Implementing pageup pagedown feature using C language

[ssubbarayan]

Dear all,
I am in the process of implementing pageup/pagedown feature in our
consumer electronics project.The idea is to provide feature to the
customers to that similar to viewing a single sms message in a mobile
device.With in the given view area if the whole message does not
fit,we need to provide the ability for users to scroll through the
entire message using pageup/pagedown or key up and key down.In our
case we have the whole contents to be displayed stored in a buffer.I
am giving below one implementation I have tried:

Note:We are using only pure C code and dont have active x controls
like tabs or pageup/down buttons as in .NET or VC++.

#include <stdio.h>
#include "string.h"


void pageup();
void pagedown();
static int pagecounter=0;
/*bytes per page calculated by trial and error
considering the viewable area in my device*/
int bytesperpage=161;
int numpages=0;
int contentremaining=0;
char* customstringcopy=NULL;
char arr[161]={0};
int main(int argc, char *argv[])
{
int totlen=0;
int pagenum=0;


char* customstring=NULL;

char* data="Bloodshed Dev-C++ is a full-featured Integrated
Development Environment (IDE) for the C/C++ programming language. It
uses Mingw port of GCC (GNU Compiler Collec.";
char* stringdata="Bloodshed Dev-C++ is a full-featured Integrated
Development Environment (IDE) for the C/C++ programming language. It
uses Mingw port of GCC (GNU Compiler Collection) as it's compiler. Dev-
C++ can also be used in combination with Cygwin or any other GCC based
compiler.";
char* stringdata1="the #bloodshed channel has recently been
created on the Undernet IRC server. I will be please to talk with you
there so feel free to join If you want have an IRC client you can
get one for Windows at mirc.com and for Linux at xchat.org";
char* stringdata2="You can subscribe to the Dev-C++ mailing list
(for asking and answering questions on Dev-C++ and C/C++ programming)
by clicking here and filling out the subscribe form there.";

totlen=strlen(stringdata)+strlen(stringdata)+strlen(stringdata2);
printf("total length is %d\n",totlen);
printf("length of data is %d\n",strlen(data) );
customstring=(char*)(malloc)(totlen+4);
customstringcopy=customstring;
memset(customstring,0,totlen+4);

memcpy(customstring,stringdata,strlen(stringdata));
customstring=customstring+strlen(stringdata);
customstring[0]='\n';
customstring++;
/*printf("%s\n",stringdata);
printf("%s\n",customstringcopy);*/

/*customstring[0]='\n';
customstring++;*/

memcpy(customstring,stringdata1,strlen(stringdata1));
customstring=customstring+strlen(stringdata1);
customstring[0]='\n';
customstring++;
/*printf("%s\n",stringdata1);
printf("%s\n",customstringcopy);*/

memcpy(customstring,stringdata2,strlen(stringdata2));
customstring=customstring+strlen(stringdata2);
customstring[0]='\n';
customstring++;
/* printf("%s\n",stringdata2);*/
printf("%s\n",customstringcopy);

numpages=totlen/bytesperpage;
printf("total number of pages is %d\n",numpages);
contentremaining=(totlen)%(bytesperpage);
if(contentremaining > 0)
{
numpages=numpages+1;

}
printf("total number of pages is %d\n",numpages);
for(pagenum=0;pagenum<=numpages;pagenum++)
{
pageup();
}
for(pagenum=numpages;pagenum>0;pagenum--)
{
pagedown();
}
system("PAUSE");
return EXIT_SUCCESS;
}

void pageup()
{


if(pagecounter<numpages)
{
pagecounter++;
printf("pagecounter value is %d\n",pagecounter);
memcpy(arr,customstringcopy,161);
printf("%s\n\n",arr);
customstringcopy=customstringcopy
+bytesperpage;
}


}

void pagedown()
{

if(pagecounter==numpages)
{
pagecounter--;
printf("pagecounter value is %d
\n",pagecounter);
customstringcopy=customstringcopy-2*(bytesperpage);
memcpy(arr,customstringcopy,161);
printf("%s\n\n",arr);
}
else
{
if(pagecounter>1)
{
pagecounter--;
printf("pagecounter value is %d
\n",pagecounter);
customstringcopy=customstringcopy-(bytesperpage);
memcpy(arr,customstringcopy,161);
printf("%s\n\n",arr);
}


}
/* if(pagecounter>0)
{

customstringcopy=customstringcopy-
bytesperpage;
memcpy(arr,customstringcopy,161);
printf("%s\n\n",arr);

} */

}

Incase you find bugs in above code please let me know ways to fix it.

I believe there are much better ways then the one I have tried here.It
would be helpful if some one could provide me some sample code for
similar features you would have come across in your product or some
sample links which shows me sample code on how to implement such a
feature.

Note:I am looking for only C code and not C#,JAVA,.NET,VC++

Looking farward for all your replies and advanced thanks for the same,
Regards,
s.subbarayan

 

[Andrew Smallshaw]

I believe there are much better ways then the one I have tried here.It
would be helpful if some one could provide me some sample code for
similar features you would have come across in your product or some
sample links which shows me sample code on how to implement such a
feature.

How did you design this code? In a sense that is a rhetorical
question because I already know how you wrote it - you opened up
your text editor and started typing, right? It shows in the code.
If it was me and I wanted to have some pride in my work I would
delete it now and start over. I'd probably end up saving time in
the long run because this code simply isn't maintainable.

I suspect you have already spent hours bashing this code into
shape. Next time, invest a little time in design first. Turn
off your computer, find a pad of paper, a pencil and a rubber and
don't go back to your computer until you know how every aspect of
your code is going to work. Your code will go together far more
quickly, and be much tidier and more compact to boot.

Returning to your code, reading between the lines it appears that
arr is supposed to represent a memory-mapped display device - this
isn't mentioned anywhere and that is one of the first things you
should specify in an early stage of designing your code. Similarly
I spent some time looking around for where you receive input from
the user, before realising that you weren't doing this, but
running through a predefined test sequence; again something that
should have been mentioned. I also take it that all those printfs
littering your code are debug code to try and get your code
working - they shouldn't be needed for something like this.

I _really_ doubt arr should be 161 characters long and your trial
and error comment does little to inspire confidence. 160 is much
more likely, quite possibly on a 40x4 matrix. If that's the case
then what about line breaks - presumably we need to wrap at word
boundaries which you aren't doing here. I can understand why you
got 161 - you need one for the trailing NULL character to treat
the display buffer as a string. Big mistake - it is a character
array, not a string, and if you start writing to arr[161] that
may well be some special function register associated with the
display.

Looking through your code in a little more detail, a few more
specific issues do come to mind.

Don't:
#include <stdio.h>
#include "string.h"

Do:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

[Always use angle brackets for standard libraries]
[malloc() is defined in <stdlib.h>]

Don't:
totlen=strlen(stringdata)+strlen(stringdata)+strlen(stringdata2);

Do:
totlen = strlen(stringdata) + strlen(stringdata1) + strlen(stringdata2);

[Silly mistake: we all make them]

Don't:
numpages=totlen/bytesperpage;
printf("total number of pages is %d\n",numpages);
contentremaining=(totlen)%(bytesperpage);
if(contentremaining > 0)
{
numpages=numpages+1;

}
printf("total number of pages is %d\n",numpages);

Do:
numpages = (totlen + (bytesperpage - 1)) / bytesperpage;
printf("total number of pages is %d\n", numpages);

[More compact and faster]
[This is a standard pattern whenever you need to round a division up]
[Test it with a pen and paper to prove to yourself it works]