问题
I\'ve singled out a failure on my program that prevents me from assigning a value to the variable addAntonymAnswer1. I\'ve tried running cin.clear() before the statement to get the thing read my yes/no answer, but the code just won\'t respond.
The program bit that\'s failing is located inside void dictionaryMenu(vector <WordInfo> &wordInfoVector) and reads
cin.clear();
cout<<\">\";
cin>>addAntonymAnswer1;
// cin reading STUCK, why!?
to get to that point of the program the user has to choose to add a word, and then add a synonym.
The input for running the program is:
dictionary.txt
1 cute
2 hello
3 ugly
4 easy
5 difficult
6 tired
7 beautiful
synonyms
1 7
7 1
3 2
antonyms
1 3
3 1 7
4 5
5 4
7 3
#include <iostream>
#include <fstream>
#include <string>
#include <sstream>
#include <vector>
using namespace std;
class WordInfo{
public:
WordInfo(){}
WordInfo(string newWord){
word=newWord;
}
~WordInfo() { }
int id() const {return myId;}
void readWords(istream &in)
{
in>>myId>>word;
}
vector <int> & getSynonyms () {
return mySynonyms;
}
vector <int> & getAntonyms() {
return myAntonyms;
}
string getWord() {
return word;
}
void dictionaryMenu (vector <WordInfo> &wordInfoVector){
cout<<endl<<\"Would you like to add a word?\"<<endl;
cout<<\"(yes/no)\"<<endl;
cout<<\">\";
string addWordAnswer;
cin>>addWordAnswer;
if (addWordAnswer==\"yes\")
// case if the guy wants to add a word
{
cout<<endl;
cout<<\"Please, write the word \"<<endl;
string newWord;
cout<<\">\";
cin>>newWord;
cout<<endl;
WordInfo newWordInfo (newWord);
int newWordId = wordInfoVector.size() +1;
newWordInfo.myId=newWordId;
cout<<\"The id of \"<<newWordInfo.word<<\" is \"<<newWordInfo.myId<<endl<<endl;
wordInfoVector.push_back(newWordInfo);
cout<<\"Would you like to define which words on the existing dictionary are\" <<endl
<<\"synonyms of \"<<newWordInfo.word<<\"?\"<<endl;
cout<<\"(yes/no)\"<<endl;
string addSynonymAnswer, addAntonymAnswer1, addAntonymAnswer2;
cout<<\">\";
cin>>addSynonymAnswer;
if (addSynonymAnswer==\"yes\")
{
cout<<endl;
cout<<\"Please write on a single line the ids for the synonyms of \"
<<newWordInfo.word<<endl<<\"starting with its id, which is \"<<newWordInfo.myId<<endl<<endl;
cout<<\"For example, to define that the synonym of the word \'cute\', which has an id 1, is\"
<<\"\'beautiful\', which has an id 7, you should write: 1 7\"<<endl<<endl;
cout<<\"In the case of \"<<newWordInfo.word<<\" you should start with \"<<newWordInfo.myId<<endl;
cin.clear();
string lineOfSyns;
cout<<\">\";
cin>>lineOfSyns;
newWordInfo.pushSynonyms(lineOfSyns, wordInfoVector);
cin.clear();
cout<<\"Would you like to define which words on the existing dictionary are\" <<endl
<<\"antonyms of \"<<newWordInfo.word<<\"?\"<<endl;
//##HERE THE CIN READING OF addAntonymAnswer1 FAILS, WHY?
cin.clear();
cout<<\">\";
cin>>addAntonymAnswer1;
// cin reading STUCK, why!?
