#include <fstream>
#include <iostream>
#include <string>
using namespace std;
int main() {
ofstream w("d:/tester.txt");
int f = 1;
int s = 2;
int t = 3;
string x = "hello";
w << f << endl << s << endl << t << endl << x ;
w.close();
ifstream r("d:/tester.txt");
r >> x;
cout << x << endl ;
s = s + 10 ;
r.close();
/* ofstream wa("d:/tester.txt");
wa << s;
wa.close();*/
}
I always get the output equal to 1 .
Why is this so ? When i asked for the string hello 1 gets displayed .
In fact, it is the output you ask for: you are reading the first string token from the file. And that happens to be the number “1” you wrote on the first line into the file.
The streaming operators don’t magically parse your file for the most suitable token; they are simply reading the next available token. And even if they did, “1” would be a perfectly valid choice for a string.
You store consecutively 1 2 3 in your file, then you fetch the first value from the file. Are you surprised that the value is 1? If you want the other values, you must call the stream in functions more than once.
The first line of tester.txt is f which is 1.
x is a string, so when you read from tester.txt using r>>x you get the first line, which is "1"
When you open a stream, you open it in a top-bottom fashion, this meaning that the stream is positioned in the beginning of the first line.
You wrote:
w << f << endl << s << endl << t << endl << x ;
So line the lines are:
f
s
t
x
And f = 1, so you are getting what you should be getting.
Isn't that what you would expect, or am I missing something?
You put the values "1" "2" "3" "hello" in the stream, in that order.
Then, you stream from that into a string. String spec says that all characters will be copied until the first valid whitespace. It will see "1" as a char, and then stop at the newline. Hence, you will get a string, the string "1";
Related
I have trapped for hours when using sstream to parser integer in a char array.
I don't know why there is one more iteration in while loop.
//main.cpp
#include <iostream>
#include <sstream>
int main()
{
char data[5] = "1 23";
//char data[4] = {'1', ' ', '2', '3'}; another attempt
std::stringstream stream;
stream << data;
int count = 1;
while (stream)
{
double x = 0;
stream >> x;
std::cout << count << " " << x << std::endl;
count++;
}
return 0;
}
the program output shows that:
1 1
2 23
3 0
I use the follow command to compile the program.
g++ main.cpp
I think that there are 2 integer,so there are only 2 iteration. I don't know why there are 3 iteration in while loop. I suppose it is because the '\0' at the end of char array, but I tried, it get the same result.
Any Suggestion? Thanks.
You are not checking stream >> x for success:
if (stream >> x)
{
std::cout << count << " " << x << std::endl;
count++;
}
will do the job.
You can also include it in the loop:
double x = 0;
while (stream >> x)
{
std::cout << count << " " << x << std::endl;
count++;
}
I think that there are 2 integer,so there are only 2 iteration. I don't know why there are 3 iteration in while loop.
First iteration: The stream is in a good state. You extract an integer. You print the integer without checking whether the extraction succeeded (it did).
Second iteration: The stream is in a good state. You extract an integer. You print the integer without checking whether the extraction succeeded (it did).
Third iteration: The stream is in a good state. You extract an integer. You print the integer without checking whether the extraction succeeded (it had not).
Fourth iteration: The stream is in a failed state. The loop ends.
The conventional pattern for stream extraction is:
while(stream >> variable) { // or if for single extraction
// use the extracted variable
This way you always check the success of the extraction before using the extracted value.
char data[4] = {'1', ' ', '2', '3'}; another attempt
This attempt has undefined behaviour as soon as you insert the non-null terminated string into the stream.
I am working on NOV14 on COdechef contest problems. and i stuck at this problem.
http://www.codechef.com/NOV14/problems/RBTREE
My algorithm working well, but i cant take the input correctly. the problem is i don't know how many number of inputs are given. but i need to store in multiple variables.
Take a look at here..
5
Qb 4 5
Qr 4 5
Qi
Qb 4 5
Qr 4 5
where 5 is the number of test cases,
can i read every test cases into variables.
if i take First test case I can take Qb to one variable, 4 to other and 5 to another.
But the problem is How to read a line which start with Qi.
