C++ program that displays on the screen item codes with corresponding
item descriptions and prices. It asks the user to enter the code of the item
purchased by a customer. It looks for a match of the item code stored in items.txt.
How can I output only a specific line from a text file after the user inputs the item code?
You need to read the file line-by-line (std::getline), extract (depending on the exact format, e.g. by searching for a whitespace in the string) and compare the code and then return the corresponding line on a match.
It is not possible to access lines from a text file directly by index or content.
This is assuming that you mean the file contains lines in the form
code1 item1
code2 item2
//...
If the code is just the index of the line, then you only need to call std::getline in a loop with a loop counter for the current index of the line.
If you do this multiple times on the same file, you should probably parse the whole content first line-by-line into a std::vector<std::string> or a std::(unordered_)map<std::string, std::string> or something similar to avoid the costly repeated iteration.
Depending on the use case, maybe it would be even better to parse the data into a database first and then query the database, even if it is only e.g. sqlite or something like that.
Related
I would like to read a text file in C++ in following manner:
Ignore the entire first line as it is simply meant as an introduction.
Only read the following lines from a specific position.
That starting position for reading is a fixed one and remains the same for every line; however, the numbers after that may be of variable length. I need to save all of these numbers from line 2 to line n into an Array.
At the moment I can read a regular 2D Array with getline.
How can I work around these things?
An example for a line I want to read could be:
Person1: 25 988.3 0.0023 7
To set the file to a position, use std::ifstream::seekg().
To set the file to the beginning of a line, you must read and count the line endings. Many text files have variable length text lines.
How can I work around these things?
You can't, unless you can ensure that all of the data lines after the first line are all the same length.
If you can't ensure that, then all you can do is read through all of the preceding lines.
An alternative I have employed in the past is to generate an 'index' of line start positions in a secondary file in binary format (so that I CAN jump directly to the right place in that file), and use that to jump to the right place in the text file. Of course that means that you need to regenerate that index file every time you replace/amend the data file.
I have to extract information from a text file.
In the text file there is a list of strings.
This is an example of a string: AAA101;2015-01-01 00:00:00;0.784
The value after the last ; is a non integer value, which changes from line to line, so every line has different lenght of characters.
I want to map all of these lines into a structured vector as I can access to a specific line anytime I need without scan the whole file again.
I did some research and I found some threads about a command called, which permit me to reach a specific line of a text file but I read it only works if any line has the same characters lenght of the others.
I was thinking about converting all the lines in the file in a proper format in order to be able to map that file as I want but I hope there is a better and quick way
You can try TStringList*. It creates a list of AnsiStrings. Then each AnsiString can be accessed via ->operator [](numberOfTheLine).
I've written a program to parse through a log file. The file in question is around a million entries or so, and my intent is to remove all duplicate entries by date. So if there's 100 unique log-ins on a date, it will only show one log-in per name. The log output I've created is in the form:
AA 01/Jan/2013
AA 01/Jan 2013
BB 01/Jan 2013
etc. etc. all through the month of January.
This is what I've written so far, the constant i in the for loop is the amount of entries to be sorted through and namearr & datearr are the arrays used for name and date. My end game is to have no repeated values in the first field that correspond to each date. I'm trying to follow proper etiquette and protocols so if I'm off base with this question I apologize.
My first thought in solving this myself is to nest a for loop to compare all previous names to the date, but since I'm learning about Data Structures and Algorithm Analysis, I don't want to creep up to high run times.
if(inFile.is_open())
{
for(int a=0;a<i;a++)
{
inFile>>name;//Take input file name
namearr[a]=name;//Store file name into array
//If names are duplicates, erase them
if(namearr[a]==temp)
{
inFile.ignore(1000,'\n');//If duplicate, skip to next line
}
else
{
temp=name;
inFile.ignore(1,' ');
inFile>>date;//Store date
datearr[a]=date;//Put date into array
inFile.ignore(1000,'\n');//Skip to next like
cout<<namearr[a]<<" "<<datearr[a]<<endl;//Output code to window
oFile<<namearr[a]<<" "<<datearr[a]<<endl;//Output code to file
}
}
}
Ughhh... You better use a Regular Expression library to easily deal with that size of a file. Check Boost Regex
http://www.boost.org/doc/libs/1_55_0/libs/regex/doc/html/index.html
You can construct a key composed of the name and the date with simple string concatenation. That string becomes the index to a map. As you are processing the file line by line, check to see if that string is already in the map. If it is, then you have encountered the name on that day once before. If you've seen it already do one thing, if it's new do another.
This is efficient because you're constructing a string that will only be found a second time if the name has already been seen on that date and maps efficiently search the space of keys to find if a key exists in the map or not.
I'm writing a payroll program in c++ and need to be able to read lines in a file, do calculations, and then overwrite the read lines in the file. IS there a function/way i can simply overwrite specific lines, insert new lines, add onto the end of an existing file?
There are no C++ functionality to "insert" or "remove" text in a text-file. The only way to do that is to read the existing text in, and write out the modified text.
If the new text fits in the same space as the old one, all you need to do is to overwrite the existing text - and of course, you can always add extra spaces before/after a comma in a .CSV file, without it becoming part of the "field". But if the new data is longer, it definitely won't work to "overwrite in place".
Adding to the end is relatively easy by using the ios_base::ate modifier. But inserting in middle still involves basically reading until you find the relevant place, and then, if the new text is longer, you have to read all the following lines before you can write the new one(s) out.
I'm new to C++ and probably have a silly question. I have an ifstream which I'd like to split approximately in half.
The file in question is a sorted csv and I wish to search on the first value of each line of the file.
Eventually the file will be very large so I am trying to avoid having to read every line of the file.
e.g.
If the file contains 7 lines I'd like to split the ifstream to give 1 stream containing the first 3 lines and 1 stream containing the last 4 lines.
First, use the answer to this question to determine the size of your file. Then divide that number by two. Read the input line by line, and write it to the first output stream; check file.tellg() after each call. Once you're past the half-way point, switch the output to the second file.
This wouldn't split the strings evenly between the files, but the total number of characters in these strings should be close enough, and it wouldn't split your file in the middle of a string.
Think of it as a relational database with one huge table. In order to find a certain piece of data, you can either do a sequential scan over the entire table, or use an index (which must be usable for the type of query you want to perform).
A typical index for a text file would be a list of offsets inside the file, sorted by the index expression. If the csv file is sorted by a specific column already, then the offsets in the index would be ascending, which is useful to know when building the index.
So basically you have to read the file once anyway, to find out where lines end; this is the index for the sort column. To find a particular element, use a binary search, using the index to find individual elements in the data set.
Depending on the data type, you can extend your index to allow for quick comparison without reading the actual data table. For example, in a word list you could keep the first four letters of the word next to the offset, which allows you to get into the right area quickly and only requires data reads for the last accesses (which you can then optimize to a sequential scan, as filesystems handle that a lot better).
The same technique can be applied to the other columns as well; the offsets stored in the index would no longer be ascending in file order, of course.
Since it is CSV data, a special case also applies: If the only index you have is in the same order as the file data itself and the end of record can be determined easily (that is, either you have a fixed record length, or there is a clear record separator, such as an EOL character), then building the actual index can be omitted and the values guessed (for fixed length records, offset is always equal to record length times offset in the index; for separated records you can just jump into the middle of a record and seek for the next terminator; be aware that there are nasty corner cases with binary search here). This does however mean that you will always be reading data pages here, which is less efficient than just reading the index.