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Hi! I'm working on the Monte Carlo simulation of the NO-CO reaction on a square surface, and when I was writing the code, I needed to specify the states of "empty" or "occupied" of the sites on the surface that CO or NO can occupy.

In my code I defined those states as follows
C
#define empty 0
#define occupied 1

So, if the site is "occupied", the chosen site is written as
C
grid[x][y] = occupied;

But when we go to check if this is effectively true, I can't do something like
C
if(grid[x][y] = occupied)
{
     thing 1;
     thing 2;
}

Since I will obtain a warning or error message. I know that if, while statements work with "==", "!=", "<", ">", but I really need to check if a chosen site of my simulation is or not occupied and I'm not sure how to write it in C

What I have tried:

The following is the code that I've been writing so far.
I've defined some booleans but I don't know if I'am using them correctly. All that I need is to know how to check properly if a chosen site of my simulation is or not occupied.

C
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <stdbool.h>

//define the size of the grid
#define xmax 3
#define ymax 3
//define the Monte Carlo Steps
#define MCSS 100
//define the states "empty" and "occupied"
#define empty 0
#define occupied 1
//define if the chosen site is occupied by por CO, NO, N, or O
#define co_occupied 2
#define no_occupied 3
#define n_occupied 4
#define o_occupied 5
//definimos la probabilidad para que un sitio este ocupado o no
#define p 0.300000
//define the probability of CO adsorption
#define xco 0.4
//define the disociation rate of NO
#define rno 0.6

//function that selects the x-coordinate of a site at random
int printrandomx(int lower, int upper, int cuenta)
	{
		int m, num1;
		for (m = 0; m < cuenta ; m++) {
		num1 = (rand() % (upper - lower + 1)) + lower;
		// printf("random x: %d \n", num1);
		}
	return num1;
	}

//function that selects the y-coordinate of a site at random
	int printrandomy(int bajo, int alto, int contar)
	{
		int n,num2;
		for (n = 0; n < contar; n++) {
		num2 = (rand() % (alto - bajo + 1)) + bajo;
		//printf("random y: %d \n", num2);
		}
	return num2;
	}


//Function that locates four nearest neighbors of the chosen site (here we are considering periodic boundary conditions)
	int get_limited_coord(int coord, int coord_max)
	{
		if (coord >= 0 && coord < coord_max) {

			return coord;

		} else if (coord >= coord_max) {

			return coord - coord_max;

			} else {

			return coord + coord_max;
			
			}
	}
	



int main(){
	FILE *fp;
	fp  = fopen ("data.txt", "w");
	srand(time(0));
	
	int N = xmax*ymax;
	int grid[xmax][ymax];
	int lower = 0, bajo=0, upper=xmax-1, alto=ymax-1, cuenta=1, contar=1;
	int count = 1, x=0, y=0;
	int right, left, up, down, upleft, upright, downleft, downright;
	double random,q,r; 
	int co = 0, no = 0, n = 0, o = 0, v = N, pco2 = 0, pn2 = 0, po2 = 0; 
	
	// Initialize lattice values to 0
	for (int i = 0; i < xmax; i++) {
		for (int j = 0; j < ymax; j++) {
			grid[i][j] = 0;
		}
	}
	
	
	step1:	
	//LOCATE AN ENTRY OF THE MATRIX RANDOMLY
			for (int i = 0; i < xmax; i++) {
					for (int j = 0; j < ymax; j++) {
						grid[i][j] = count++;
						printf("%d ", grid[i][j]);
					}
					printf("\n");
				}
	
			x = printrandomx(lower, upper, cuenta);
			y = printrandomy(bajo, alto, contar);
			printf("(x,y)=(%d,%d)\n\n",x,y);


			for (int i = 0; i < xmax; i++)
			{
				for (int j = 0; j < ymax; j++ )
				{

					if(x == i && y == j){ grid[x][y] == count; }
					// else{ printf("FAILED\n");}
				}

			}
			printf("The coordinates are (%d,%d) and the solicited matrix entry is: %d\n\n", x, y, grid[y][x]);

			if (x > xmax || x < 0 || y > ymax || y < 0) {
				printf ("Invalid coordinates given\n");
				return 1;
			}
		
		
			
	 //EVALUATE IF THE CHOSEN SITE IS EITHER EMPTY OR OCCUPIED 
	 printf("IS IT OCCUPIED???\n\n\n");
	
	check:			if (grid[y][x] == occupied){
					
					printf("It's occupied, the trial ends. Choose another site again\n\n");
					goto step1;
					
				}
				else{ 
					if (grid[y][x] == empty){

						printf("It's empty. Let's go to generate a random number to choose the adsorbates\n\n");
						goto step2;
					}
					
				}
				
				
				
	seekneighbors: printf("FIND THE NEIGHBORS\n\n");
	//THESE PRINT THE FOUR NEAREST NEIGHBORS OF THE CHOSEN SITE
	//Looking up the nearest 8 neighbors (If the chosen entry is not on any edge of the matrix)
			right = grid[get_limited_coord(y , ymax)][get_limited_coord(x+1, xmax)];
			left = grid[get_limited_coord(y , ymax)][get_limited_coord(x-1, xmax)];
			up = grid[get_limited_coord(y-1, ymax)][get_limited_coord(x , xmax)];
			down = grid[get_limited_coord(y+1, ymax)][get_limited_coord(x , xmax)];
			//printf("The coordinates are (%d,%d) and the solicited matrix entry is: %d\n", x, y, grid[y][x]);
			//Printing the eight nearest neighbors
			printf("Right: %d\nLeft: %d\nUp: %d\nDown: %d\n\n",right,left,up,down);
	goto step3;
	
