C Language, Problems assigning structure members to pointers

Problems assigning structure members to pointers

Author	Message
Hillary Woo #1 / 4	Problems assigning structure members to pointers Hello, I have been having problems assigning pointers to structure members. I have been able to assign a pointer to the first member of my structure but have not been able to assign a pointer to a specific member. Thanks a lot. // structure definition static struct symval { char symbol[MAX_SYMBOL_LEN]; double value; Quote: }; // declared array of structures and parallel pointer static struct symval current[MAX_NUM_SYMBOLS]; static struct symval *ptrstr[MAX_NUM_SYMBOLS]; // assignments: The first assignment works fine. I think this is because I assigning the first member and do not have to // worry about the member syntax. In the second assignment I have tried to add the ampersand, but I get a compile error // "incompatible types". Without the ampersand I don't get any errors but I don't get the assignment I want either. I am at // my wits end! Please help. ptrstr[middle] = &current[num_symbols]; ptrstr[middle]->value = current[num_symbols].value;
Wed, 26 May 2004 07:02:18 GMT

Morris Dove #2 / 4	Problems assigning structure members to pointers Quote: > Hello, > I have been having problems assigning pointers to structure members. I have > been able to assign a pointer to the first > member of my structure but have not been able to assign a pointer to a > specific member. Thanks a lot. > // structure definition > static struct symval { > char symbol[MAX_SYMBOL_LEN]; > double value; > }; > // declared array of structures and parallel pointer > static struct symval current[MAX_NUM_SYMBOLS]; > static struct symval ptrstr[MAX_NUM_SYMBOLS]; > // assignments: The first assignment works fine. I think this is because I > assigning the first member and do not have to > // worry about the member syntax. In the second assignment I have tried to > add the ampersand, but I get a compile error > // "incompatible types". Without the ampersand I don't get any errors but I > don't get the assignment I want either. I am at > // my wits end! Please help. > ptrstr[middle] = &current[num_symbols]; > ptrstr[middle]->value = current[num_symbols].value; Hillary... ptrstr[] is an array of pointers to symval{}. The first assignment is OK because you are assigning an address of (pointer to) a symval{}, as advertized. In the second assignment ptrstr[middle] (set by the preceeding assignment) is used to reference the value element of that structure (using ptrstr[middle]->value) and you can, indeed, store the value element from another symval{} into it. It doesn't make sense to attempt: ptrstr[middle]->value = &current[num_symbols].value; because you would be attempting to store a pointer to a double* into a symval{} element that you promised to put only double into! -- Morris Dovey West Des Moines, Iowa USA
Wed, 26 May 2004 06:18:41 GMT

Russ Bobbi #3 / 4	Problems assigning structure members to pointers Quote: >Hello, >I have been having problems assigning pointers to structure members. I have >been able to assign a pointer to the first >member of my structure but have not been able to assign a pointer to a >specific member. Thanks a lot. >// structure definition >static struct symval { static isn't needed here. Code outside this compilation unit won't see the structure definition. static is only relevant to the visibility of variables not the visibility of variable types. Quote: >char symbol[MAX_SYMBOL_LEN]; >double value; >}; >// declared array of structures and parallel pointer >static struct symval current[MAX_NUM_SYMBOLS]; >static struct symval ptrstr[MAX_NUM_SYMBOLS]; >// assignments: The first assignment works fine. I think this is because I >assigning the first member and do not have to >// worry about the member syntax. In the second assignment I have tried to >add the ampersand, but I get a compile error >// "incompatible types". Without the ampersand I don't get any errors but I >don't get the assignment I want either. It's incorrect to add the ampersand and does, indeed, result in an incompatible type assignment. ptrstr[middle]->value is a double, so you want to assign it a double. Placing the & before the rvalue makes the assignment a pointer to double. Quote: > I am at >// my wits end! Please help. > ptrstr[middle] = &current[num_symbols]; > ptrstr[middle]->value = current[num_symbols].value; What you've provided looks correct. For instance, this code works: #include <stdio.h> #include <string.h> struct symval { char symbol[10]; double value; Quote: }; static struct symval current[10]; static struct symval ptrstr[10]; int main(void) { strcpy(current[0].symbol,"hello"); current[0].value=5.5; ptrstr[0] = &current[0]; ptrstr[0]->value=10.5; printf("%s; %f\n",ptrstr[0]->symbol,ptrstr[0]->value); return 0; Quote: } Output: hello; 10.5 It would be nice to see the actual code you're using as I suspect the problem is hidden there. Try putting together the smallest compilable piece of code that reproduces your problem. Russ -- Replace all the N's with i's to reply.
Wed, 26 May 2004 07:34:16 GMT

Mal Ka #4 / 4	Problems assigning structure members to pointers Quote: > Hello, > I have been having problems assigning pointers to structure members. I have > been able to assign a pointer to the first > member of my structure but have not been able to assign a pointer to a > specific member. Thanks a lot. > // structure definition > static struct symval { > char symbol[MAX_SYMBOL_LEN]; > double value; > }; > // declared array of structures and parallel pointer > static struct symval current[MAX_NUM_SYMBOLS]; > static struct symval *ptrstr[MAX_NUM_SYMBOLS]; > // assignments: The first assignment works fine. I think this is because I > assigning the first member and do not have to > // worry about the member syntax. In the second assignment I have tried to > add the ampersand, but I get a compile error > // "incompatible types". Without the ampersand I don't get any errors but I > don't get the assignment I want either. I am at > // my wits end! Please help. > ptrstr[middle] = &current[num_symbols]; > ptrstr[middle]->value = current[num_symbols].value; This looks legitimate but pointless. What were you expecting? After ptrstr[middle] = &current[num_symbols]; ptrstr[middle]->value and current[num_symbols].value refer to the same location so you are copying a structure member back into itself. You can't really see that the assignment has occured and a very smart compiler is in any case allowed to skip the redundant operation! Malcolm Kay
Wed, 26 May 2004 10:08:25 GMT

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