I have a constraint to read the input strings character by character. So I'm checking for \n after each string. But the program is not terminating.
Here's the problem I'm facing in a very short code:
#include <iostream>
using namespace std;
int main()
{
char c;
while(cin >> c)
{
char x;
cin >> x;
while(x != '\n')
{
// print the characters
cin >> x;
}
}
return 0;
}
In the above code, c will have the first character of the string while x will have the rest of characters one by one.
Input Case:
banananobano
abcdefhgijk
Radaradarada
I have a constraint to read the input strings character by character
One way of reading character by character, is via std::basic_istream::get.
If you define
char c;
then
std::cin.get(c);
will read the next character into c.
In a loop, you could use it as
while(std::cin.get(c))
<body>
cin is whitespace delimited, so any whitespace (including \n) will be discarded. Thus, x will never be
Use getline for reading line from the input stream and then use istringstream to get formatted input from the line.
std::string line;
std::getline(cin, line);
std::istringstream iss(line);
while ( iss >> c) {
print the characters;
}
Related
Is there a default delimiter for stringstream? From my research, I understood that I can use it to split a string using space and comma as delimiters. But can I use other delimiters for stringstream?
Here is a C++ code snippet :
vector<int> parseInts(string str) {
// Complete this function
stringstream ss(str);
vector<int> res;
char ch;
int x;
while(ss){
ss >> x >> ch;
res.push_back(x);
}
return res;
}
This code works without me mentioning any specific delimiter. How does that happen?
There is no "delimiter" for streams at all. operator>>, on the other hand, implements its reading by delimiting on whitespace characters. For other delimiter characters, you can use std::getline() instead, eg:
vector<int> parseInts(string str) {
// Complete this function
istringstream iss(str);
vector<int> res;
int x;
string temp;
char delim = '-'; // whatever you want
while (getline(iss, temp, delim)) {
if (istringstream(temp) >> x) { // or std::stoi(), std::strtol(), etc
res.push_back(x);
}
}
return res;
}
This code works without me mentioning any specific delimiter. How does that happen?
streams don't know anything about delimiters. What is happening is that, on each loop iteration, you are calling ss >> x to read the next available non-whitespace substring and convert it to an integer, and then you are calling ss >> ch to read the next available non-whitespace character following that integer. The code doesn't care what that character actually is, as long as it is not whitespace. Your loop runs until it reaches the end of the stream, or encounters a reading/conversion error.
I was solving a question on hackerrank and came across this problem involving string streams.
https://www.hackerrank.com/challenges/c-tutorial-stringstream/problem
For Extracting data, hackerrank has given an example:
stringstream ss("23,4,56");
char ch;
int a, b, c;
ss >> a >> ch >> b >> ch >> c; // a = 23, b = 4, c = 56
However, when I try to export it to a vector, I have to escape the ',' using:
stringstream ss(str);
vector<int> vect;
int i;
while (ss >> i)
{
vect.push_back(i);
if (ss.peek() == ',')
ss.ignore();
}
Why can't I use the extraction operation to get the required word here? Shouldn't the stream escape the ','(Sorry for the noob-level question)?
operator>> extracts the next delimited token, only so far as characters actually belong to the requested data type. So, when using operator>> to read an int, it will extract only digits, not letters, punctuation, etc. That means a comma following a number has to be read separately.
In the first example:
ss >> a reads the first int in the stream
then >> ch reads the comma after it
then >> b reads the next int
then >> ch reads the comma after it
then >> c reads the next int
In the second example:
ss >> i reads the next int in the stream, breaking the loop if fails or EOF
then ss.peek() checks if a comma exists (since the last int doesn't have one), and if found then ss.ignore() skips past it
goto #1
If you try to use operator>> to read a comma that doesn't exist, it will set the stream's eofbit state and fail the extraction. If you use while (ss >> i >> ch), the while would evaluate as false when the last int is reached. Even though ss >> i would succeed, >> ch would fail, and thus i would not be added to the vector.
In theory, you could replace if (ss.peek() == ',') ss.ignore(); inside the loop with char ch; ss >> ch instead. The end effect would be the same, at least for a string like "23,4,56". But, let's say you were given something like "23 4 56" instead. The first example would fail to handle that correctly, but the second example would handle it just fine when using peek()+ignore(), but not when using ss >> ch.
I think you can use this code to escape the ','
std::string valstr;
while (std::getline(ss, valstr, ','))
{
vect.push_back(std::stoi(valstr));
}
Basically I first takes an integer as input and then test case follows. My each test case is an string. I am suppose to print the string back if the starting patten of string matches "HI A" and it is case-insensitive. I wrote the code below to accomplish to this. My problem is that when I press enter after each input, getline takes newline character as new input. I have tried to tackle this by using extra getline after each input but the issue is still there. Program gets stuck in the loop even though I have put a break condition. What am I doing wrong?
