Programming Forums > Application Development > C++

can342man · Jan 27th, 2006, 4:27 PM

Hi,

I have been struggling to understand why this simple program is generating the exact same number 100 times in the output.dat file. If I display the results on the screen (cout), the program works fine, generating numbers from 1 to 99 in order.

Is it true that a variable will only pass the first value to a function but not the rest of the values?

How would I go about trying to dump all the numbers generated from 1 to 99 into the output file by calling the function fileout()?

There are no compilation errors in this program, just that it produces the wrong result when the numbers are sent to the fileout() function.

(Toggle Plain Text)

//Output of data to file "output.dat"

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

int fileout(int j);

int main () {

        int i;
        for (i=0; i<100; i++) {
                fileout(i);
        }

}

int fileout(int j)
{
        ofstream out("output.dat");
        if(!out) {
                cout << "Cannot open file." << "\n";
                return 1;
        }
        for (int k=0; k<100; k++) {
                out << j << "\n";
        }
        out.close();
        return 0;
}

Really appreciate it if anybody has come across something like this before.

Thanks.

jayme · Jan 27th, 2006, 4:51 PM

(Toggle Plain Text)

       for (int k=0; k<100; k++) {
                out << j << "\n";
        }

In English, if k is less than 100, output j to the screen (or file in your case) one more time. How many times do you want the output saved to the .dat file? If only one time then use

(Toggle Plain Text)

       for (int k=0; k<1; k++) {
                out << j << "\n";
        }

Jason Isom · Jan 27th, 2006, 5:30 PM

Quote:

Originally Posted by can342man

Hi,

I have been struggling to understand why this simple program is generating the exact same number 100 times in the output.dat file. If I display the results on the screen (cout), the program works fine, generating numbers from 1 to 99 in order.

Is it true that a variable will only pass the first value to a function but not the rest of the values?

How would I go about trying to dump all the numbers generated from 1 to 99 into the output file by calling the function fileout()?

There are no compilation errors in this program, just that it produces the wrong result when the numbers are sent to the fileout() function.
.

(Toggle Plain Text)

snip

Really appreciate it if anybody has come across something like this before.

Thanks.

Trace your program on paper. It'll probably become more clear.

First off your main problem (although not your only problem...)

(Toggle Plain Text)

for (int k=0; k<100; k++) {
    out << j << "\n";
}

You're outputting the same value 100x. Namely the value of the variable that you passed to the fileout function. You probably intended to use k?

Anyways, I'm a little confused on what you're trying to accomplish. You have this in main...

(Toggle Plain Text)

int i;
for (i=0; i<100; i++) {
        fileout(i);
}

So basically you're calling fileout(...) 100x with the values 0-99.

In fileout(...) you have another loop that loops 100x with the values 0-99, but it's only outputting the value you passed to the function.

An important thing you have to realize though, is that when you open a file with

(Toggle Plain Text)

ofstream out("output.dat");

you effectively overwrite whatever USED to be there. If you want to append to an already existing file, you need to use ios::app

You need to figure out whether you want the loop to be in main, or fileout. You can technically do it either way, but your program is rather trivial so I can't figure out what way you SHOULD do it. If you're trying to understand the appending of files, just take out the loop in fileout(...), use ios::app and append the integer that you pass to the function to the file. If you don't care about appending, take out the loop in main and take out the parameter of your fileout function and just use the loop in fileout()

Kilo · Jan 27th, 2006, 6:25 PM

[php]
#include <iostream>
#include <fstream>
using namespace std;

int fileout(int j);

int main () {

int i;
ofstream out("output.dat");

if(!out) {
cout << "Cannot open file." << "\n";
return 1;
}

for (i=0; i<100; i++) {
fileout(i);
}

out.close();

}

void fileout(int j)
{
out << j << "\n";
}
[/php]

Jason Isom · Jan 27th, 2006, 7:31 PM

Quote:

Originally Posted by Kilo

snip

Actually, that won't work either because of scoping rules. You could do that if you pass a reference to the ofstream. Not to mention your prototype differences from the definition.

Try changing the function to

(Toggle Plain Text)

void fileout(ofstream& out, int j)

and calling it with

(Toggle Plain Text)

fileout(out, i);

Kilo · Jan 27th, 2006, 10:03 PM

lol your so right.. i just did a quick edit.. my fault to all!

