string search and copy in a file !

Hello,
I have a text file which contains this...
Mass Flow Rate = 1.06984
Total Press Ratio = 1.05424
Adiabatic Efficiency = 0.730169
Exit Flow Coeff Cr2/U2rms = 0.438395
Ps2/Pa = 0.990686
i want to seach for this "Adiabatic Efficiency = 0.730169" and print it.
this is my code

Code:

---

#include <fstream>
#include <iostream>
#include <string>
using namespace std;

void main()
{
        string c;
        ifstream reads("R0_1st.txt");
        reads.seekg(ios::beg);
        reads.unsetf(ios::skipws);
        while(!reads.eof())
        {
        getline (reads,c);
        cout<<c<<endl;
        }
}

---

this code prints all the file
i just want to seach for the string before i print
Thanks

>void main()
int main(). You have no excuses for not doing it.

>reads.seekg(ios::beg);
Why do you think this is needed?

>while(!reads.eof())
This is the wrong way to read from a stream. The eofbit is only set after you try and fail to read, so you'll end up processing the last successfully read record twice.

>i just want to seach for the string before i print
Well, you're already using std::strings, so it's pretty straightforward:

Code:

---

while ( getline ( reads, c ) ) {
  if ( c.find ( "Adiabatic Efficiency" ) != string::npos )
    cout<< c <<'\n';
}

---

you can consider me a noob

why would i need "int main()" and not "void main()" for me it's just a matter of not writing return 0; at the end !! i never knew it means something :S

and if i'm reading the file the wrong way
what is the right way ?!

thank you for replying that was very useful

>> int main(). You have no excuses for not doing it.

I don't know why so many "higher up" programmers HATE that.
They take it as an offence and as lazyness.
I just don't get it.

So you might say it's there for other programs to get it's return code, so, many just use return 0; everywhere, wether error or no.

>why would i need "int main()" and not "void main()"
Because int is right and void is wrong. Eventually you'll be directed to the C++ standard, where these things are documented, but for now just take our word for it. :)

>for me it's just a matter of not writing return 0; at the end !!
Remember that I said there are no excuses. C++ automatically returns 0 even if you omit the return statement. This is a perfectly legal C++ program that returns 0:

Code:

---

int main()
{
}

---

>and if i'm reading the file the wrong way
>what is the right way ?!
I showed you. You use the return value of getline. It returns the stream, which when used in a conditional statement tells you whether the stream is in a failure state or not. The failure state includes end-of-file, so all you need to do is:

Code:

---

while ( getline ( my_stream, my_string ) ) {
  // Work with a line
}

---

You can do the same thing with basically every input method. Another common one is:

Code:

---

while ( my_stream>> my_var ) {
  // Use the variable
}

---

>I don't know why so many "higher up" programmers HATE that.
Because it's wrong. The C++ standard explicitly states that main returns an int. If an implementation doesn't provide for void main then the entire program is has undefined behavior. And undefined behavior is the worst possible thing you can have in C++.

>They take it as an offence and as lazyness.
I'm sorry if it seems that way. We're just on a quest to avoid undefined behavior and promote correct C++.

First hit on google: http://users.aber.ac.uk/auj/voidmain.shtml

And return 0 is implicit in main as defined by the standard (so you can omit a return statement in main)

I didn't know what string::npos was there for
so i searched and this is what i found

Quote:

---

An unsigned integral value initialized to –1 that indicates either "not found" or "all remaining characters" when a search function fails.

---

is it right ?
it sounds reasonable.
and if so what does it mean by " all remaining characters "?

Thanks

It's not correct that definition. First there is no guarantee that the value of string::npos is -1. But also how can an unsigned integral possibly be -1? It can't.

npos is defined by the standard as simply being greater than any possible index. That is, it will hold a value that is greater than the last index of the largest possible string. npos is what is returned by several of the string find functions when there is no match.

how come it's not right?
i found it on msdn library ! :s
http://msdn.microsoft.com/library/de...ng_members.asp
sorry for askin too much but can you explain to me what does it do in Prelude's code?

