Arrow and point operator in C

Question

Below I have two code samples. In the first I can only work with the dot . operator and in the second I can only work with the arrow -> operator. But why? Do I not have in both cases pointers?

The code for arrow -> operators:

#include <stdlib.h>
#include <stdio.h>

typedef struct{
    double **plist;
} ParticleList;

void sendPar(ParticleList *pl, int *n, int np){
    pl -> plist = malloc(sizeof(double*) * np);
    for(int p=0; p<np; p++){
        pl->plist[p] = malloc(sizeof(double) * n[p]);
    }

    for(int p=0; p<np; p++){
        for(int k=0; k<n[p]; k++){
            pl->plist[p][k] = k+1;
        }
    }
}

int main(){
    ParticleList pl;
    int np = 3;
    int n[np];
    n[0] = 2;
    n[1] = 4;
    n[2] = 7;   
    sendPar(&pl, n, np);
}

The code for dot . operators:

#include <stdlib.h>
#include <stdio.h>

typedef struct{
    double **plist;
} ParticleList;

void sendPar(int *n, int np){
    ParticleList pl;
    pl.plist = malloc(sizeof(double*) * np);
    for(int p=0; p<np; p++){
        pl.plist[p] = malloc(sizeof(double) * n[p]);
    }

    for(int p=0; p<np; p++){
        for(int k=0; k<n[p]; k++){
            pl.plist[p][k] = k+1;
        }
    }
    free(pl.plist);
}

int main(){
    int np = 3;
    int n[np];
    n[0] = 2;
    n[1] = 4;
    n[2] = 7;
    sendPar(n, np);
}

Are both versions valid or is there any difference?

" Do I not have in both cases pointers?" No, do you not see the difference between `ParticleList *pl` and `ParticleList pl`? — itsme86, Dec 22 '16 at 15:57
A useful related question: http://stackoverflow.com/questions/2575048/arrow-operator-usage-in-c?rq=1 — David Hoelzer, Dec 22 '16 at 16:02
Please note that the dot `.` and arrow `->` operators bind very tightly and should not be surrounded by spaces. — Jonathan Leffler, Dec 22 '16 at 18:41

score 2 · Answer 1 · edited Dec 22 '16 at 18:44

2

In the 1st case, you do are using a pointer; thus using the arrow operator -> is correct:

void sendPar(ParticleList *pl, int *n, int np){
    pl->plist

In the 2nd case, you are NOT using a pointer but a value; thus using the DOT . operator is correct (necessary):

ParticleList pl;
pl.plist =

That's it — except that you don't put spaces around either the dot or arrow operator.

edited Dec 22 '16 at 18:44

Jonathan Leffler

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answered Dec 22 '16 at 16:01

Tytou

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2

score 1 · Answer 2 · answered Dec 22 '16 at 15:57

1

In the first example, pl is a pointer of type ParticleList. To dereference the members of a struct type pointer you use the ->.

In your second code example, pl is a struct of ParticleList, not a pointer. To dereference struct members, you use a dot.

answered Dec 22 '16 at 15:57

David Hoelzer

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Arrow and point operator in C

2 Answers2