Ooble · Jan 28th, 2006, 8:31 AM

You don't need a function do do something as trivial as out << num << std::endl;. I'd personally do it like this:

(Toggle Plain Text)

#include <iostream>
#include <fstream>

int writeNumbers(std::string filename, int start, int end);

int main ()
{
	std::string filename;
	int start, end;
	
	std::cout << "Enter the name of the file you want to write to: ";
	std::cout.sync(); // flushes the output buffer to make sure the string is shown
	std::getline(cin, filename); // gets the entire line and stores it in "filename"
	
	std::cout << "Enter the starting value: ";
	std::cout.sync();
	std::cin >> start; // gets the starting value and stores it in "start"
	
	std::cout << "Enter the ending value: ";
	std::cout.sync();
	std::cin >> end;
	
	writeNumbers(filename, start, end);
	
	return 0;

}

bool writeNumbers (std::string filename, int start, int end)
{
	std::ostream out(filename);
	
	if (start < end)
	{
		for (int i = start; i <= end; i++)
		{
			out << i << std::endl;
		}
	}
	else
	{
		for (int i = start; i >= end; i--)
		{
			out << i << std::endl;
		}
	}
	
	out.close();
}

I'll let you add the error-checking.

Cache · Jan 28th, 2006, 9:57 AM

@Ooble: I don't want to sound like an ass, but there are a few basic mistakes in the code you posted above. Although you did only say it's "like" the way you would do it, at least.

Take a look at the return type in the prototype of 'writeNumbers' - then look at the return type in it's definition. Your missing a 'std::' on one of your cin's. And when you open the file stream, perhaps you meant: std:: ofstream out( filename.c_str() ); (I put the space to avoid a smiley face).

There's one more thing, but it's more of question: in the code you posted above, you synchronize the output buffer using 'std::cout.sync();'. Is that valid? Dev-C++ spits and error about this. The error: 'struct std::ostream' has no member named 'sync'

HaCkeR · Jan 28th, 2006, 11:41 AM

Well i have not done any coding in a while so this proberly isnt the best or the cleanest. but it works

(Toggle Plain Text)

//Output of data to file "output.dat"
 
#include <iostream>
#include <fstream>
 
using namespace std;
 
int main () {
int i=0;
 
ofstream out("output.dat");
if(!out) {
	cout << "Cannot open file." << "\n";
	return 1;
}
 
while (i < 100) {
	out << i << endl;
	i++;
}
 
out.close();
 
return 0;
}

Ooble · Jan 28th, 2006, 1:01 PM

Quote:

Originally Posted by Cache

@Ooble: I don't want to sound like an ass, but there are a few basic mistakes in the code you posted above. Although you did only say it's "like" the way you would do it, at least.

Take a look at the return type in the prototype of 'writeNumbers' - then look at the return type in it's definition. Your missing a 'std::' on one of your cin's. And when you open the file stream, perhaps you meant: std:: ofstream out( filename.c_str() ); (I put the space to avoid a smiley face).

There's one more thing, but it's more of question: in the code you posted above, you synchronize the output buffer using 'std::cout.sync();'. Is that valid? Dev-C++ spits and error about this. The error: 'struct std::ostream' has no member named 'sync'

I forgot to add: "not tested, but the essence is there". It's supposed to be a bool in both cases, by the way, though that's really up to whoever's coding the error-checking.

Oh, and I couldn't remember which one was ANSI C++ and which one was the old version that works with iostream.h. Do this instead:

(Toggle Plain Text)

std::cout << std::flush;

Jan 27th, 2006, 4:27 PM	#1
can342man Newbie Join Date: Jan 2006 Location: London, Ontario, Canada Posts: 16 Rep Power: 0	Simple output problem Hi, I have been struggling to understand why this simple program is generating the exact same number 100 times in the output.dat file. If I display the results on the screen (cout), the program works fine, generating numbers from 1 to 99 in order. Is it true that a variable will only pass the first value to a function but not the rest of the values? How would I go about trying to dump all the numbers generated from 1 to 99 into the output file by calling the function fileout()? There are no compilation errors in this program, just that it produces the wrong result when the numbers are sent to the fileout() function. (Toggle Plain Text) //Output of data to file "output.dat" #include <iostream> #include <fstream> using namespace std; int fileout(int j); int main () { int i; for (i=0; i<100; i++) { fileout(i); } } int fileout(int j) { ofstream out("output.dat"); if(!out) { cout << "Cannot open file." << "\n"; return 1; } for (int k=0; k<100; k++) { out << j << "\n"; } out.close(); return 0; } //Output of data to file "output.dat" #include <iostream> #include <fstream> using namespace std; int fileout(int j); int main () { int i; for (i=0; i<100; i++) { fileout(i); } } int fileout(int j) { ofstream out("output.dat"); if(!out) { cout << "Cannot open file." << "\n"; return 1; } for (int k=0; k<100; k++) { out << j << "\n"; } out.close(); return 0; } Really appreciate it if anybody has come across something like this before. Thanks. __________________ Greatness courts failure and solitude. --- Anonymous

