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.
Related
I have these 2 codes:
char a[256];
cin>>a;
cout<<a;
and
char a[256];
cin.get(a,256);cin.get();
cout<<a;
and maybe, relative to the second one without cin.get();
char a[256];
cin.get(a,256);
cout<<a;
My question is (first one) : for a char array, what should i use? cin or cin.get()? And why should i use cin.get(); with no parameter after my char initialisation?
And my second question is: my c++ teacher taught me to use every time cin.get() for initialisation chars and AFTER every initialisation char array or int array or just int or whatever, to again put cin.get(); after it. That's what i wanted to ask initially.
So, now i got these 2:
In this case, without cin.get() after the integer initialisation, my program will break and i can't do anymore my char initialisation.
int n;
cin>>n;
char a[256];
cin.get(a,256); cin.get(); // with or without cin.get();?
cout<<a;
And the correct one:
int n;
cin>>n; cin.get();
char a[256];
cin.get(a,256); cin.get(); // again, with or without?
cout<<a;
So, what's the matter? Please someone explain for every case ! Thank you.
They do different things, so choose whichever does what you want, or the better alternatives given below.
The first cin>>a; reads a single word, skipping over any leading space characters, and stopping when it encounters a space character (which includes the end of the line).
The second cin.get(a,256);cin.get(); reads a whole line, then consumes the end-of-line character so that repeating this will read the next line. cin.getline(a,256) is a slightly neater way to do this.
The third cin.get(a,256) reads a whole line but leaves the end-of-line character in the stream, so that repeating this will give no more input.
In each case, you'll get some kind of ill behaviour if the input word/line is longer than the fixed-size buffer. For that reason, you should usually use a friendlier string type:
std::string a;
std::cin >> a; // single word
std::getline(std::cin, a); // whole line
The string will grow to accommodate any amount of input.
The problem, most likely, is in the way you enter the values later on. The cin.get() after every initialization is there to "grab" the newline character that gets put in the stream every time you press enter.
Say you start entering your values like this:
2
a b c d...
Assuming you have pressed enter after 2, the newline character was also put on the stream. When you call cin.get() after that, it will grab and discard the newline character, allowing the rest of your code to properly get the input.
To answer your first question, for an array, you should use cin.get instead of the overloaded operator >> cin>> as that would only grab a single word, and it would not limit the amount of characters grabbed, which could lead to an overflow and data corruptions / program crashing.
On the other hand, cin.get() allows you to specify the maximum number of characters read, preventing such bugs.
For a char array use cin.get() because it counts whitespace whereas cin does not. More importantly, cin.get() sets the maximum number of characters to read. For example:
char foo[25]; //set maximum number of characters
cout << "Please type in characters for foo" << endl;
cin.get(foo,25);
cout << ' ' << foo;
In this case, you can only type in 24 characters and a null terminator character \0.
I hope this answers your first question.
I would personally use a string.
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'm having a hard time understanding why while (cin.get(Ch)) doesn't see the EOF. I read in a text file with 3 words, and when I debug my WordCount is at 3 (just what I hoped for). Then it goes back to the while loop and gets stuck. Ch then has no value. I thought that after the newline it would read the EOF and break out. I am not allowed to use <fstream>, I have to use redirection in DOS. Thank you so much.
#include <iostream>
using namespace std;
int main()
{
char Ch = ' ';
int WordCount = 0;
int LetterCount = 0;
cout << "(Reading file...)" << endl;
while (cin.get(Ch))
{
if ((Ch == '\n') || (Ch == ' '))
{
++WordCount;
LetterCount = 0;
}
else
++LetterCount;
}
cout << "Number of words => " << WordCount << endl;
return 0;
}
while (cin >> Ch)
{ // we get in here if, and only if, the >> was successful
if ((Ch == '\n') || (Ch == ' '))
{
++WordCount;
LetterCount = 0;
}
else
++LetterCount;
}
That's the safe, and common, way to rewrite your code safely and with minimal changes.
(Your code is unusual, trying to scan all characters and count whitespace and newlines. I'll give a more general answer to a slightly different question - how to read in all the words.)