They clearly made 2 mistakes.

First, if it is unsigned, it can't be negative. Second the typedef defines npos as being of type std::size_type. if std::size_type is unsigned (as they say it is) then it can't be negative.

may be -1 means null ?

No, it doesn't. -1 means -1 :)
Null is on some contexts intepreted as 0.

If Microsoft implementation decides to treat npos as being -1, it's their business. Although it will not be according to what the standard defines. But they can't say it's an unsigned integral. It would have to be a signed integral.

As for the second part of your previous question (sorry missed it before), Prelude is testing against npos to see if the string was found. The find function either returns the index at which the string was found, or that special value mapped to string::npos that means it was not found.

>First, if it is unsigned, it can't be negative.
But MSDN doesn't say that it's negative, only that it's initialized with -1. That's different.

>sorry for askin too much but can you explain to me what does it do in Prelude's code?
The find member function returns npos if the string wasn't found.

Thank you so much guys.
But now i'm confused about that (-1) part.

> But now i'm confused about that (-1) part.

Well, prelude just opened my eyes.

What happens is that integrals wrap around when they reach their limits. For instance, if an int variable had as minimum value 0 and maximum possible value 100, when you give it a value out of bounds, it will wrap. So, -1 becomes 100, -2 becomes 99. Conversely, 101 becomes 0, 102 becomes 1,...

So in fact, I'm wrong and the documentation found in MSDN is right (albeit perhaps confusing to the distracted like me). By initializing to -1, they are in fact giving it the maximum possible value.

>But now i'm confused about that (-1) part.
Don't worry about it, it's irrelevant to the task at hand. However, if you're interested, string::size_type is an unsigned type, meaning that it cannot have a negative value (as Mario said). However, if you assign a negative value to an unsigned type, the value wraps around to a large positive value. So if you say:

Code:

---

unsigned int x = -1;

---

You're actually setting x's value to the largest positive value that an unsigned integer can hold.

Say using void main() is wrong, why does such a large company like MS use it?

>Say using void main() is wrong, why does such a large company like MS use it?
Because they're arrogant enough to believe that they can rewrite the standard without ISO's permission.

Perhaps because MS designed their operating systems to work where main is declared as void, and they don't give a damn about standards?

Quote:

---

Originally Posted by Cool-August

Say using void main() is wrong, why does such a large company like MS use it?

---

They don't believe that Windows qualifies as a "hosted environment"? :p

>> Because they're arrogant enough to believe that they can rewrite the standard without ISO's permission.

I hate to say it, but, who ever gave the ISO the right over c++?

>> I'm sorry if it seems that way. We're just on a quest to avoid undefined behavior and promote correct C++.

Wouldn't you if you where directly told "void is just plain lazy".?
And you can usualy tell when someone takes offence to somthing someone else has said.

I will be honest, the only reason I make it int rather than void is because it's one of dev-c++'s requierments. Although, it's a habit now, so I use int even in visual-c++.

EDIT:

I also noticed that you seem to think that I am supporting void main().
I'm not.
I just don't bash or support somthing untill I have a good reason.

Your program itself is expected to return a value. Would you expect this to do something meaningful?

Code:

---

#include <stdio.h>

void foo(void) {}

int main()
{
  printf("return value = %d\n", foo());
  return 0;
}

---

>I hate to say it, but, who ever gave the ISO the right over c++?
ANSI, via the C community. In other words, we did.

>Wouldn't you if you where directly told "void is just plain lazy".?
Wouldn't I what? Your sentence doesn't constitute a complete thought.

>I will be honest, the only reason I make it int rather than void is because it's one of dev-c++'s requierments.
And you never wondered why it was a requirement in Dev-C++?

>I also noticed that you seem to think that I am supporting void main().
Not really, I'm simply answering your questions in an objective manner.

>I just don't bash or support somthing untill I have a good reason.
The ISO standard defines C++ for the entire world. It's accepted as the one and final authority when it comes to language questions. It also says that void main is wrong. Is that not a good enough reason?