Jan 27th, 2006, 6:25 PM	#4
Kilo Expert Programmer Join Date: Nov 2005 Location: In Pink Clam? Posts: 542 Rep Power: 0	[php] #include <iostream> #include <fstream> using namespace std; int fileout(int j); int main () { int i; ofstream out("output.dat"); if(!out) { cout << "Cannot open file." << "\n"; return 1; } for (i=0; i<100; i++) { fileout(i); } out.close(); } void fileout(int j) { out << j << "\n"; } [/php] __________________ "When in Rome, Do as the Romans Do" "Beauty is in the eye of the BEER holder" "Save your breath your going to need it for your blow up doll later" SearchLores.org

Jan 27th, 2006, 10:03 PM	#6
Kilo Expert Programmer Join Date: Nov 2005 Location: In Pink Clam? Posts: 542 Rep Power: 0	lol your so right.. i just did a quick edit.. my fault to all! __________________ "When in Rome, Do as the Romans Do" "Beauty is in the eye of the BEER holder" "Save your breath your going to need it for your blow up doll later" SearchLores.org

Jan 28th, 2006, 8:31 AM	#7
Ooble I eat cake for breakfast. Join Date: Jul 2004 Location: In my box. Posts: 4,434 Rep Power: 9	You don't need a function do do something as trivial as out << num << std::endl;. I'd personally do it like this: (Toggle Plain Text) #include <iostream> #include <fstream> int writeNumbers(std::string filename, int start, int end); int main () { std::string filename; int start, end; std::cout << "Enter the name of the file you want to write to: "; std::cout.sync(); // flushes the output buffer to make sure the string is shown std::getline(cin, filename); // gets the entire line and stores it in "filename" std::cout << "Enter the starting value: "; std::cout.sync(); std::cin >> start; // gets the starting value and stores it in "start" std::cout << "Enter the ending value: "; std::cout.sync(); std::cin >> end; writeNumbers(filename, start, end); return 0; } bool writeNumbers (std::string filename, int start, int end) { std::ostream out(filename); if (start < end) { for (int i = start; i <= end; i++) { out << i << std::endl; } } else { for (int i = start; i >= end; i--) { out << i << std::endl; } } out.close(); } #include <iostream> #include <fstream> int writeNumbers(std::string filename, int start, int end); int main () { std::string filename; int start, end; std::cout << "Enter the name of the file you want to write to: "; std::cout.sync(); // flushes the output buffer to make sure the string is shown std::getline(cin, filename); // gets the entire line and stores it in "filename" std::cout << "Enter the starting value: "; std::cout.sync(); std::cin >> start; // gets the starting value and stores it in "start" std::cout << "Enter the ending value: "; std::cout.sync(); std::cin >> end; writeNumbers(filename, start, end); return 0; } bool writeNumbers (std::string filename, int start, int end) { std::ostream out(filename); if (start < end) { for (int i = start; i <= end; i++) { out << i << std::endl; } } else { for (int i = start; i >= end; i--) { out << i << std::endl; } } out.close(); } I'll let you add the error-checking. __________________ Me :: You :: Them

Jan 28th, 2006, 11:41 AM	#9
HaCkeR Hobbyist Programmer Join Date: Nov 2005 Location: UK Posts: 131 Rep Power: 0	Well i have not done any coding in a while so this proberly isnt the best or the cleanest. but it works (Toggle Plain Text) //Output of data to file "output.dat" #include <iostream> #include <fstream> using namespace std; int main () { int i=0; ofstream out("output.dat"); if(!out) { cout << "Cannot open file." << "\n"; return 1; } while (i < 100) { out << i << endl; i++; } out.close(); return 0; } //Output of data to file "output.dat" #include <iostream> #include <fstream> using namespace std; int main () { int i=0; ofstream out("output.dat"); if(!out) { cout << "Cannot open file." << "\n"; return 1; } while (i < 100) { out << i << endl; i++; } out.close(); return 0; }

Jan 28th, 2006, 9:57 AM	#8
Cache Hobbyist Join Date: Sep 2005 Posts: 261 Rep Power: 4	@Ooble: I don't want to sound like an ass, but there are a few basic mistakes in the code you posted above. Although you did only say it's "like" the way you would do it, at least. Take a look at the return type in the prototype of 'writeNumbers' - then look at the return type in it's definition. Your missing a 'std::' on one of your cin's. And when you open the file stream, perhaps you meant: std:: ofstream out( filename.c_str() ); (I put the space to avoid a smiley face). There's one more thing, but it's more of question: in the code you posted above, you synchronize the output buffer using 'std::cout.sync();'. Is that valid? Dev-C++ spits and error about this. The error: 'struct std::ostream' has no member named 'sync'

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Programming Forums > Application Development > C++

Simple output problem