MHB Why Use a Helper Variable When Inserting an Element in an Array?

[mathmari]

The first is correct! (Smile)

But the second is not. (Worried)
Sentinal is a data member of StarS[k], and not of last->next.
You should set StarS[k].Sentinal to last->next, which is by now the same as plansystem.Alternatively, you could do at the end: [m]StarS[k].Sentinal = plansystem[/m].
That works because [m]plansystem[/m] is always added to the end, wherever that is. So afterwards it is the last node in the list, which is where [m]Sentinal[/m] should point. (Nerd)

Do you mean that we could write it as followed??

int Beginning(int solid, int ss){
        int i, k=-1;
        /* Add a planetary system to the list of the planetary system of the star system */
        plansys_t *plansystem = calloc(1, sizeof(plansys_t));
        
        plansystem->solid=solid;
        plansystem->asteroids=NULL;
        plansystem->next=NULL;
	for(i=0; i<Sfreep; i++){
		if(StarS[i].ss==ss){
			k=i;
			if (StarS[k].plasy == NULL) {
                                   /* List is empty: replace with new plansystem */
                                   StarS[k].plasy = plansystem;
                        } else {
                                    /* Find last node */
                                    plansys_t *last = StarS[k].plasy;
                                    while (last->next != NULL) {
                                             last = last->next;
                                    }
                                    /* Append new plansystem to the last node */
                                    last->next = plansystem;
                        }
                        StarS[k].Sentinal = plansystem;
                }
        }
	return k;
}

(Wondering)

The way the code is indented, it is hard to follow the flow of the program, and understand if and where something is wrong and should be changed. (Sweating)

Better is:

int Beginning(int solid, int ss){
    int i, k = -1;
    /* Add a planetary system to the list of the planetary system of the star system */
    plansys_t *plansystem = calloc(1, sizeof(plansys_t));
    
    plansystem->solid = solid;
    plansystem->asteroids = NULL;
    plansystem->next = NULL;
    for(i=0; i<Sfreep; i++){
        if(StarS[i].ss == ss){
            k=i;
            if (StarS[k].plasy == NULL) {
                /* List is empty: replace with new plansystem */
                StarS[k].plasy = plansystem;
                StarS[k].Sentinel = StarS[k].plasy;
            } else {
                /* Find last node */
                plansys_t *last = StarS[k].plasy;
                while (last->next != NULL) {
                    last = last->next;
                }
                /* Append new plansystem to the last node */
                last->next = plansystem;
                last->next->Sentinel = last;
            }
        }
    }
    return k;
}

Like this it becomes clear where the for-loop starts and ends, where the first if-statement is, and where the nested if-else statement is (and that there is one). (Whew)

Ok! (Blush)

 

[I like Serena]

Do you mean that we could write it as followed?? (Wondering)

Yep! (Nod)

 

[mathmari]

The structures have the following form:

View attachment 3540

I am looking again at the program and I got stuck...

asteroid_t *p=StarS[k].plasy->asteroids;
while(p != NULL && p->as != as){
		p=p->next;
}
if(p != NULL && p->as == as){
		k=i;				
}

After [m]p=p->next;[/m] where does [m]p[/m] point?? At the next node of the planetary system, plasy, or the next asteroid?? (Wondering)

 

[I like Serena]

Hey!

After [m]p=p->next;[/m] where does [m]p[/m] point?? At the next node of the planetary system, plasy, or the next asteroid?? (Wondering)

asteroid_t *p=StarS[k].plasy->asteroids;

After the initial assignment, [m]p[/m] points to the first asteroid of the first planetary system of star system [m]k[/m].

while(p != NULL && p->as != as){
		p=p->next;
}

After p=p->next, p points to the next asteroid of the first planetary system. (Nerd)

 

[mathmari]

Ok! (Malthe)