#include <iostream>
#include <string>
using namespace std;
int main(){
int N;
cin >>N;
string nl;
getline(cin,nl);
for (int i=0;i<N;i++){
string s;
getline(cin,s);
//cout <<"string"<<s<<endl;
int flag=0;
if ((s.at(0)=='h'||s.at(0)=='H')&&(s.at(1)=='i'||s.at(1)=='I')&&(s.at(2)==' ')&&(s.at(3)=='a'||s.at(3)=='A')) flag=1;
if (flag==1) cout << s;
//cout << "not " <<s;
string ne;
cout << "i="<< i<<endl;
if (i==N-1) {break;}
getline(cin,ne);
}
}
Here is sample input:
5
Hi Alex how are you doing
hI dave how are you doing
Good by Alex
hidden agenda
Alex greeted Martha by saying Hi Martha
Output should be:
Hi Alex how are you doing
ignore() function does the trick. By default, it discards all the input suquences till new line character.
Other dilimiters and char limit can be specified as well.
http://www.cplusplus.com/reference/istream/istream/ignore/
In your case it goes like this.
cin >> N;
cin.ignore();
Your cin >>N stops at the first non-numeric character, which is the newline. This you have a getline to read past it, that's good.
Each additional getline after that reads the entire line, including the newline at the end. By putting in a second getline you're skipping half your input.
So, your real problem isn't that getline eats newlines, but that your second getline(cin, ne) is eating a line...
And that is because you mistakenly think that you need two getline operations to read one line - or something like that. Mixing "linebased" and "itembased" input does have confusing ways to deal with newlines, so you do need something to "skip" the newline left behind frin cin >> N;, but once you have got rid of that, you only need ONE getline to read up and including the newline at the end of a line.
I am writing this answer with the hopes that it may help someone else out there that wants a very simple solution to this problem.
In my case the problem was due to some files having different line endings such as '\r' vs. '\n'. Everything worked fine in windows but then it failed in Linux.
The answer was actually simple. I created a function removeNewLineChar after each line was read in. That way the char was removed. The removeNewLineChar takes in the line that was read in and copies it over character by character into a new string but it avoids copying either of the newline characters.
Here is an example:
string trim(string line)
{
string newString;
for (char ch : line)
{
if (ch == '\n' || ch == '\r')
continue;
newString += ch;
}
return newString;
}
//some function reading a file
while (getline(fin, line)) {
line = trim(line);
//... do something with the line
line = "";
}
you just need to accept the fact that getline will give you '\n' at the end. One solution is remove '\n' after getting it. Another solution is do not write the additional 'endl'. for example, for your problem, you can use this code
int N;
cin >> N;
string line;
getline(cin, line); // skip the first new line after N.
for (int i = 0; i < N; i++) {
string line;
getline(cin, line);
string first4 = line.substr(0, 4);
// convert to upper case.
std::transform(first4.begin(), first4.end(), first4.begin(), std::ptr_fun<int, int>(std::toupper)); // see http://en.cppreference.com/w/cpp/algorithm/transform
if (first4 == "HI A") {
cout << line; // do not include "<< endl"
}
}
cin.ignore() worked for me.
void House::provideRoomName()
{
int noOfRooms;
cout<<"Enter the number of Rooms::";
cin>>noOfRooms;
cout<<endl;
cout<<"Enter name of the Rooms::"<<endl;
cin.ignore();
for(int i=1; i<=noOfRooms; i++)
{
std::string l_roomName;
cout<<"Room"<<"["<<i<<"] Name::";
std::getline(std::cin, l_roomName);
}
}
std::string line;
std::cin>>std::ws; // discard new line not processed by cin
std::getline(std::cin,line);
From Notes section https://en.cppreference.com/w/cpp/string/basic_string/getline
When consuming whitespace-delimited input (e.g. int n; std::cin >> n;) any whitespace that follows, including a newline character, will be left on the input stream. Then when switching to line-oriented input, the first line retrieved with getline will be just that whitespace. In the likely case that this is unwanted behaviour, possible solutions include:
An explicit extraneous initial call to getline
Removing consecutive whitespace with std::cin >> std::ws
Ignoring all leftover characters on the line of input with cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
My understanding is that the operator >> returns the left operand, so cin >> x returns cin which has a value of false if we encounter an EOF or other errors. Now my guess is that since cin skips whitespace, characters like \n don't get stored. What I'd like to do is something like this:
int x;
while((cin >> x) and x != '\n')
// do stuff
So that when cin encounters a newline in the stream, it exits the input loop (as it would normally do with say, EOF).
Using >> operator directly won't work, because as you say it skips whitespace including newlines.
What you can do is to read a single line using std::getline, and then you can read all input from the line with std::stringstream.
std::string line;
if (std::getline(std::cin, line)) {
std::istringstream ss(line);
int x;
while (ss >> x) {
//....
}
}
My code has to identify whitespace characters using cin, so when I use space as an input it should identify the space. How do I do this?
You can use std::noskipws to disable the whitespace skipping that std::cin does by default:
#include <iostream>
#include <iomanip>
int main() {
char c;
std::cin >> std::noskipws;
while (std::cin >> c) {
if (c == ' ')
std::cout << "A space!" << std::endl;
}
return 0;
}
string str;
getline(cin, str); // get the whole line
If you want to deal with c-strings you could use the mentioned cin.getline(....) which is different from strings getline.
Cin breaks on whitespace, of any kind. If you need to read an entire line, you need to use the get line function:
getline(cin, line);
Where line is a std::string. This will still cut off any new lines or carriage returns.
To test the string for spaces examine every character in the string and compare it to the space character " ". That is left as an exercise for the reader ;)
Use cin.getline to read the line with the space.
http://www.cplusplus.com/reference/iostream/istream/getline/