if (addAntonymAnswer1==\"yes\"){ }
else if (addAntonymAnswer1==\"no\"){
// END DICTIONARY MENU
}
}
else if (addSynonymAnswer==\"no\"){
cout<<\"Would you like to define which words on the existing dictionary are\" <<endl
<<\"antonyms of \"<<newWordInfo.word<<\"?\"<<endl;
cout<<\">\";
cin>>addAntonymAnswer2;
if (addAntonymAnswer2==\"yes\"){ }
else if (addAntonymAnswer2==\"no\"){
// END DICTIONARY MENU
}
}
} // if addWordAnswer == \"no\"
else if (addWordAnswer==\"no\"){
// ######RETURN TO MAIN MENU############
}
}
void pushSynonyms (string synline, vector<WordInfo> &wordInfoVector){
stringstream synstream(synline);
vector<int> synsAux;
// synsAux tiene la línea de sinónimos
int num;
while (synstream >> num) {synsAux.push_back(num);}
int wordInfoVectorIndex;
int synsAuxCopyIndex;
if (synsAux.size()>=2){ // takes away the runtime Error
for (wordInfoVectorIndex=0; wordInfoVectorIndex <wordInfoVector.size(); wordInfoVectorIndex++)
{
if (synsAux[0]==wordInfoVector[wordInfoVectorIndex].id()){
// this is the line that\'s generating a Runtime Error, Why?
for (synsAuxCopyIndex=1; synsAuxCopyIndex<synsAux.size(); synsAuxCopyIndex++){
// won\'t run yet
wordInfoVector[wordInfoVectorIndex].mySynonyms.push_back(synsAux[synsAuxCopyIndex]);
}
}
}
}// end if size()>=2
} // end pushSynonyms
void pushAntonyms (string antline, vector <WordInfo> &wordInfoVector)
{
stringstream antstream(antline);
vector<int> antsAux;
int num;
while (antstream >> num) antsAux.push_back(num);
int wordInfoVectorIndex;
int antsAuxCopyIndex;
if (antsAux.size()>=2){ // takes away the runtime Error
for (wordInfoVectorIndex=0; wordInfoVectorIndex <wordInfoVector.size(); wordInfoVectorIndex++)
{
if (antsAux[0]==wordInfoVector[wordInfoVectorIndex].id()){
// this is the line that\'s generating a Runtime Error, Why?
for (antsAuxCopyIndex=1; antsAuxCopyIndex<antsAux.size(); antsAuxCopyIndex++){
// won\'t run yet
wordInfoVector[wordInfoVectorIndex].myAntonyms.push_back(antsAux[antsAuxCopyIndex]);
}
}
}
}// end if size()>=2
}
//--dictionary output function
void printWords (ostream &out)
{
out<<myId<< \" \"<<word;
}
//--equals operator for String
bool operator == (const string &aString)const
{
return word ==aString;
}
//--less than operator
bool operator <(const WordInfo &otherWordInfo) const
{ return word<otherWordInfo.word;}
//--more than operator
bool operator > (const WordInfo &otherWordInfo)const
{return word>otherWordInfo.word;}
public:
vector<int> mySynonyms;
vector <int> myAntonyms;
string word;
int myId;
};
//--Definition of input operator for WordInfo
istream & operator >>(istream &in, WordInfo &word)
{
word.readWords(in);
}
//--Definition of output operator
ostream & operator <<(ostream &out, WordInfo &word)
{
word.printWords(out);
}
int main() {
string wordFile;
cout<<\"enter name of dictionary file: \"<<endl;
getline (cin,wordFile);
ifstream inStream (wordFile.data());
if(!inStream.is_open())
{
cerr<<\"cannot open \"<<wordFile<<endl;
exit(1);
}
vector <WordInfo> wordInfoVector;
WordInfo aword;
while (inStream >>aword && (!(aword==\"synonyms\")))
{
wordInfoVector.push_back(aword);
}
inStream.clear();
vector <int> intVector;
string synLine;
while (getline(inStream, synLine)&&(synLine!=(\"antonyms\"))){
aword.pushSynonyms(synLine, wordInfoVector);
}
int theIndex;
string antLine;
while (getline(inStream,antLine)){
aword.pushAntonyms(antLine, wordInfoVector);
}
cout<<endl<<\"the words on the dictionary are: \"<<endl;
int h=0;
while (h<wordInfoVector.size()){
cout<<wordInfoVector[h]<<endl;
h++;
}
aword.dictionaryMenu(wordInfoVector);
system(\"PAUSE\");
return 0;
}
回答1:
cin.clear() doesn't clear the standard input. What it does is clearing error bits, like eofbit, failbit and others, and sets the stream into a good state. Maybe you expected it to clear out anything in it? If the user typed
yes no
Just before, and you
cin >> someStringVariable;
It will read up to no and the stream will still contain
no
The call to clear then clears any error bits being active. Then, your
cin>>addAntonymAnswer1;
Will read the no that wasn't eaten by the previous read, and the action immediately returns, not waiting for new input. What you should do is doing a clear followed by an ignore, up to the next newline. You tell it the amount of characters it should ignore maximally. That amount should be the highest number possible:
cin.clear();
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
Doing that will make the stream empty, and a following read will wait for you to type something in.
