bad because there isn't any double to read from. When the stream has gone bad it won't become good again unless you do something, e.g., using std::cin.clear() to clear the state flags.
Of course, just clearing the state flags won't make much of a difference because it would have read all viable numbers: the format of valid doubles is a superset of the format of valid ints (well, OK, unless they are using base 16). That is, you'll need some sort of separator, probably a non-space, non-digit character. Your use of ignore() should skip over a separator and up to the next newline (although the magic value isn't INT_MAX but std::numeric_limits<std::streamsize>::max()).
It is unclear what you are trying to input but if you use something like
1 2 3 exit 4 5 6
the first three values would be read as double and everything else would be ignored. That is, you may want to be ignore characters a bit more careful, e.g., clear the input and keep trying to read an int and clear() and ignore() until this is successful:
// skip separator:
std::cin.clear();
int input;
while (!(std::cin >> input)) {
std::cin.clear();
std::cin.ignore(); // ignore the next character only
}
do {
list.push_back(input);
} while (std::cin >> input);
I have this code snippet that is supposed to test whether the user enters an integer or not. This works if the user enters letters, but not decimals and I'm left wondering why that is. Here's my code snippet:
Student student;
int id;
while(!(cin >> id))
{
cout << "\nERROR: Please enter a Positive Whole Number" << endl;
cin.clear();
cin.ignore ();
cout << "Enter Student ID: ";
}
Entering A will make it iterate through the while loop, but if I enter 12.5 it drops out of the while loop and keeps going. Isn't it testing whether it will parse to integer or not? Why is it accepting 12.5 but not characters?
cin>>id will succeed as long as it finds something it can convert to an int ("12", in this case). When it reaches something it can't convert, it stops, but if it's read an int already, that counts as success.
To check that everything it read was digits, you might want to do something like using std::getline to read a line of input into a string, then use std::isdigit to test whether those are all digits. Testing a conversion to int (by itself) will only tell you that it found something that could be read as an integer, but won't tell you if that was followed by other things that couldn't be converted to an int.
I rewrote this loop in several ways, with nested Ifs and do whiles, yet behavior is the same. It behaves as expected as long as the user does not enter a character or a string. Once the user does it just goes on spinning the loop until I CTRL+C it.
From what I have researched, when a variable is a number and the user inputs a char or a string, they just get converted into their ASCII numbers, in which case the while check should work. The number should be larger than allowed and the user should be prompted for a new value right? Why does it keep looping infinitely?
Width is declared as a float.
void setWidth ()
{
std::cout << "\nPlease enter the width (use numbers greater than 0 and no greater than 20.0).\n";
std::cin >> width;
while (width <= 0 || width > 20)
{
std::cin.clear();
std::cin.ignore();
std::cout << "You have entered a number outside of the allowed range.\nPlease enter a number greater than 0 and no greater than 20.\n";
std::cin >> width;
}
}
Like I said, for numbers it works great, doubles, negatives, whatever. But something like "asdf" or "a" will put it in infinitely spinning loop.
It seems like I've tried everything. Why does this happen? I mean I know why it loops, it's because the number is not between 0 and 20, but why does it not ask user for input? I do clear the buffer.
The line std::cin >> width; fails because the input isn't a number. It also doesn't consume any of the input, so you are stuck in an infinite loop.
To avoid this, you should read the input using std::getline(), then try to convert it (std::ostringstream is one option), handling and reporting failures accordingly.
The default for cin.ignore() is to ignore just a single character.
If you want to ignore longer strings, you have to add extra parameters for that, perhaps cin.ignore(1000, '\n') which skips up to 1000 characters or the next newline (whichever comes first).
cin::clear() "Sets a new value for the error control state" 1, but the remaining input is still here and still read.
Then I guess the actual behavior depends on the compiler since when I compile it with g++ 4.6.3 and type the input "abc", it only loops three times and then wait for another input.
To empty the cin buffer you may rather see How do I flush the cin buffer?
Try checking the failbit on cin
Ok, thanks for all the help guys... I finally managed to get it to work with cin (not getline), by doing exactly what ive been doing, except I made a clearBuffer() function. So instead of clearing the buffer from within the getWidth function, the getWidth function calls another function.. thereby leaves the getWidth function to execute some code... then comes back to run the rest of it...
For some reason when it goes outside of the function it works fine and strings and chars trigger the error.. but if the cin.clear and cin.ignore are kept within the function then I have that problem.
So the final code looks like this.
void clearBuffer()
{
std::cin.clear();
std::cin.ignore(80, '\n'); //Ignore the first 80 characters up to an Enter character.
}
void setWidth ()
{
std::cout << "\n\t\tPlease enter the width.\n(use numbers greater than 0 and no greater than 20.0).\n";
float temp = NULL; //Using temp here so that we dont write invalid characters to an actual variable.
std::cin >> temp;
clearBuffer();
while (temp <= 0 || temp > 20)
{
std::cout << "\nERROR: You have entered width outside of the allowed range.\nPlease enter a number greater than 0 and no greater than 20.\n";
std::cin >> temp;
clearBuffer();
}
if(temp > 0 && temp <= 20)
width=temp;
}