I need some help with a code.
I need to take this information to my c++ code from another file, the last one is just like this:
Human:3137161264 46
This is what I wrote for it, it takes the word "Human" correctly but then it takes random numbers, not the ones written on the file I just wrote:
struct TSpecie {
string id;
int sizeGen;
int numCs; };
__
TSpecie readFile(string file){
TSpecie a;
ifstream in(file);
if (in){
getline(in,a.id,':');
in >> a.sizeGen;
in >> a.numCs;
}
else
cout << "File not found";
return a; }
Hope you can solve it and thanks for your help
3137161264 causes integer overflow leading to Undefined Behaviour.
So unsigned int sizeGen would be enough for this case, but consider long long (unsigned) int sizeGen too.
Edit 1: As pointed out by #nwp in comments to your question, you can also check your stream if any error has occured:
//read something and then
if (!in) {
// error occured during last reading
}
Always test whether input was successful after reading from the stream:
if (std::getline(in, a.id, ':') >> a.sizeGen >> a.NumCs) {
// ...
}
Most likely the input just failed. For example, the first number probably can't be read successful. Also note that std::getline() is an unformatted input function, i.e., it won't skip leading whitespace. For example the newline after the last number read is still in the stream (at least, since your use of std::getline() finishes on a colon, it will only create an odd ID).
Hey I know I can just use a string to read from a text file. However I need to use a char array. Like If I was using a string I would do this
while (!input.eof()){
input >> s;
}
I am unsure how I would go about this if I don't know the length of the string. I know I can use getLine, but I'll prefer to use input.
I'm thinking that maybe I can use a loop to check until it reaches "\0"?
Anyway I have a feeling this question has been asked before, but if it has I can't find it. So sorry if that is the case.
You can consider istream::getline. Note that it can be use for C++ string and it must have a length limit for C string.
I think you should avoid check eof directly in while condition. It only returns true it reach end-of-file. So if you have multiple line, you read it, then do some calculate, the consequence can be unexpected when it reach the end-of-file right at reading step. So the check of EOF should be placed right after reading from stream like my example.
int main()
{
ifstream input("filename.txt");
const int MAX = 10000;
char characters[MAX];
while (true) {
input.getline(characters, MAX - 1, '\n');
if (input.eof())
break;
}
}
I'm trying to create a student database system for a school project. I'm trying to create a function that will search a .txt file for the student id and return all of the other variables on the string. This is working great if I search for the id of the student on the first line of the txt file but isn't capturing anything if I search for a student on another line. Am I missing something obvious?
The student data is 16 strings delimited by commas on each line. The student ID is the first string.
Thanks for any assistance!
StudentType findStudent(int studentToFind)
{
ifstream inFile;
inFile.open("students.txt");
string currentLine;
string dataRead[16];
istringstream is;
int currentStudent;
if (inFile)
{
while (getline(inFile, currentLine))
{
is.str(currentLine);
for (int i = 0; i < 16; i++)
{
getline(is, dataRead[i], ',');
}
currentStudent = stoi(dataRead[0]);
if (currentStudent == studentToFind)
{
/*
Do stuff here
*/
inFile.close();
return foundStudent;
}
cin.ignore(); // Not sure if this is needed but I was trying to
// clear the \n char if that was causing the issue
}
}
}
First : you aren't using cin, so get rid of cin.ignore().
Second : you should make sure you ALWAYS close infile at the end... so I would suggest not returning early or closing early, but using a break statement to exit your loop and then have a single return of whether you found it or not.
Third: Now that you removed all the 'gorp' we can finally hone in on the problem ... effectively the question is do we read all the lines?
Well let's check that, try printing out currentLine each time at the beginning of the while loop, if you know currentLine is updated properly, is is getting updated each time? yes...
ok then look at your next loop let's print out currentStudent each time... does currentStudent print the right value for each line? i.e. is the getline write into dataRead[i] actually writing what you think it should be to the right space?
Did you find the problem yet?
This is the kind of problem you need to learn how to solve yourself using print statements and a debugger. That what its for. If you are in visual studio run in debug mode and step through it... if not, use gdb. learn it and get used to it, you'll be using it a lot!
good luck
I'm tring to do a simple exercise here, but i need to understand how EOF works first.
void main()
{
char s1[1000];
while (scanf("%s", s1)!=EOF)
;
printf("%s",s1);
}
The idea is to have multiple lines in input, and display them.
The problem I have is that if I put
Hello World
This is stackoverflow
When printf is called, it only prints
stackoverflow
Why isn't it printing everything and how do I make it print?
Regards
Remove the semicolon ;:
while (scanf("%s", s1)!=EOF)
printf("%s",s1);
Note that this will still exhibit odd behavior at end of file depending on how it ends exactly. Furthermore, it splits the input into words, which are separated by spaces or new lines. You may want to simply split into lines.
So you may be better served with for instance:
while (gets(s1)!=NULL)
puts(s1);
This code fragments reads your input line by line until end-of-file.
