Quickest Way to Read First Line from File

Quickest Way to Read First Line from File

Well, you could do:

$f = fopen($file, 'r');
$line = fgets($f);
fclose($f);

It's not one line, but if you made it one line you'd either be screwed for error checking, or be leaving resources open longer than you need them, so I'd say keep the multiple lines

Edit

If you ABSOLUTELY know the file exists, you can use a one-liner:

$line = fgets(fopen($file, 'r'));

The reason is that PHP implements RAII for resources.

That means that when the file handle goes out of scope (which happens immediately after the call to fgets in this case), it will be closed.

How to extract the first line from a text file?

To read the first line of a file, you can do:

con <- file("file.txt","r")
first_line <- readLines(con,n=1)
close(con)

To write it out, there are many options. Here is one:

cat(first_line,file="first_line.txt")

How to get the first line of a file in a bash script?

head takes the first lines from a file, and the -n parameter can be used to specify how many lines should be extracted:

line=$(head -n 1 filename)

Read only first line from a text file

You can make use of File.ReadLines together with Enumerable.First.
This guarantees you to only read the first line from the file.

using System.Linq; 

...

string line1 = File.ReadLines("MyFile.txt").First(); // gets the first line from file.

The difference to File.ReadAllLines is, that File.ReadLines makes use of lazy evaluation and doesn't read the whole file into an array of lines first.

Linq also makes sure of properly disposing the FileStream.

What is the most efficient way to get first and last line of a text file?

docs for io module

with open(fname, 'rb') as fh:
    first = next(fh).decode()

    fh.seek(-1024, 2)
    last = fh.readlines()[-1].decode()

The variable value here is 1024: it represents the average string length. I choose 1024 only for example. If you have an estimate of average line length you could just use that value times 2.

Since you have no idea whatsoever about the possible upper bound for the line length, the obvious solution would be to loop over the file:

for line in fh:
    pass
last = line

You don't need to bother with the binary flag you could just use open(fname).

ETA: Since you have many files to work on, you could create a sample of couple of dozens of files using random.sample and run this code on them to determine length of last line. With an a priori large value of the position shift (let say 1 MB). This will help you to estimate the value for the full run.

How to read first N lines of a file?

Python 3:

with open("datafile") as myfile:
    head = [next(myfile) for x in range(N)]
print(head)

Python 2:

with open("datafile") as myfile:
    head = [next(myfile) for x in xrange(N)]
print head

Here's another way (both Python 2 & 3):

from itertools import islice

with open("datafile") as myfile:
    head = list(islice(myfile, N))
print(head)

How to skip reading the first line of file?

When I saw this post it reminded me of a similar task completed at uni. I have rewritten your code to perform the same task but using classes instead of structs. I have also included a way to sort the vector by using the function here.

I have included the "ignore first line" method @Scheff's Cat mentioned.

Here it is:

#include <iostream>
#include <fstream>
#include <sstream>
#include <limits>
#include <string>
#include <vector>

using namespace std;

class studentrecord{
    string firstname, lastname, grade;        
    public:
    studentrecord(string firstname, string lastname, string grade){
        this -> firstname = firstname;
        this -> lastname = lastname;
        this -> grade = grade;
    }

    friend ostream& operator<<(ostream& os, const studentrecord& studentrecord) {
        os << "\n\t" << studentrecord.firstname << "\t" << studentrecord.lastname << "\t" << studentrecord.grade;
        return os;
    }
};

void displayRecords(vector <studentrecord*> records){
    for(int i = 0; i < records.size(); i++){
        cout << *records[i];
    }
}

int main(){
    //read in file
    ifstream infile;
    infile.open("student-file.txt");
    infile.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    if (!infile.is_open()){
            cout << "Error opening student records file" <<endl;
            exit(1);
    }
    vector <studentrecord*> records;
    string firstname, lastname, grade;
    while (infile >> firstname >> lastname >> grade;) {
        records.push_back(new studentrecord(firstname, lastname, grade));
    }

    displayRecords(records);

    return 0;
}

To sort the vector so that it prints in order of either first name, last name or grade I used the following functions:

bool sortfirstname(studentrecord* A, studentrecord* B) {
    return (A->getfirstname() < B->getfirstname());
}

bool sortlastname(studentrecord* A, studentrecord* B) {
    return (A->getlastname() < B->getlastname());
}

bool sortgrade(studentrecord* A, studentrecord* B) {
    return (A->getgrade() < B->getgrade());
}

sort(records.begin(), records.end(), (sortfirstname));
sort(records.begin(), records.end(), sortlastname);
sort(records.begin(), records.end(), sortgrade);

If you wanted to sort by first name you would call the sort(records.begin(), records.end(), (sortfirstname)); function and then the displayrecords() function.

The advantage of using classes stored in vectors is that you don't have to state the size of the vector containing the details about students since you can keep adding information to the end of the vector using the vector.push_back() function. It also makes sorting the data contained easier.

If anything isn't clear, let me know and I can give you a hand.

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