>> The ISO standard defines C++ for the entire world.
>> It's accepted as the one and final authority when
>> it comes to language questions. It also says that
>> void main is wrong. Is that not a good enough reason?

Sure it is, but untill now, I didn't have that reason.

>Wouldn't you if you where directly told "void is just plain lazy".?
Wouldn't I what? Your sentence doesn't constitute a complete thought.

I'm sorry, I read it wrong:

Quote:

---

ME: They take it as an offence and as lazyness.
YOU: I'm sorry if it seems that way.
ME: Wouldn't you if you where directly told "void is just plain lazy".?

---

I read it as this:

Quote:

---

ME: They take it as an offence and as lazyness.
YOU: I'm sorry you see it that way.
ME: Wouldn't you if you where directly told "void is just plain lazy".?

---

>Sure it is, but untill now, I didn't have that reason.
Now you do. Learning is what this forum is all about, right? :)

Sorry one more question i didn't wanna create another thread for it
it's small.
Now i found the string i was looking for in a txt file
I want to replace this string with another string.
I've tried but i couldn't do it.
can someone help me?
Thanks

> They don't believe that Windows qualifies as a "hosted environment"?
Heh - the flipside of the symbiotic relationship is a parasitic environment :p

> Say using void main() is wrong, why does such a large company like MS use it?
Probably because it hails from when they (and Borland) implemented DOS compilers, and DOS was so utterly useless at doing anything meaningful with the status returned that nobody really bothered. But like other poisonous meme viruses, it manages to maintain a fan-base which makes it a lot harder to kill off.

Of course, if you want to carry on down the road of "what my compiler will let me get away with", don't expect a lot of sympathy when your code is lying in intensive care due to all your use of undefined behavour.

Quote:

---

Originally Posted by Shady

I want to replace this string with another string.

---

Replacing something in the middle of the file is a doomed endeavor in at least 90% of the cases - for example in yours.

You need to read the complete file, change what you want to change in memory, and then write the complete file back.

Here is my code
it compiles
but nothing is replaced !!! nothing is written in the file !!! nothing happens to "new.txt"

Code:

---

#include <fstream>
#include <iostream>
#include <string>
using namespace std;

int main()
{
        string c,d = "";
        fstream reads("R0_1st.txt",ios::in);
        fstream writes("new.txt",ios::in|ios::out);
        reads.unsetf(ios::skipws);
       
        while ( getline ( reads, c ) ) {
                cout<<c<<endl;
                if ( c.find ( "Adiabatic Efficiency" ) != string::npos )
                       
                {       
                        cout<<">>>>>>>>>>>>>>>"<<endl; //Just to make sure it got here
                        while( getline (writes,d) )
                        {
                                if ( d.find ("replace here") != string::npos )
                                {
                                        cout<<"<<<<<<<<<<<<<<<"<<endl; //Just to make sure it got here
                                        writes<<c;
                                }
                        }
                }
        }

        return 0;
}

---

Why don't you just do read from one file, write to another file and stop monkeying around trying to do the near impossible of writing updates to the middle of a file

Quote:

---

Originally Posted by Salem

Why don't you just do read from one file, write to another file and stop monkeying around trying to do the near impossible of writing updates to the middle of a file

---

If you read my code
that's what I did !
I just said
"writes<<c;"
that should put the string in the file where ever the put pointer is.
but nothing happens !

But writes is an update file - you're busy reading AND writing it.
Just simplify the problem to one file for reading, and one for writing.

> fstream writes("new.txt",ios::out); // NO ios::in

If I did that then I didn't do something new.
But let me put it this way

"writes" has data.
I want to write the data found from "reads" in the last line of "writes" i.e add data to writes without deleting old data.
I'm thinking about incrementing the put pointer till the eof
but i'm not sure how to do that !

Thanks for helping, much appreciated

> I want to write the data found from "reads" in the last line of "writes" i.e add data to writes without deleting old data.
Then by far the simplest is to copy the N-1 lines from the old version of "writes" to the new version of "writes".
For the last line, read it from old "writes", make whatever changes you need to make then write the changed line to the new version of "writes".