Thread: Array of Structures & Pointers

Hi,

I am a bit stumped by a small issue and I am sure that it is something simple I am overlooking and could use some direction. Below I have put my code where I define an array of structures and a pointer to it. What I am doing is taking the input and assigning a name to each Citizen in newCits. However when I am testing the code and I print out the names, they all come out the same. Any help is much appreciated!

Code:

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


typedef struct {
char *name;
} Citizen;


void mainMenu(Citizen *c);
void uploadMenu(Citizen *c);
void citizenUpload(Citizen *c);
void printCitizens(Citizen *c);


void inputFlush();


int main(){


Citizen newCits[5];
Citizen *p = newCits;
mainMenu(p);


return (0);


}


void mainMenu(Citizen *c){


int input;


printf("Welcome to the HGSM Main Menu! What would you like to do?\n(1) Access Upload Menu\nSelection: ");
scanf("%d", &input);
inputFlush();


    if (input == 1){
    uploadMenu(c);
    }


}


void uploadMenu(Citizen *c){


int input;


printf("Welcome to the HGSM Upload Menu. What would you like to do?\n(1) Upload Citizen Data\nSelection: ");
scanf("%d", &input);
inputFlush();


    if (input == 1){
    citizenUpload(c);
    }


}


void citizenUpload(Citizen *c){


int input, i;
char citInput[20];


printf("Welcome to the Citizen Upload Menu. How many Citizens do you want to Upload?\nNumber of Citizens: ");
scanf("%d", &input);
inputFlush();


    for (i = 0; i < input; i++){
        printf("Enter Citizen Information in the format (Name, District #, Year of Birth, Sex):\nInput: ");
        scanf("%s", citInput);
        inputFlush();
        (c+i) -> name = citInput;
    }


    printCitizens(c);


}


void printCitizens(Citizen *c){


int i;


    for (i = 0; i < 2; i++){
        printf("%s\n", (c+i) -> name);
    }


}


void inputFlush(){


int ch;


    while ((ch = getchar()) != '\n' && ch != EOF);


}

You do not allocate memory for the name.You should do it with malloc.
Also in the print function notice that you have i<2 and not i<3

Anyway, I solved this problem on my own, just sharing information for anyone who might be interested. The problem was not the loop counter or a malloc for char name, the problem was with not directly accessing the array/pointer that I wanted to add the information.

The way I had this set up was an array that held 5 structures of type Citizen that I had defined. The problem therein was I only passing a pointer to the Citizen structures I wanted to add information, when the function actually required a double pointer:

Code:

void citizenUpload(Citizen **c);

Then in order to add the information to the individual fields of a structure you have to again use some pointer arithmetic to access the address of the data and overwrite it:

Code:

 strcpy((*c+i) -> name, "I want this string in there!");

Well hopefully this will help someone else who is having some trouble with pointers and structures, but I would highly recommend reading more into pointers, double pointers and general pointer arithmetic as it proved very helpful to me.

Cheers.

If i was a beginner and wanted to use your instructions i might write something like this

Code:

#include <stdio.h>
#include <string.h>

int main(void)
{
    char* strPointer;

    strcpy(strPointer,"Some boys thing they know everything...");

    printf("%s\n",strPointer);

    return 0;
}

output

Code:

Macintosh-c8bcc88e5669-9:~ usi$ pico px.c
Macintosh-c8bcc88e5669-9:~ usi$ gcc -Wall px.c -o px
Macintosh-c8bcc88e5669-9:~ usi$ ./px
Segmentation fault: 11

I hope you understand why in your case it worked
......

The loop counter was not a problem.You do not access all the elements. It is similar to have this

Code:

#include <stdio.h>

int main(void)
{
    int array[5] = {0,1,2,3,4};
    int i;

    for(i = 0 ; i < 4 ; i++)
       printf("%d\n",array[i]);

    return 0;
}

output

Code:

Macintosh-c8bcc88e5669-9:~ usi$ pico px.c
Macintosh-c8bcc88e5669-9:~ usi$ gcc -Wall px.c -o px
Macintosh-c8bcc88e5669-9:~ usi$ ./px
0
1
2
3

But ok maybe you want this