Type casts ?

kai85 · #1 19-Jun-2005, 11:58

hi,
i have a few questions relating to the two programs written below,
i'd be greatful if u couuld reply as soon as possible.
(i have an exam in less than 12 hours !)

CPP / C++ / C Code:

#include<iostream.h>
#include<fstream.h>

main(int argc,char *argv[])
{
	if(argc!=4)
	{
		cout<<"Usage:change<filename><byte><char>\n";
		return 1;
	}

	fstream out(argv[1],ios::in|ios::out);
	if(!out)
	{
		cout<<"cannot open output file to write in"<<endl;
		return 1;
	}
	out.seekp(atoi(argv[2]),ios::beg);// this is the line  i don't 
                                                     //   understand    !!!!                                             
	out.put(*argv[3]);
	out.close();
	return 0;
}

what does" atoi " do?

here's the second program:

CPP / C++ / C Code:

#include<iostream.h>
#include<fstream.h>



main()
{
	float fnum[4]={99.76,45.87,-3454.34,545.1};
	int i;

	ofstream out("numbers");
	if(!out){
		cout<<"cannot open output file to write in"<<endl;
		return 1;
	}
	out.write([b](unsigned char*)[/b]&fnum,sizeof fnum);//and the other line i 
                                                                      //don't understand!!!!
	out.close();
	for( i=0;i<4;i++)
		fnum[i]=0;


	ifstream in("numbers");
	in.read((unsigned char*)&fnum,sizeof fnum);
	cout<<in.gcount()<<" bytes read"<<endl;
	for(i=0;i<4;i++)
		cout<<fnum[i]<<" "<<endl;
	in.close();
	return 0;
}

i don't understand the bold part, why do these kinda programs that use the read and write functions use type casts???

anyone?simple ,easy to understand explenation????
thanks.

Dave Sinkula · #2 19-Jun-2005, 13:21

Quote:

Originally Posted by kai85

CPP / C++ / C Code:

out.seekp(atoi(argv[2]),ios::beg);// this is the line  i don't 
                                                     //   understand    !!!!

what does" atoi " do?

It converts a string to an integer.

Quote:

Originally Posted by kai85

CPP / C++ / C Code:

out.write((unsigned char*)&fnum,sizeof fnum);//and the other line i 
                                                                      //don't understand!!!!

i don't understand the bold part, why do these kinda programs that use the read and write functions use type casts???

Because write expects a pointer to a char; fnum is not a char.

kai85 · #3 19-Jun-2005, 19:35

Quote:

Originally Posted by Dave Sinkula

It converts a string to an integer.

but i thought we were supposed to enter an integer on the command line.
like 12 0r ... ???
i'm not sure because for some reason(don't ask me) the program doesn't compile

Quote:

Because write expects a pointer to a char; fnum is not a char.

so this line of code would have to compile fine ,right?

CPP / C++ / C Code:

:
:
:
char characterarray[5]={'a','s','d','f''}
out.write(&characterarray,sizeof characterarray);
:
:
:

again ,i can't be sure because the programs don't compile.
thanks,
kai85
BTW what does atol do?

Dave Sinkula · #4 19-Jun-2005, 22:21

Quote:

Originally Posted by kai85

but i thought we were supposed to enter an integer on the command line.
like 12 0r ... ???

You do enter text, right? Even though the text does represent a number.

Quote:

Originally Posted by kai85

BTW what does atol do?

Same as atoi, different data type.

Google works quite well for such questions. Its answer arrive more quickly.

kai85 · #5 20-Jun-2005, 10:15

ur right , i shouldve searched in google , but i ... nope no excuses.. i just didn't think of google.(this place is so good it just pops into mind when coming across problems)
thanx anyway.

leetster · #6 20-Jun-2005, 23:30

In your second program you should change

Code:

 float fnum[4]={99.76,45.87,-3454.34,545.1};

to:

Code:

 float fnum[5]={99.76,45.87,-3454.34,545.1};

. Because it always holds one value less then what its array number is =)
(or so i remember)

davekw7x · #7 21-Jun-2005, 01:21

Quote:

Originally Posted by leetster

In your second program you should change

Code:

 float fnum[4]={99.76,45.87,-3454.34,545.1};

to:

Code:

 float fnum[5]={99.76,45.87,-3454.34,545.1};

. Because it always holds one value less then what its array number is =)
(or so i remember)

No; if you declare an array of four floats, it holds four floats. (The index values for such an array go from fnum[0] to fnum[3] --- maybe that's what you were thinking of.)

Regards,

Dave

kai85 · #8 21-Jun-2005, 06:37

Quote:

In your second program you should change
Generic Code:
float fnum[4]={99.76,45.87,-3454.34,545.1};
to:
float fnum[5]={99.76,45.87,-3454.34,545.1};
Because it always holds one value less then what its array number is =)
(or so i remember)

yes thats what u do when u declare an array of characters ,one extra for the null character .
and thats what ive done in the other program where ive declared an array of chars .
anyway has anyone tried compiling the 2 first programs?
do they compile? i know im doing sth wronge. i just have no clue what it is!!!

davekw7x · #9 21-Jun-2005, 08:12

Quote:

Originally Posted by kai85

yes thats what u do when u declare an array of characters ,one extra for the null character .
and thats what ive done in the other program where ive declared an array of chars .

When you declare an array of, say, 7 chars. If you want to put a "string" in it, the maximum string length is 7, including the terminating null.

Quote:

Originally Posted by kai85

anyway has anyone tried compiling the 2 first programs?
do they compile? i know im doing sth wronge. i just have no clue what it is!!!

Well, we have no clue as to what you tried and what happened. Here are a few observations:

Program number 1 needs to include the stdlib header if you are using Microsoft Visual C version 6 (that's where atoi() is prototyped).

Of course, program number 2 needs to lose the [ B ] and [ /B ] thingies on the out.write() line. If you are using GNU g++, you will find that it is very picky about argument types in function calls. The first argument to the out.write() function should be cast as (const char *), not (unsigned char *), and the first argument to the out.read() function should be cast as (char *) not as (unsigned char *).

With these tweaks, the programs compile and execute on my Windows XP box with Borland, Microsoft, and GNU compilers.

As a matter of style, you should get into the habit of including C++ standard headers. <iostream> instead of <iostream.h>, for example. This also requires some consideration of namespace issues, as has been covered numerous times in threads on this forum. Even though the old style headers still work on all C++ compilers that I have seen, that won't necessarily always be the case.

In a similar vein, the main() program should be defined as "int main" instead of using the legacy implicit "int" declaration. That is:

CPP / C++ / C Code:

int main(int argc, char *argv[])

There are some other style issues, but basically, they seem to work (that is, they do not crash the system with the compilers that I used, and they do produce expected output --- if you know what to expect).

Regards,

Dave

kai85 · #10 23-Jun-2005, 02:52

Quote:

Originally Posted by davekw7x

When you declare an array of, say, 7 chars. If you want to put a "string" in it, the maximum string length is 7, including the terminating null.

this is what i ment too.
Ok I did what u said , the second one compiles fine.
with the first program this is what I do:
In command line I locate the directory I’ve saved the programs in, then type:
Cd debug, then type this line:hs458.exe change out 1 z .
I’ve only just learnt I/O so I’m not sure ... after that I open the debug directory
and open the Out file .. in that there’s some numbers and at the end there’s two z’s.
Is this what I’m supposed to see?
sorry if my questions seem stupid ...

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Type casts ?

google?