Well, first of all, if you write C++, you should use C++ streams. Here's the code for input (which you can adjust for your own needs):
#include <iostream>
#include <fstream>
int main() {
std::ifstream file;
file.open("data.in");
int lines = 0;
file >> lines;
std::string query_type;
for (int i = 0; i < lines; i++) {
file >> query_type;
if (query_type == "Qi") {
std::cout << query_type << std::endl;
} else {
int x = 0;
int y = 0;
file >> x >> y;
std::cout << query_type << " " << x << " " << y << std::endl;
}
}
file.close();
return 0;
}
You'll need to check what you've read at each step, and then determine whether or not you need to read the numbers in.
So read two characters, and if the characters you've read are "Q" and "i", you don't need to read any numbers, and you can just step on to the next line. Otherwise, you should read the two numbers before going to the next line.
Here is a small code:
char a;
while(std::cin >> a) {
std::cout << a << " is pressed\n";
}
When I type in "w", i get "w is pressed".
When I type in "www", i get "w is pressed" 3 times in a row.
Can someone please explain why this happens?
Thanks
When you use std::cin to read a char variable it reads one character at a time. That is why you get 3 iterations in the while loop for input www.
There is a queue of inputs. if you entered too much, your input waits in patient...
The first part of the answer is on the first line of your code.
char a;
Variable a is a single char, an 8-bit value typically used to store a code representing a display character. If the display is ASCII, then (value) 0 = no character, (value) 32 = space, value 48 = (character) '0', etc.
std::cin is an instance of class std::istream, it has various members and operator overloads to deal with different types. In the case of a char, you are calling
std::istream::operator(char)
Which reads one char, exactly one, from the input stream and returns.
#include <iostream>
int main()
{
char a, b, c;
std::cin >> a >> b >> c;
std::cout << "a = " << a << ", b = " << b << ", c = " << c << '\n';
return 0;
}
----------------- EDIT -----------------------
Based on juanchopanza's comment : I edit the title
Based on jrok's comment : I'm using ofstream to write, and ifstream to read.
I'm writing 2 programs, first program do the following tasks :
Has a vector of integers
convert it into array of string
write it in a file
The code of the first program :
vector<int> v = {10, 200, 3000, 40000};
int i;
stringstream sw;
string stringword;
cout << "Original vector = ";
for (i=0;i<v.size();i++)
{
cout << v.at(i) << " " ;
}
cout << endl;
for (i=0;i<v.size();i++)
{
sw << v[i];
}
stringword = sw.str();
cout << "Vector in array of string : "<< stringword << endl;
ofstream myfile;
myfile.open ("writtentext");
myfile << stringword;
myfile.close();
The output of the first program :
Original vector : 10 200 3000 40000
Vector in string : 10200300040000
Writing to File .....
second program will do the following tasks :
read the file
convert the array of string back into original vector
----------------- EDIT -----------------------
Now the writing and reading is fine, thanks to Shark and Jrok,I am using a comma as a separator. The output of first program :
Vector in string : 10,200,3000,40000,
Then I wrote the rest of 2nd program :
string stringword;
ifstream myfile;
myfile.open ("writtentext");
getline (myfile,stringword);
cout << "Read From File = " << stringword << endl;
cout << "Convert back to vector = " ;
for (int i=0;i<stringword.length();i++)
{
if (stringword.find(','))
{
int value;
istringstream (stringword) >> value;
v.push_back(value);
stringword.erase(0, stringword.find(','));
}
}
for (int j=0;j<v.size();i++)
{
cout << v.at(i) << " " ;
}
But it can only convert and push back the first element, the rest is erased. Here is the output :
Read From File = 10,200,3000,40000,
Convert back to vector = 10
What did I do wrong? Thanks
The easiest thing would be to insert a space character as a separator when you're writing, as that's the default separator for operator>>
sw << v[i] << ' ';
Now you can read back into an int variable directly, formatted stream input will do the conversion for you automatically. Use vector's push_back method to add values to it as you go.
Yes, this question is over a year old, and probably completely irrelevant to the original asker, but Google led me here so it might lead others here too.
When posting, please post a complete minimal working example, having to add #include and main and stuff is time better spent helping. It's also important because of your very problem.