	//ESCOGEMOS CUAL ESPECIE SE VA A ADSORBER
	//genero un numero aleatorio q tal que si es menor que xco, se adsorbe CO y si es mayor que xco, se adsorbe NO
	
	step2: q = (double) rand () / RAND_MAX;
	printf("random q= %lf\n\n",q);
	
	if(q < xco || q == xco){
		//se adsorbe CO
		printf("CO is adsorbed\n\n");
		grid[y][x] = co_occupied;
		co = co + 1;
		v = v - 1;
		printf("adsorbed CO ---> co = %d\n\n",co);
		
		goto seekneighbors;
		
		step3:
		printf("****THIS PART WORKS****\n\n");
	
		right = o_occupied;
		left = o_occupied;
		up = o_occupied;
		down = o_occupied;
		bool flag1 = 0;
		bool flag2 = 0;
		bool flag3 = 0;
		bool flag4 = 0;
		flag1 = (right == o_occupied);
		flag2 = (left == o_occupied);
		flag3 = (up == o_occupied);
		flag4 = (down == o_occupied);
		
		if(flag1 || flag2 || flag3 || flag4){
			pco2 = pco2 + 1;
			v = v + 2;
			printf("produced co2 = %d\n\n",pco2);
		} else{
			printf("CO2 IS NOT PRODUCED\n\n");
		  }
	} else {
		//se escoge NO
		printf("q = %lf is greater than xco = %lf. We choose NO\n\n", q, xco);
		r = (double) rand () / RAND_MAX;
		printf("r= %lf\n\n", r);
		
		if(r < rno){
			printf("r= %lf is less than rno= %lf\n\n We seek another neighbor to decide if NO can disociate\n\n", r,rno);
			bool flag5 = 0;
			bool flag6 = 0;
			bool flag7 = 0;
			bool flag8 = 0;
			
			flag5 = (right == empty);
			flag6 = (left == empty);
			flag7 = (up == empty);
			flag8 = (down == empty);
			
			if( flag5 || flag6 || flag7 || flag8 ){
				n = n + 1;
				o = o + 1;
				v = v - 2;
				
				printf("adsorbed N ---> n= %d\n adsorbed O ---> o= %d\n",n, o);
			}else{
				printf("****THE TRIAL ENDS, START AGAIN****");
				goto step1;
			}
		}
		
	}
	
	fclose(fp);
	return 0;
}


It seems that my code doesn't even passes through the following chunk:

//EVALUATE IF THE CHOSEN SITE IS EITHER EMPTY OR OCCUPIED
    printf("IS IT OCCUPIED???\n\n\n");

               //random = (double) rand () / RAND_MAX;
               //printf("random = %lf\n\n",random);
               // Include particle based on the probability
   check:          if (grid[y][x] == occupied){

                   printf("It's occupied, the trial ends. Choose another site again\n\n");
                   goto step1;

               }
               else{
                   if (grid[y][x] == empty){

                       printf("It's empty. Let's go to generate a random number to choose the adsorbates\n\n");
                       goto step2;
                   }

               }
Why could that happen?
I'll greatly appreciate if you could give me some hints to continue. Thanks for your time :)
Posted
Updated 29-Jan-23 8:24am
v7

1 solution

A single equals character is an assignment operator:
C
grid[x][y] = 1;

look at what you did in the if statement:
C
if(grid[x][y] = 1)

You're trying to do another assignment, not a comparison. Your if statement should be:
C
if(grid[x][y] == 1)
 
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Comments
Auyik 29-Jan-23 13:39pm    
Hi! thank you for your answer. I was thinking about using "==" but doesn't that action replace the value of the chosen site? In my code grid[x][y] is the chosen entry in the surface I'm working with.
Dave Kreskowiak 29-Jan-23 13:42pm    
You didn't read my post, did you? Again, using a single = is an assignment. The expression value on the right is assigned to the expression on the left.

Two = characters, ==, is the equality operator. This compares the result of the expression on the right of the operator to the result of the expression on the left of it.
Auyik 29-Jan-23 14:28pm    
OK, thanks. I made some modifications to my code, but it never passes through the following part of the code (where I'm trying to verify the occupation of a site):

//EVALUATE IF THE CHOSEN SITE IS EITHER EMPTY OR OCCUPIED
    printf("IS IT OCCUPIED???\n\n\n");

               //random = (double) rand () / RAND_MAX;
               //printf("random = %lf\n\n",random);
               // Include particle based on the probability
   check:          if (grid[y][x] == occupied){

                   printf("It's occupied, the trial ends. Choose another site again\n\n");
                   goto step1;

               }
               else{
                   if (grid[y][x] == empty){

                       printf("It's empty. Let's go to generate a random number to choose the adsorbates\n\n");
                       goto step2;
                   }

               }
Why it would happen?
Dave Kreskowiak 29-Jan-23 14:42pm    
Why are you doing a second if statement if there only two possible states, occupied and empty?
Shouldn't that if statement look like this:
    //EVALUATE IF THE CHOSEN SITE IS EITHER EMPTY OR OCCUPIED
    printf("IS IT OCCUPIED???\n\n\n");

   check:      if (grid[y][x] == occupied)
               {
                   printf("It's occupied, the trial ends. Choose another site again\n\n");
                   goto step1;
               }
               else
               {
                   // If it's not occupied, it must be empty, so why check for it?
                   printf("It's empty. Let's go to generate a random number to choose the adsorbates\n\n");
                   goto step2;
               }
Auyik 29-Jan-23 14:45pm    
Oh, thank you very much. You're right. Let me check my code one more time.

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