I need help

**zbonzbon** · 11-05-2005

Hi People,

I am trying to understand of how pointer to struct works. And I am running into a problem.

Not sure why the last line "printf("Struct pointer with Name: %s\n",e2[0]->Name);" is showing :-

Struct pointer with Name: This is a second struct.

I thought that it would show:-

Struct pointer with Name: This is the first struct.

However, it does not. Would any one pls help me out.

Thanks,

zbonzbon

Code:

**********************************************************
 * Array.c - Learning how array works.
 **********************************************************/


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


int main(){

	struct example{
		char Name[40];
	};

	char *pt_Name[2]= { 
		"This is the first struct",
		"This is a second struct"
	};

	struct example *e2[2];
	int i;

	for(i=0; i<2; i++){

		struct example e;
		strcpy(e.Name,pt_Name[i]);
		e2[i]=&e;


	}

	printf(" %s\n",e2[0]->Name);

	return 0;
}

**dwks** · 11-05-2005

Array.c - Learning how array works.

You know about structures before arrays?

That code shouldn't work properly. You're taking the address of a local variable.

**dwks** · 11-05-2005

Code:

	struct example *e2[2];
	int i;

	for(i=0; i<2; i++){

		struct example e;
		strcpy(e.Name,pt_Name[i]);
		e2[i]=&e;


	}

	printf(" %s\n",e2[0]->Name);

See? e2[]'s elements point to a variable declared in the for loop, and then you try to print it after the for loop, when e no longer exists!

Get rid of the pointers; make *e2 into e2, and try it that way.

**zbonzbon** · 11-05-2005

Hi dwks,

1. I was just finishing to learn how array in C works and then I just use the same file to add the structure to it. That is why you see - Array.c - Learn how array works.

2. What do you mean by local variable ? Are you refer to *e2 ? Would you pls give me a little more explaining.

Thanks,

zbonzbon

**dwks** · 11-05-2005

e is a variable local to the for loop. It's scope ends when the for loop ends. But you take it's address, assign it to a variable, and try to access it after the for loop! e no longer exists.

**zbonzbon** · 11-05-2005

Hi dwks,

Ah. I got it. Thanks

Regards,

zbonzbon

**dwks** · 11-05-2005

So, did you fix it? Here's a program that uses an array:

Code:

#include <stdio.h>

int main(void) {
    char *s[] = {"false", "true"};
    int x;

    printf("\nEnter a number: ");
    scanf("%i", &x);
    if(!x) puts(s[0]);
    else puts(s[1]);

    return 0;
}

**zbonzbon** · 11-05-2005

Hi dwks,

1. Yes, I just change *e2 into a normal an array of structs - struct example e2[2]. Then it would work nicely. However, one important point I learn today (actually from you) is that - Do not use pointer to point to a local variable.

2. Thanks for the program I will try it.

Thanks,

zbonzbon

**zbonzbon** · 11-05-2005

Hi dwks,

BTW, I sometimes saw a code and it starts liked this:

int main(argc, char argv[])

I kind of understand that - argc is an argument count on the command line.

argv[] is where it stores the commands ?....

Another the words is that I am not quite 100% understand it.

Would you help ?

Thanks,

zbonzbon

**dwks** · 11-05-2005

Actually, you probably meant

Code:

int main(argc, char *argv[])

I kind of understand that - argc is an argument count on the command line.

argv[] is where it stores the commands ?....

argv[0] is usually the program's name, that is, the path to the executable. The rest of argv[] is the arguments passed on the command line. argc is the number of items in argv[].

Code:

#include <stdio.h>

int main(int argc, char *argv[]) {  /* or **argv */
    int x;
    printf("argc: %i\n", argc);

    for(x = 0; x < argc; x ++) {
        printf("argv[%i]: %s\n", x, argv[x]);
    }

    return 0;
}

Code:

C:\DWK>testargs
argc: 1
argv[0]: C:\DWK\TESTARGS.EXE

C:\DWK>testargs hippo testing
argc: 3
argv[0]: C:\DWK\TESTARGS.EXE
argv[1]: hippo
argv[2]: testing

C:\DWK>testargs "This is a string with spaces in it" hippo
argc: 3
argv[0]: C:\DWK\TESTARGS.EXE
argv[1]: This is a string with spaces in it
argv[2]: hippo

C:\DWK>

**zbonzbon** · 11-05-2005

Hi dwks,

1. Thanks a lot for your help.

2. Another hard topic that I also did not understand is - Link list. I have been trying to read and read. But most are very hard understand. Can you give me a basic simple program with link list and explain what they are?

Thanks again,

zbonzbon

**dwks** · 11-05-2005

Well, I could. But read the FAQ first. Then I can, if you want.

**zbonzbon** · 11-05-2005

Hi dwks,

Thanks, I will read it now and I will post my questions later.

Regards,

zbonzbon

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