I am in a bind right now and the most frustrating thing about this is that I know what the problem is but, I cannot fix it :(...
My goal is to ultimately use getline to read lines of strings from redirected input (from a text file) and keep going until EOF is reached.
Example text file (contents):
Hello World!
Good Bye.
My source code(only includes the section where it will not work):
while (!(getline(std::cin, s_array)).eof()){ // it won't read second line
//do some awesome stuff to the first line read!
}
As far as I know, getline reads everything upto the newline and stops so how do we get it to keep reading because it always stops at Hello World!.
Use while (getline(std::cin, s_array)) { } instead.
std::getline() returns istream&, and istream::operator void*() makes it evaluated as false whenever any error flag is set.
You should definitely read Joseph Mansfield's blog post titled "Don't condition input on eof()" which describes this pitfall in details and provides a well justified guideline.
Related
I have a program I am writing, I have already completed it, but the requirement/specification for it says that:
In the "Command Prompt", if someone runs your program like this:
Peter David < savednames.txt
it should print out the names inside savednames.txt that matches each of "Peter" and "David"
I have written the program but my own is interactive, i.e it asks for the file, then the name you want to search for, and then it prints the matches.
It works perfectly fine, but I don't understand what the running of the program in command prompt like this: "Peter David < savednames.txt" means. I am using C++ on Microsoft Windows.
Please I need your help to explain it and how to implement it in my code...is it some kind of operator overloading or ...I don't understand!
On the command line, < is used for input redirection. The shell opens the file whose name follows the < and copies its contents into standard input for the program.
So if you called program input.txt, you could open the file and read its contents using std::ifstream or whatever; if you called program < input.txt, you could just read the file's contents from stdin using cin.
You can also do the same for output. Instead of opening a file and writing to it in your code, write to stdout and call your program as program > output.txt.
yourprogramname Peter David < savednames.txt
means that your program will be called with Peter as first argument, David as second argument, and its standard input will be connected to a stream reading out of savednames.txt.
You just need to read standard input one row at a time, and process it according to the arguments you received.
You just need to modify the code you already have to take the names from the command line (is there a limit? Could there be three names? Or four? You need to consider that) and to read the data from standard input stream, without any need of opening a file.
Im using scanf because we must use it.
the problem is the following :
(thats just an example of the problem):
int main() {
char ch [10]={0};
scanf("%s",ch);
printf("%s",ch);
}
if i run the program and enter for example : word^Z
when ^Z is EOF.
the program stays in place, stuck in the scanf, althogh i did type word then ctrl+z then Enter. but somehow it stays in the scanf, its the same thing with redirection, like its not a problem with ctr+z or anything.
i hope that i can get some help
thanks in advance,
totally apprecaite it :)
scanf uses whitespace as a delimiter to store the read data into various fields. From the command line, entering ControlZ, then Enter only puts the EOF character into the input stream and scanf() continues waiting for whitespace. If you hit Enter again, scanf will receive the whitespace character, and everything including the EOF will be stored into the ch array.
Here's a sample run. The first line is the input, and the second line is the output.
Hello^Z
Hello→
I mam trying to complete a college assignment in C++ and am having trouble with what should be a very basic operation. I am trying to read a string of characters from the keyboard. This is the relevant code:
string t;
cout << endl << "Enter title to search for: ";
getline(cin, t, '\n');
I understand, that the last line is supposed to read the input buffer (cin , in this instance) and store the character in the 't' string until it reaches a new line character and then continue the program flow.
However, when I run my code in XCode, it just sort of jumps over the getline function and treats 't' as an empty string.
What's going on? I tried using cin >> t but that just read characters forever - Why cant I get this to behave?
The reason that the input operation apparently is skipped, is most probably (that means, ignoring possible peculiarities of a bugsy XCode IDE) that you have performed some input earlier and left a newline in the input buffer.
To fix that, make sure that you have emptied the input buffer after each input operation that logically should consume a line of input.
One easy way is to always use getline into a string, and then use e.g. an istringstream if you want to convert a number specification to number type.
Cheers & hth.,
From the docs page it looks like you want
cin.getline(t,256,'\n');
or something similar.
This sounds like an issue with the way Xcode is running your program. Try running your program directly from the terminal, and see if this is sufficient to fix your issue.
I am working on a simple application written in C. I am working in a Unix environment.
My application is doing some simple I/O. I use printf to prompt the user for some input and then use scanf to get that input.
The problem is, I don't know how to tell my application that I am ready to proceed after entering in a value. Typing 'enter' provides a newline '\n' which makes sense. Control-d does allow scanf to capture my input but seems to ignore any subsequent scanf instructions.
Can someone help me out?
printf("Enter name\n");
scanf("%s",input);
printf("%s",input);
printf("enter more junk\n")
scanf("%s",morestuff); /* cntrl+d skips this*/
Check the return value from scanf(). Once it has gotten EOF (as a result of you typing control-D), it will fail each time until you clear the error.
Be cautious about using scanf(); I find it too hard to use in the real world because it does not give me the control over error handling that I think I need. I recommend using fgets() or an equivalent to read lines of data, and then use sscanf() - a much more civilized function - to parse the data.
See also a loosely related question: SO 3591642.
[EDIT: This answer is incorrect, as I stated below, I'm learning as well]
Have you tried CTRL-Z?
That sends EOF to scanf, which, according to its man page, should make scanf move to the next field. As you've entered only a string as the input format, that should terminate the scanf.
I can't test this right now, but you can give it a shot.
Man page is here:
http://www.slac.stanford.edu/comp/unix/package/rtems/doc/html/libc/libc.info.scanf.html
I am reading in data from a file that has three columns. For example the data will look something like:
3 START RED
4 END RED
To read in the data I am using the following check:
while (iFile.peek() != EOF) {
// read in column 1
// read in column 2
// read in column 3
}
My problem is that the loop usually does an extra loop. I am pretty sure this is because a lot of text editors seem to put a blank line after the last line of actual content.
I did a little bit of Googling and searched on SO and found some similar situations such as Reading from text file until EOF repeats last line however I couldn't quite seem to adapt the solution given to solve my problem. Any suggestions?
EOF is not a prediction but an error state. Hence, you can't use it like you're using it now, to predict whether you can read Column 1, 2 and 3. For that reason, a common pattern in C++ is:
while (input >> obj1 >> obj2) {
use(obj1, obj2);
}
All operator>>(istream& is, T&) return the inputstream, and when used in boolean context the stream is "true" as long as the last extraction succeeded. It's then safe to use the extracted objects.
Presuming iFile is an istream:
You should break out of the loop on any error, not only on EOF (which can be checked for with iFile.eof(), BTW), because this is an endless loop when any format failure sets the stream into a bad state other that EOF. It is usually necessary to break out of a reading loop in the middle of the loop, after everything was read (either successfully or not), and before it is entered.
To make sure there isn't anything interesting coming anymore, you could, after the loop, reset the stream state and then try to read whitespace only until your reach EOF:
while( !iFile.eof() )
{
iFile >> std::ws;
string line;
std::getline(iFile,line);
if(!line.empty()) error(...);
}
If any of the reads fail (where you read the column data), just break out of the while loop. Presumably you are then at the end of the file and reading the last 'not correct' line
Maybe you'll consider it a good idea to handle whitespace and other invalid input then. Perhaps some basic validation of columns 1,2,3 would be desirable as well.
Don't worry about the number of times that you loop: just validate your data and handle invalid inputs.
Basically, check that you have three columns to read and if you don't decide if it's because the file is over or because of some other issue.