The safest way to check if a stream is finished if if(stream). Beware of if(stream.good()) - it doesn't always work as expected and will sometimes quit too early. The last >> into a char will not take us to EOF, but the last >> into an int or string will take us to EOF. This inconsistency can be confusing. Therefore, it is not correct to use good(), or any other test that tests EOF.
string word;
while(cin >> word) {
++word_count;
}
There is an important difference between if(cin) and if(cin.good()). The former is the operator bool conversion. Usually, in this context, you want to test:
"did the last extraction operation succeed or fail?"
This is not the same as:
"are we now at EOF?"
After the last word has been read by cin >> word, the string is at EOF. But the word is still valid and contains the last word.
TLDR: The eof bit is not important. The bad bit is. This tells us that the last extraction was a failure.
The Counting
The program counts newline and space characters as words. In your file contents "this if fun!" I see two spaces and no newline. This is consistent with the observed output indicating two words.
Have you tried looking at your file with a hex editor or something similar to be sure of the exact contents?
You could also change your program to count one more word if the last character read in the loop was a letter. This way you don't have to have newline terminated input files.
Loop Termination
I have no explanation for your loop termination issues. The while-condition looks fine to me. istream::get(char&) returns a stream reference. In a while-condition, depending on the C++ level your compiler implements, operator bool or operator void* will be applied to the reference to indicate if further reading is possible.
Idiom
The standard idiom for reading from a stream is
char c = 0;
while( cin >> c )
process(c);
I do not deviate from it without serious reason.
you input file is
this is fun!{EOF}
two spaces make WordCount increase to 2
and then EOF, exit loop! if you add a new line, you input file is
this is fun!\n{EOF}
I took your program loaded it in to visual studio 2013, changed cin to an fstream object that opened a file called stuff.txt which contains the exact characters "This is fun!/n/r" and the program worked. As previous answers have indicated, be careful because if there's not a /n at the end of the text the program will miss the last word. However, I wasn't able to replicate the application hanging in an infinite loop. The code as written looks correct to me.
cin.get(char) returns a reference to an istream object which then has it's operator bool() called which returns false when any of the error bits are set. There are some better ways to write this code to deal with other error conditions... but this code works for me.
In your case, the correct way to bail out of the loop is:
while (cin.good()) {
char Ch = cin.get();
if (cin.good()) {
// do something with Ch
}
}
That said, there are probably better ways to do what you're trying to do.
This is the code I have been trying to execute on TurboC++ 3.0 (Yes, I know it's ancient but can't help it), when the program goes into the loop, it skips the value of y every time including the first attempt. Any help would be appreciated but please avoid rubbing salt into wounds by asking why TurboC++ 3.0. Thanks in advance.
void main()
{
int x, z;
char y[10];
for (int i=0;i<5;i++)
{
cout<<"\nX:";
cin >> x;
cout<<"\nY:";
cin.getline(y,10);
cout<<"\nZ:";
cin>>z;
}
for(i=0;i<5;i++)
{
cout<<x<<"\n";
cout.write(y, 10)<<"\n";
cout<<z<<"\n\n";
}
}
and even if I use cin.get(var) where var is a character, i still get weird results like a heart, diamond or even a smiley.
You get weird results because you are not terminating your c style string with a null character.('\0').
The problem you are facing is because , fail bit or eof bit is set. To remove that, do the following:-
You can use
cin.clear() ;
to clear if any error bits are set and then use
cin.ignore(100, '\n') ;
// 100 is just a random no, change it depending on your size of input.
to ignore any irrelevant characters int the stream.
or you can do the following:-
after cin>>x just type cin.ignore(), it will flush out any newline characters present in the buffer .
it skips the value of y
cin >> x reads the input until it finds something that's not a digit - in this case, the end-of-line character. That character is left in the stream.
getline reads the input until it finds an end-of-line character (or the end of the stream). Since you've left one in the stream, it finds it straight away and doesn't read anything.
You can call cin.ignore(-1,'\n') to ignore the remainder of the first line after reading x (assuming your prehistoric library behaves like the modern one).
i still get weird results like a heart, diamond or even a smiley
cout.write(y, 10) is wrong - there are up to 9 valid characters in y, followed by the null terminator. You want cout << y to treat it as a null-terminated string and print only the valid characters.
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;
}