Another problem arises if you have got a cin >> followed by a getline: The cin will leave any whitespace (also newlines) after its read token, but getline will stop reading after it hits such a newline. I see you have put clear after nearly everything. So i want to show you when you need it and when not. You don't need it when you sequence multiple cin >>. Assume you have in your buffer: "foo\nbar\n". Then you do the following reads
cin >> a; // 1
cin >> b; // 2
After the first, your buffer will contain "\nbar\n". That is, the newline is still in. The second cin>> will first skip all whitespace and newlines, so that it can cope with \n being at the front of bar. Now, you can also sequence multiple getline calls:
getline(cin, a);
getline(cin, b);
Getline will throw away the \n that it reads at the line end, but won't ignore newlines or whitespace at the begin. So, after the first getline, the buffer contains "bar\n". The second getline will correctly read "bar\n" too. Now, let's consider the case where you need the clear/ignore:
cin >> a;
getline(cin, b);
The first will leave the stream as "\nbar\n". The getline then will see immediately the \n at the begin, and will think it read an empty line. Thus, it will immediately continue and not wait for anything, leaving the stream as "bar\n". So, if you have a getline after a cin>> you should first execute the clear/ignore sequence, to clear out the newline. But between getline or cin>>'s, you should not do it.
回答2:
It is "stuck" because it is waiting for input. cin is attached to the standard input handle for the program, you have to type something and hit enter.
回答3:
cin >> ... reads from standard input until it finds a whitespace character. When you enter, say, 8 5 for the list of synonyms, 8 gets read into lineOfSyns and nothing more. When the program reaches cin >> addAntonymAnswer1, 5 gets read into addAntonymsAnswer1. Your program behaves unexpectedly since it expects yes or no but it got 5.
Look at using cin.getline() instead of >>. See, for example, sections 18.2 and 18.3 of this page.
回答4:
In your program you ask the user:
Please write on a single line the ids for the synonyms of test
starting with its id, which is 8
For example, to define that the synonym of the word 'cute', which has an id 1, i
s'beautiful', which has an id 7, you should write: 1 7
In the case of test you should start with 8
Then you attempt to read in the line the user typed with
cin>>lineOfSyns;
However, this only reads up to the first whitespace. So the 2nd number typed in by the user is still in the cin buffer when the
cin>>addAntonymAnswer1;
line is executed, so that data gets read into the addAntonymAnswer1 string. The user never gets an opportunity to type in 'yes' or 'no' and your tests for those values fail.
You should consider changing to using the string overload for getline() to do your reads:
getline( cin, stringvar);
This would probably be better than using cin.getline(), as that does not have an overload that takes string - with that member function, you need to read into an array of chars, which is far less flexible than reading into a string.
来源：https://stackoverflow.com/questions/478258/why-is-this-cin-reading-jammed