Pointer to array structure

Question

I have a small program which when compiling throws me the following errors

error #2168: Operands of '+' have incompatible types 'struct agenda' and 'int'.

error #2113: Left operand of '.' has incompatible type 'int'.

error #2088: Lvalue required.

This is the code that I have done

#include <stdio.h>

struct agenda{
    int order, cellular;
    char name[30], last_name[30], street[30], city[30], mail[50];
}contact[10];

int main(void)
{
    struct agenda *ptrcontact;
    ptrcontact = &contact[0];

    (*ptrcontact+3).order = 3;

    printf("\n\n %d", (*ptrcontact).order);
    return 0;
}

because it throws these errors and how to fix them?

what is *ptrcontact? Its contact[0]. You should try something like: (ptrcontact + 3)->order = 3; — DOOM
– DOOM, Commented Feb 8, 2015 at 14:16
(*ptrcontact+3) gives you whatever ptrcontact points at (a struct agenda) and adds 3 (int) to that. * has higher precedence than +, so in order to get the third index, you have to either write: *(ptrcontact + 3) or ptrcontact[3]. — Beko
– Beko, Commented Feb 8, 2015 at 14:18

Sourav Ghosh · Accepted Answer · 2015-02-08 14:38:03Z

6

You need to change

(*ptrcontact+3).order = 3;

to

ptrcontact[3].order = 3;

or, at least,

(*(ptrcontact+3)).order = 3;

or,

(ptrcontact + 3)->order = 3;

Otherwise, as per the precedence rule, * has higher precedence over +, causing the error.

Just to add to that, ptrcontact is a pointer ( to struct agenda) and can be used as an operand to + operator.

OTOH, *ptrcontact is of type struct agenda and cannot be used as an operand to + operator.

edited Feb 8, 2015 at 14:38

answered Feb 8, 2015 at 14:14

Sourav Ghosh

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Comments

Lev Kuznetsov · Accepted Answer · 2015-02-08 14:19:15Z

2

You are dereferencing the pointer which yields the struct and obviously you can't add anything to that. Dereference operator has the highest priority, you need to do it like this: (*(ptr + 3)).order or use the arrow instead of star dot: (ptr + 3) -> order

answered Feb 8, 2015 at 14:19

Lev Kuznetsov

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1 Comment

Lev Kuznetsov Over a year ago

Man that really took me this long to type up

Roman Nikitchenko · Accepted Answer · 2015-02-08 15:24:26Z

0

Your problem here is operations priority:

 (*ptrcontact+3).order = 3;

This derefers ptrcontract and then tries to add number to dereferred structure. Which gives you exact situation you report.

My recommendations:

Either avoid address ariphmetics in such cases. Operate array indexes.

int baseIndex = 0;
contact[baseIndex + 3].order = 3;

Or if you really have to do so, hide address arithmetic from outside:

(pcontact + 3)->order = 3;

And finally learn C language operations priority or, to do it once (but some C people don't like C++), C++ operations priority

edited Feb 8, 2015 at 15:24

answered Feb 8, 2015 at 14:34

Roman Nikitchenko

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5 Comments

Iharob Al Asimi Over a year ago

This is a c question and the provided link will contain operators that are not even valid in the c language. en.cppreference.com is a c++ resource, not a c resource.

Roman Nikitchenko Over a year ago

Exactly. But all valid operations priorities are as C programmer expects ;-). This is just to not learn them again when C++ starts.

Iharob Al Asimi Over a year ago

I don't think every c programmer has to learn such an ugly language.

Roman Nikitchenko Over a year ago

:-))) I agree with you somewhat. I've migrated to Java then Scala after C++11. Actually C++ is now too complex and too broad for single language - this results 'several languages in one'. But reality is - C programmers often learn C++ and (which makes the deal) then they return to C and become real programmers - after this they start to be language-agnostic. :-|

Roman Nikitchenko Over a year ago

OK, I agreed C priorities at least should be referred first.

Akilesh Reddzz · Accepted Answer · 2015-02-08 16:48:56Z

0

the error is in the lines (*ptrcontact+3).order = 3; and printf("\n\n %d", (*ptrcontact).order);. In this instructions use -> instead of . the errors will be solved.

answered Feb 8, 2015 at 16:48

Akilesh Reddzz

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