I found in this code on codeforces . I am not that expert please guide me the use of these lines of code
The question just reads an input string of integers of maximum length 1000
ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
#ifndef ONLINE_JUDGE
freopen("test.in", "r", stdin);
#endif
The first line is basically done to speedup reading files since these features are not generally required in competitive coding (take this with a grain of salt):
Usage of ios_base::sync_with_stdio can be found in Using scanf() in C++ programs is faster than using cin?
Usage of cin.tie(0), cout.tie(0) can be found in Why do we need to tie std::cin and std::cout?
As for using ONLINE_JUDGE it has been explained in codeforces blog. Basically, when codeforces runs the code online it adds the ONLINE_JUDGE flag. In your case if you are running the code at home you can ignore ONLINE_JUDGE flag and it will read your test file test.in. The same code while running in Codeforces will have ONLINE_JUDGE set and will ignore the freopen("test.in", "r", stdin); line and run its own test cases.
Related
I'm doing this challenge thing and this is one of the levels:
An agent on Level 05 has told us about another big hack he's working on. Apparently someone broke into a popular shopping site, stole all the usernames and passwords and was going to post them online. Luckily, we got to them first and recovered the details. Why is this important? Well, it seems one of the Yakoottees was a member of that site.
He typically uses one of these three usernames: kazuya, kaz_whizz, kazuya99. We've put the recovered data on one of our servers. We've given you access, so see if you can find him on there. If we knew the password he uses maybe we can use that later.
Tip: The flag is his password.
I think I need to read this file called "182k_accounts_rip.txt". The directory is "/root/site_pwned" i think.
Maybe I need to read the file to get the password? The 'cat', 'less', 'more' and 'tail' command do not work. Maybe I need to see the current password with the user names mentioned? Please help
To read the file you can invoke vi or vim command from program using system function or execve family function as
int main(int argc, char *argv[]) {
system("vim 182k_accounts_rip.txt");
perror("system");
/* do the stuff here like use grep to find the pattern*/
return 0;
}
I don't find a way to check the free space available in a device using Haxe, Openfl, Lime or another library.
I would like to avoid download data that will exceed the size recommended for an app in each device.
What do you do to check that?
Try creating a file of that size! Then either delete it or reopen and write (not append) over its contents.
I don't know whether all platforms Haxe supports will work fine with this trick, but this algorithm is reported to work in many places and languages (I personally tested it in Ruby and saw the same suggestion for C++/.NET). To check whether X bytes of disk space are available:
open a new file for writing
seek X-1 bytes from the beginning
write a byte of data (whatever you want, 0, 42...)
close the file (probably unrelated to the task at hand, but don't forget to do that anyway)
If there's insufficient disk space, you'll likely get an exception at some point in this algorithm. You'll have to find out what errors to expect and process them properly.
Using ihx I've found this is working and requires nothing but Haxe Standard Library:
haxe interactive shell v0.3.4
type "help" for help
>> import sys.io.*;
>> var f = File.write('loca', true)
sys.io.FileOutput : { __f => #abstract }
>> f.seek(39999, FileSeek.SeekBegin)
Void : null
>> f.writeByte(0)
Void : null
>> f.close()
Void : null
After these manipulations, I had a file named loca of exactly 40000 bytes in my working directory.
By the way, be careful when doing things like these in ihx since it re-runs the entire session with the last entered line appended each time.
Ongoing experimentation:
However, when there's insufficient disk space, it may not fail with errors. In this case you'll have to check the real size with sys.FileSystem.stat(path).size. And don't forget to delete the file if there's not enough space.
I have a perl script that traverses a set of directories and when it hits one of them it blows up with an Invalid Argument and I want to be able to programmatically skip it. I thought I could start by finding out the file type with the file command but it too blows up like this:
$ file /sys/devices/virtual/net/br-ex/speed
/sys/devices/virtual/net/br-ex/speed: ERROR: cannot read `/sys/devices/virtual/net/br-ex/speed' (Invalid argument)
If I print out the mode of the file with the perl or python stat function it tells me 33060 but I'm not sure what all the bits mean and I'm hoping a particular one would tell me not to try to look inside. Any suggestions?
To understand the stats number you got, you need to convert the number to octal (in python oct(...)).
Then you'll see that 33060 interprets to 100444. You're interested only in the last three digits (444). The first digit is file owner permissions, the second is group and the third is everyone else.
You can look at each of the numbers (in your case all are 4) as 3 binary bits in this order:
read-write-execute.
Since in your case owner, group & other has 4, it is translated (for all of them) to 100 (in binary) which means that only the read bit is on for all three - meaning that all three can only read the file.
As far as file permissions go, you should have been successful reading /sys/devices/virtual/net/br-ex/speed.
There are two reasons for the read to fail:
- Either speed is a directory, (directories require execute permissions to read inside).
- Or it's a special file - which can be tested using the -f flag in perl or bash, or using os.path.isfile(...) in python.
Anyhow, you can use the following links to filter files & directories according to their permissions in the 3 languages you mentioned:
ways to test permissions in perl.
ways to test permissions in python.
ways to test permissions in bash.
Not related to this particular case, but I hit the same error when I ran it on a malicious ELF (Linux executable) file. In that case it was because the program headers of the ELF was intentionally corrupted. Looking at the source code for file command, this is clear as it checks the ELF headers and bails out with the same error in case the headers are corrupted:
/*
* Loop through all the program headers.
*/
for ( ; num; num--) {
if (pread(fd, xph_addr, xph_sizeof, off) <
CAST(ssize_t, xph_sizeof)) {
file_badread(ms);
return -1;
}
TLDR; The file command checks not only the magic bytes, but it also performs other checks to validate a file type.