To find the asteroid with identifier "as" in the star system I had written the following part:

	int i, k=-1;
	asteroid_t *p=NULL;
	asteroid_t *ast=NULL;
	for(i=0; i<Sfreep; i++){
			ast=StarS[i].plasy->asteroids;
			while(ast != NULL && ast->as != as){
				ast=ast->next;
			}
			if(ast != NULL && ast->as == as){
				k=i;				
			}
	}

	if(k == -1){
		return -1;
	}
		
	p=StarS[k].plasy->asteroids;
	
	if(p == NULL){
		return -1;
	}

But in this way do we not look only the asteroids of the first planetary system?? (Wondering)

Should it be as followed??

	for(i=0; i<Sfreep; i++){
                        Plan_Syst=StarS[i].plasy;
                        while(Plan_Syst != NULL){ 
                                ast=Plan_Syst->asteroids;
			        while(ast != NULL && ast->as != as){
				        ast=ast->next;
			        }
			        if(ast != NULL && ast->as == as){
				        k=i;	
                                        A=ast;			
			       }
                               Plan_Syst=Plan_Syst->next; 
	                }
        }
	if(k == -1){
		return -1;
	}
		
	p=A;
	
	if(p == NULL){
		return -1;
	}

(Wondering)

 

[I like Serena]

Ok! (Malthe)

But in this way do we not look only the asteroids of the first planetary system?? (Wondering)

Should it be as followed??

(Wondering)

Yep. To search all asteroids it should be like that. (Wasntme)

 

[mathmari]

Yep. To search all asteroids it should be like that. (Wasntme)

Ok! (Malthe) I have also an other question...

int nrStarS = 0;

int Constitution(int ss) {
        int i;

        // Add a star system to the the array of stars
        starsy_t *starsystem = &StarS[nrStarS];
        nrStarS++;

        // Initialize the new star system
        starsystem->ss = ss;
        starsystem->plasy=StarS[i].Sentinel;
	for(i=0; i<max; i++){
		starsystem->ffplan[i].fp=INT_MAX;
		starsystem->ffplan[i].ff=NULL;
	}

        // Return the index of the star system that was added
	return (nrStarS - 1);
}

int main(){
	Constitution(12);
	for(i=0; i<nrStarS; i++){
		printf("The elements are: %d \n", StarS[i].ss);
	}
	return 0;
}

Could you explain to me why we write [m]starsystem = &StarS[nrStarS];[/m] and not [m]&StarS[nrStarS]=starsystem[/m] ?? (Wondering)

 

[I like Serena]

Could you explain to me why we write [m]starsystem = &StarS[nrStarS];[/m] and not [m]&StarS[nrStarS]=starsystem[/m] ?? (Wondering)

The structure [m]StarS[nrStarS][/m] is an entry in a global array. Its address is fixed and cannot change. That means it makes no sense to try and assign anything to [m]&StarS[nrStarS][/m].

The pointer [m]starsystem[/m] is a helper variable. We let it point to the location of [m]StarS[nrStarS][/m], so that we can manipulate it more easily, and so that the code reads better. That's why we assign the address [m]&StarS[nrStarS][/m] to it. (Nerd)

 

[mathmari]

The structure [m]StarS[nrStarS][/m] is an entry in a global array. Its address is fixed and cannot change. That means it makes no sense to try and assign anything to [m]&StarS[nrStarS][/m].

The pointer [m]starsystem[/m] is a helper variable. We let it point to the location of [m]StarS[nrStarS][/m], so that we can manipulate it more easily, and so that the code reads better. That's why we assign the address [m]&StarS[nrStarS][/m] to it. (Nerd)

How can the array [m]StarS[/m] have the value of the new starsystem at the position nrStarS, when we assign the [m]&StarS[nrStarS][/m] to [m]starsystem[/m] ?? (Wondering)

[m]nrStarS[/m] is the first free position of the array, so if we assign [m]&StarS[nrStarS][/m] to the new [m]starsystem[/m] doesn't it get the value [m]NULL[/m] ?? (Wondering)