Why your second code isn't working is all in this block
for (int i=0;i<stringword.length();i++)
{
if (stringword.find(','))
{
int value;
istringstream (stringword) >> value;
v.push_back(value);
stringword.erase(0, stringword.find(','));
}
}
istringstream (stringword) >> value interprets the data up to the comma as an integer, the first value, which is then stored.
stringword.find(',') gets you the 0-indexed position of the comma. A return value of 0 means that the character is the first character in the string, it does not tell you whether there is a comma in the string. In that case, the return value would be string::npos.
stringword.erase deletes that many characters from the start of the string. In this case, it deletes 10, making stringword ,200,3000,40000. This means that in the next iteration stringword.find(',') returns 0.
if (stringword.find(',')) does not behave as wished. if(0) casts the integer to a bool, where 0 is false and everything else is true. Therefore, it never enters the if-block again, as the next iterations will keep checking against this unchanged string.
And besides all that there's this:
for (int j=0;j<v.size();i++)
{
cout << v.at(i) << " " ;
}
it uses i. That was declared in a for loop, in a different scope.
The code you gave simply doesn't compile, even with the added main and includes. Heck, v isn't even defined in the second program.
It is however not enough, as the for condition stringword.length() is recalculated every loop. In this specific instance it works, because your integers get an extra digit each time, but let's say your input file is 1,2,3,4,:
The loop executes normally three times
The fourth time, stringword is 4, stringword.length() returns 2, but i is already valued 3, so i<stringword.length() is invalid, and the loop exits.
If you want to use the string's length as a condition, but edit the string during processing, store the value before editing. Even if you don't edit the string, this means less calls to length().
If you save length beforehand, in this new scenario that would be 8. However, after 4 loops string is already empty, and it executes the for loop some more times with no effect.
Instead, as we are editing the string to become empty, check for that.
All this together makes for radically different code altogether to make this work:
while (!stringword.empty())
{
int value;
istringstream (stringword) >> value;
v.push_back(value);
stringword.erase(0, stringword.find(',')+1);
}
for (int i = 0; i < v.size(); i++)
{
cout << v.at(i) << " " ;
}
A different way to solve this would have been to not try to find from the start, but from index i onwards, leaving a string of commas. But why stick to messy stuff if you can just do this.
And that's about it.
I am learning C++ for the first time. I have no previous programming background.
In the book I have I saw this example.
#include <iostream>
using::cout;
using::endl;
int main()
{
int x = 5;
char y = char(x);
cout << x << endl;
cout << y << endl;
return 0;
}
The example makes sense: print an integer and the ASCII representation of it.
Now, I created a text file with these values.
48
49
50
51
55
56
75
I am writing a program to read this text file -- "theFile.txt" -- and want to convert these numbers to the ASCII value.
Here is the code I wrote.
#include <iostream>
#include <fstream>
using std::cout;
using std::endl;
using std::ifstream;
int main()
{
ifstream thestream;
thestream.open("theFile.txt");
char thecharacter;
while (thestream.get(thecharacter))
{
int theinteger = int(thecharacter);
char thechar = char(theinteger);
cout << theinteger << "\t" << thechar << endl;
}
system ("PAUSE");
return 0;
}
This is my understanding about the second program shown.
The compiler does not know the exact data type that is contained in "theFile.txt". As a result, I need to specify it so I choose to read the data as a char.
I read the each digit in the file as a char and converted it to an integer value and stored it in "theinteger".
Since I have an integer in "theinteger" I want to print it out as a character but char thechar = char(theinteger); does not work as intended.
What am I doing incorrect?
You are reading char by char, but you really (I think) want to read each sequence of digits as an integer. Change your loop to:
int theinteger;
while (thestream >> theinteger )
{
char thechar = char(theinteger);
cout << thechar << endl;
}
+1 For a very nicely formatted & expressed first question, BTW!
You are reading one char at a time from the file. Hence, if your file contains:
2424
You will first read the char "2" from the file, convert it to an int, and then back to a char, which will print "2" on cout. Next round will print "4", and so on.
If you want to read the numbers as full numbers, you need to do something like:
int theinteger;
thestream >> theinteger;
cout << char(theinteger) << endl;