To read everything (or as much as your buffer can hold), you can use:
char s1[1000] = "";
fread(s1, sizeof(s1) - 1, 1, stdin);
puts(s1);
However, my preferred method of reading a text file is:
using namespace std;
string line;
while (getline(cin, line))
{
cout << line << endl;
}
That is because usually I want to process a file line by line, and getline with a string ensures the line buffer is always big enough.
You probably want this:
char s1[1000][20];
int i = 0 ;
while (!feof(stdin))
fgets(s1[i++], 20, stdin) ;
int j ;
for (j = 0; j < i; j++)
printf("%s\n", s1[j]);
Here you can enter at most 1000 lines that are maximum 19 characters long.
What you have is a loop that reads words into a buffer until it reaches EOF (and does nothing with those words), followed by a printf to print the contents of the buffer. The printf is after the loop (not in it), so executes once after the loop completes. At that time, the buffer will contain the last word read, so that is what gets printed.
The EOF return test means "nothing more to be read", which isn't necessarily an end of file (might be an error condition of some kind), but in practice that distinction can be ignored. Looping until your reading function returns EOF or NULL (depends on function) is good practice.
If you want to print each word as it is read, you need to put a printf in the loop.
If you want to store the words for later processing, you need to store them somewhere. That means declaring some storage space, or allocating space on the heap, and some bookkeeping to track how much space you've used/allocated.
If you want lines rather than words, you should use fgets instead of scanf("%s". Note that fgets returns NULL rather than EOF when there's nothing more to be read.
Because it only prints the last thing that is read from the file ("stackoverflow"). This is caused by the semicolon after the end of your while(...); - this means that you are doing while(...) { /* do nothing */} - which is probably not what you wanted
Also, printf("%s",s1)!='\0'; makes no sense at all. For one thing, printf returns the number of characters printed - '\0' is the value zero written as a character constant. And of course, doing != 0 of the result without some sort of use of the comparison is pretty much pointless too.
Use fgets instead of scanf if you want to read one line at at time. scanf will stop reading when it finds a whitespace. fgets will read till the end of the line.
Use fgets(). Simple and sweet
char buf[1000];
while (fgets(buf, sizeof buf, stdin) != NULL) {
fputs(buf, stdout);
}
Here is how end-of-file works in C. The input channels are called input streams; disk files and stdin are both input streams. The "end-of-file" state is a flag that a stream has, and that flag is triggered when you try to read from a stream, but it turns out there are no more characters in the stream, and there never will be any more. (If the stream is still active but just waiting for user input for example, it is not considered to be end-of-file; read operations will block).
Streams can have other error states, so looping until "end-of-file" is set is usually wrong. If the stream does go into an error state then your loop will never exit (aka. "infinite loop").
The end-of-file state can be checked by feof. However, some input operations also can signal an error as well as, or instead of, returning the actual data they were intended to read. These functions can return the value EOF. Usually these functions return EOF in both cases: end-of-file, and stream error. This is different to feof which only returns true in the case of end-of-file.
For example, getchar() and scanf will return EOF if it was end-of-file, but also if the stream is in an error state.
So it is OK to use getchar()'s result as a loop condition, but not feof on its own.
Also, it is sometimes not OK to use scanf() != EOF as a loop condition. It's possible that there is no stream error, but just that the data you requested wasn't there. For example, if you scan for "%d" but there are letters in the stream. Instead, it's better to check for successful conversion (scanf returns the number of successful conversions it performed). Then when you exit your loop, you can go on to call feof and ferror to see whether it was due to end-of-file, or error, or just unexpected input.
I am writing a program which reads data from different files, which are given as input strings, and stores them into a vector of vectors. The problem I am not able to debug the loop which reads different files. I have closed the ifstream object, cleared the string using empty function... but still it just terminates when i give second file name as input.
I am copying the code for your perusal. It is a function called by another another function. Transposectr transposes a matrix.
code:
vector<vector<float> > store1,store2;
ifstream bb;
string my_string;
float carrier;
vector<float> buffer;
cout<<"enter the file name"<<endl;
getline(cin,my_string);
while (my_string!="end")
{
bb.open(my_string.c_str());
while (!bb.eof())
{
bb >> carrier;
if (bb.peek() == '\n' || bb.eof() )
{
buffer.push_back(carrier);
store1.push_back(buffer);
buffer.clear();
}
else
{
buffer.push_back(carrier);
}
}
bb.close();
buffer.clear();
transposectr1(store1);
storex.push_back(store1[1]);
storey.push_back(store1[0]);
store1.clear();
my_string.empty();
cout<<"done reading the file"<<endl;
cout<<"enter the file name"<<endl;
getline(cin,my_string);
}
I'm really not clear what you are trying to do. But I have one golden ruile when it comes to using istreams:
Never use the eof() function!
It almost certainly does not do what you think it does. Instead you should test if a read operation succeeded.
int x;
while( in >> x ) {
// I read something successfully
}
You might also want to avoid peek() too. Try re-writing your code with this advice in mind.
Add
bb.clear();
after the bb.close() you may get the right thing. bb.close() doesn't reset the cursor I think.
Neil Butterworth is right
Never use the eof() function!
This link explains why.