Programming Forums > Application Development > C++

splitterdevildoll · Jun 14th, 2006, 7:13 PM

I am trying to a sample self-avoiding random walk routine, but my compiler doesn't compile and it gives me error messages due to this line.

Quote:

int ipos[4] = {};

error 86 - Incomplete expression
error 142 - Void type not permitted here
error 192 - An object of type 'void' cannot be assigned to an object of int

Here's the program:

Quote:

#include <iostream.h>
#include <stdlib.h>
#include <math.h>

void do_walk (int maxstep, int& nstep, double& rsquared, double& weight ){
const int MAXSTEP=30;
int map[ MAXSTEP*2][MAXSTEP*2]={0};

// start point
int completed=0;
weight=1.0;

int x = MAXSTEP;
int y = MAXSTEP;
int npoint = 1;
map[x][y] = npoint;
do {
int xnew=x;
int ynew=y;

// Check which sites are available
int ipos[4] = {};
int npos = 0;

if( map[x-1][y] == 0 ) {
ipos[npos]=0;
npos++;
}
if( map[x+1][y] == 0 ) {
ipos[npos]=1;
npos++;
}
if( map[x][y-1] == 0 ) {
ipos[npos]=2;
npos++;
}
if( map[x][y+1] == 0 ) {
ipos[npos]=3;
npos++;
}
if( npos == 0 ){
completed =1;
} else {

if( npoint > 1) weight *= npos/3.0;
int i = (int)(npos *(double)rand()/(RAND_MAX+1.0));
switch ( ipos[i] ) {
case 0: xnew-= 1; break;
case 1: xnew+= 1; break;
case 2: ynew-= 1; break;
case 3: ynew+= 1; break;
}

npoint++;
map[xnew][ynew] = npoint;
x = xnew;
y = ynew;
if ( npoint == maxstep+1 )completed=1;
}

} while ( !completed );
//
// Print a window around the centre of the array
//
for ( int i=12; i<2*MAXSTEP-12; i++ ){
for ( int j=12; j < 2*MAXSTEP-12; j++ ){
cout.width(3);
cout << map[i][j];
}
cout << endl;
}
nstep = npoint-1;
rsquared = pow( x-MAXSTEP,2.0) + pow( y-MAXSTEP, 2.0 );
}

int main(){
int maxstep=50,nstep;
double rsquared,weight;
srand(7654321);
for (int i=1; i<10; i++ ){
do_walk(maxstep,nstep,rsquared,weight);
cout << endl << "Nsteps: " <<nstep << " Rsquared: " <<rsquared<<" Weight: " << weight << endl;
}
return 0;
}

Any help is appreciated. Thank you.

jayme · Jun 14th, 2006, 7:14 PM

Try replacing with this?

(Toggle Plain Text)

int ipos[4] = '\0';

I'm only guessing you're trying to null the position by having "int ipos[4] = {};", and the above code would be the correct way.

Or if you mean to null the entire array..

(Toggle Plain Text)

int ipos[4];
for (int i = 0; i < 4; i++) ipos[i]='\0';

splitterdevildoll · Jun 14th, 2006, 7:24 PM

It compiles now, but I cannot get the program to work. It says it's a run time error.

Here's a link of what the sample output should look like.

http://hepwww.ph.qmul.ac.uk/sim/sarw...file=sarw.html
It's the very last output in the link.

DaWei · Jun 14th, 2006, 8:36 PM

You need to read a thread on how to post a question. Your first post was excellent, but your second one gives no information to work with. Information is key, both in getting answers and in debugging. A sample of the proper output is essentially worthless, since your program is running off into the weeds and barfing in its shoes. One guess is that your run-time error is a memory violation. There's a good chance that you are violating the dimensions of an array somewhere. You could also be trying to modify memory that isn't your own by abusing a pointer. Collect all the information your system gives you regarding the error. Shoot, you don't even mention your platform, OS, and compiler. If you have a debugger, step through the program or set breakpoints at various places. The latter method allows you to do a binary search to zero in on the offending statements, if you play your cards right.

The Dark · Jun 14th, 2006, 8:57 PM

Worked for me under VC2005
Final ouput:
Nsteps: 50 Rsquared: 200 Weight: 0.0438957

As DaWei says, you need to give more information.

Jun 14th, 2006, 8:36 PM	#4
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	You need to read a thread on how to post a question. Your first post was excellent, but your second one gives no information to work with. Information is key, both in getting answers and in debugging. A sample of the proper output is essentially worthless, since your program is running off into the weeds and barfing in its shoes. One guess is that your run-time error is a memory violation. There's a good chance that you are violating the dimensions of an array somewhere. You could also be trying to modify memory that isn't your own by abusing a pointer. Collect all the information your system gives you regarding the error. Shoot, you don't even mention your platform, OS, and compiler. If you have a debugger, step through the program or set breakpoints at various places. The latter method allows you to do a binary search to zero in on the offending statements, if you play your cards right. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

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Jun 14th, 2006, 7:24 PM	#3
splitterdevildoll Newbie Join Date: Aug 2005 Posts: 12 Rep Power: 0	It compiles now, but I cannot get the program to work. It says it's a run time error. Here's a link of what the sample output should look like. http://hepwww.ph.qmul.ac.uk/sim/sarw...file=sarw.html It's the very last output in the link.

Jun 14th, 2006, 8:57 PM	#5
The Dark Expert Programmer Join Date: Jun 2005 Posts: 894 Rep Power: 4	Worked for me under VC2005 Final ouput: Nsteps: 50 Rsquared: 200 Weight: 0.0438957 As DaWei says, you need to give more information.

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