This program puzzles me. The goal of this program is to count the number of newlines in a file and output it in command prompt. Learnyounode then runs their own check on the file and sees if their answer matches your answer.
So I start with the answer :
var fs = require('fs');
var filename = process.argv[2];
file = fs.readFileSync(filename);
contents = file.toString();
console.log(contents.split('\n').length - 1);
learnyounode verifies that this program correctly counts the number of new lines. But when I change the program to any of the following, it doesn't print out the same number as learnyounode prints out.
file = fs.readFileSync(C:/Nick/test.txt);
file = fs.readFileSync(test.txt);
Shouldn't nodejs readFileSync be able to input an address and read it correctly?
Lastly, this program is supposed to print out the # of newlines in a program. Why does both the correct program and learnyounode print out the same number that is different from the amount of newlines everytime I run this program?
For example, the number of newlines in test.txt is 3. But running this program prints out a different number everytime, like 45, 15, 2, etc. Yet at the same time, it is verified as a correct program by learnyounode because both their answers match! What is going on?
EDIT:
test.txt looks like this
ok
testing
123
So, I tried your program on my local machine and your program works fine. I am not an expert on learnyounode. I just tried it after your question but I think I understand how it works. As such, here are the answers to your questions:
Shouldn't nodejs readFileSync be able to input an address and read it correctly?
This method from nodejs is working fine. You can try printing the contents of the file and you'll see that there are no problems.
Why does both the correct program and learnyounode print out the same number that is different from the amount of newlines everytime I run this program.
learnyounode is running your program with a different filename as input each time. It verifies the output of your program by running its own copy of correct code against the same file.
But when I change the program to any of the following, it doesn't print out the same number as learnyounode prints out.
That is because at this point, your code is processing a fixed file whereas learnyounode is still processing different files on each iteration.
This tripped me up too. If you read the learnyounode instructions closely they explicitly say...
"The full path to the file to read will be provided as the first command-line argument."
This means they are providing the path to their own file.
When you use process.argv[2], this is passing in the 3rd array item (the learnyounode test txt file) into your script. If you run a console.log(process.argv); you'll see the full array object looks something like this:
[ '/usr/local/bin/node',
'/Users/user/pathstuff/learnyounode/firstio.js',
'/var/folders/41/p2jvc80j26l7nty0sk0zs1z40000gn/T/_learnyounode_1613.txt' ]
The reason the validation numbers begin to mismatch when you substitute your own text file for their is because your file always has 3 lines whereas their unit tests keep passing in different length files via process.argv.
Hope that helps.
when you are using process.argv[2] in learnyounode, the argument is provided by learnyounode automatically, so it prints different number of lines like 45, 15, 2 etc at multiple times verification.
If you remember the second challenge "BABYSTEPS" carefully this was given:
learnyounode will be supplying arguments to your program when you run
learnyounode verify program.js so you don't need to supply them yourself.
That's why different line numbers at program.js verification on multiple times.
there are two different ways.
if you run program like:
node program_name.js
than you need to add path to text file:
node program_name.js text_file.txt
in this case make sure that files are in the same directory.
or you can run it with command:
learnyounode program_name.js
and than default text file will be provided by learnyounode. You can watch content of this text file by using
console.log(buffer)
Problem statement says
The full path to the file to read will be provided as the first
command-line argument.
So you've to pass the path/to/file as an argument.
Remember process.argv
you should use the following method to execute .js files
node program_name.js /path/to/text_file_name
rather than
learnyounode run program_name.js /path/to/text_file_name
on this method, Node.js will run your program with specify files of you enter on the command-line-interface.
wish this answer can help you programming. :)
I make several calls to a function that reads data from an input file. Everything works fine in debug mode, but when I try to run the executable from release mode, the line with fopen crashes the program after a few calls. My code is:
From header file:
#define presstankdatabase "presst_database.txt"
In function:
FILE *fidread;
fidread = fopen(presstankdatabase,"r");
if (fidread==NULL) {
printf("Failed to open pressurant tank database: %s\n",presstankdatabase);
return 1;
}
In debugging, I've inserted comment lines just before and just after the line starting with fidread =, and after several calls the program crashes and I get the message "A problem caused the program to stop working correctly. Please close the program." The comment just before the fopen call is displayed, but the comment just after does not. My understanding of fopen is that is should return either a pointer or NULL, but it crashes before it even gets to the check. The only thing I can think of is that somehow I'm having memory problems, but I don't know how that would fit in with fopen crashing. Does anyone know what might be going on? Thanks!
EDIT 1: I increased the size of three variables, and the only places they're used (except in printf() calls), are as shown below.
char *constid = (char*)malloc(sizeof(char)*20);
Used like so:
strcpy(constid,"Propellant");
strcpy(constid,"Propellant tank");
strcpy(constid,"Pressurant tank");
If the variables are sized to 20, as shown above, it crashes. But if they're larger (I've tried 120 and 100), the program runs. The variables aren't used in any other places other than fprintf() or printf() calls.
presstankdatabase should be a pointer to a string containing the filename to open. If fopen() crashes then that pointer is probably invalid (or NULL). Without any more code it is not possible to debug it further. Use the VC debugger to see what's happening...
EDIT:
Another common cause of this is a filename string that suddenly stops being NULL-terminated.
You should add a printf() call to print the filename before opening. It will most probably fail to produce the expected output. If not, then you have a more interesting form of memory corruption that will take some more work to weed out.
EDIT 2:
If the printf() call shows the correct string, then you probably have memory corruption somewhere else in your code that has mangled some internal structure of the C library. A common cause is going beyond the end (or the beginning for that matter) of a static array or a region provided by malloc().