how i change it: left operand must be l-value

[qwerwan5]

a small code , problem with " l-value"

#include<iostream>
#include<sstream>
#include<fstream>
#include<cstdlib>
#include<cmath>
#include<string>
#include<iomanip>

using namespace std;

const NX=200;
const NY=100;
const U=0.1;

int i,j,n,dx,dy;
double ux0[NX+1][NY+1],uy0[NX+1][NY+1],ux[NX+1][NY+1],uy[NX+1][NY+1];

void init();
void evolution();
void output(int m);

int main()
{
	using namespace std;

	init();
	for(n=0;n<10000;n++)
	{
		evolution();
		if(n>0)
		{
			if(n%100==0) cout<<"The ux and uy of [NX/2][NY/2] is:"<<ux[NX/2][NY/2]<<" "<<uy[NX/2][NY/2]<<endl;
			if(n%1000==0) output(n);
		}
	}

	system("pause");
	return 0;
}

void init()
{
	dx=1;
	dy=1;
	
	for(i=0;i<=NX;i++)
		for(j=0;j<=NY;j++)
		{
			ux[i][j]=0;
			uy[i][j]=0;
			ux[0][j]=U;
		}
}

void evolution()
{
	for(i=1;i<NX;i++)
		for(j=1;j<NY;j++)
		{
			ux0[i][j]=ux[i][j];
			uy0[i][j]=uy[i][j];
			
			ux[i+1][j]-2*ux[i][j]+ux[i-1][j]+uy[i][j+1]-2*uy[i][j]+uy[i][j-1]=0;
			
		}

	
	for(i=0;i<=NX;i++)
	{
		ux[i][0]=ux[i][NY]=0;
		uy[i][0]=uy[i][NY]=0;
	}
	for(j=0;j<=NY;j++)
	{
		ux[0][j]=U;
		uy[0][j]=0;
		ux[NX][j]=ux[NX-1][j];
		uy[NX][j]=uy[NX-1][j];
	}

}

void output(int m)
{
	ostringstream name;
	name<<"FDM_"<<m<<".dat";

	ofstream out(name.str().c_str());
	for(i=0;i<=NX;i++)
		for(j=0;j<=NY;j++)
		{
			out<<double(i)<<" "<<double(j)<<" "<<ux[i][j]<<" "<<uy[i][j]<<endl;
		}
}

[AbstractionAnon]

Lines 11-13 are missing type specifiers.

Line 63: What exactly do you think this line does?

ux[i+1][j] - 2*ux[i][j] + ux[i-1][j] + uy[i][j+1] - 2*uy[i][j] + uy[i][j-1] = 0;

You have an expression on the left side of the equal sign. You then try to assign 0 to that expression. The left side of the assignment must resolve to an addressible variable (L-value). It can not be an expression.

how i change it?

I can't tell you that, because I don't understand what you're trying to do.