Assigning Array of Structs to a typedef struct

Question

How do I assign to a typedef struct an array of another struct with a similar structure.

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

typedef struct {
    int age;
    int height;
} Person[3];

struct internalStruct {
    int age;
    int height;
};

int main(void) {

    //Possible
    Person bob = {{7,5},{4,2},{4,3}};

    //Is it possible to assign array to struct?
    struct internalStruct intr[3] = {{4,32},{2,4},{2,4}};
    Person job = intr; // Does not work :(. 
    printf("%d", jon[0].height);
    return 0;
}

Is there any reason not to do struct internalStruct { int age; int height; }; typedef struct internalStruct Person[3]; ? — Chris Lutz
– Chris Lutz, Commented Nov 7, 2011 at 22:08
I am just trying to understand if the memory structures are the same (typedef of array of 3 is the same memory structure as well just a regular array of 3 struct . I am not actually coding in C. — Anonymous
– Anonymous, Commented Nov 9, 2011 at 21:18

Keith Thompson · Accepted Answer · 2011-11-07 23:24:02Z

You cannot assign to an array in C. You can initialize an array when you declare it, but an array expression cannot appear on the left side of an assignment operator.

If you want to copy the value of an array object into another array object, you can use an explicit loop to assign each element (assuming the element type is assignable), or you can use memcpy(). (Note that a call to memcpy() needs to specify the number of bytes to copy; use sizeof for this.)

And your typedef Person:

typedef struct {
    int age;
    int height;
} Person[3];

is ill-advised. A Person object (variable) isn't a person; it's an array of 3 persons (people?).

My advice: Drop the typedef and just use a struct tag (as you already do for struct internalStruct) and don't try to create a special name for the array type:

struct Person {
    int age;
    int height;
};

...

struct Person bob[] = {{7,5},{4,2},{4,3}};

(This is still confusing, since bob is three people.)

And struct Person (as I've defined it here) and struct internalStruct are two distinct types. If you're trying to assign between these two types, it probably indicates a design flaw in your code; objects you're assigning to each other should be of the same type.

Recommended reading: the comp.lang.c FAQ, especially section 6 (arrays and pointers).

sharpner · Accepted Answer · 2011-11-07 20:20:50Z

0

I would not suggest this tough, since you might run into memory leaks when the two structs are different:

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

typedef struct {
    int age;
    int height;
} Person[3];

struct internalStruct {
    int age;
    int height;
};

int main(void) {

    //Possible
    Person bob = {{7,5},{4,2},{4,3}};

    //Is it possible to assign array to struct?
    struct internalStruct intr[3] = {{4,32},{2,4},{2,4}};
    Person* jon= (Person *)intr; // Does not work :(.
    printf("%d", jon[0]->height);
    return 0;
}

edited Nov 7, 2011 at 20:20

answered Nov 7, 2011 at 20:03

sharpner

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2 Comments

Anonymous Over a year ago

That does not work. I need to be able to assign to intr to job

sharpner Over a year ago

ah that's your problem, well it is probably not a good idea to switch struct types but you could do this with pointers, i edited my first answer. note the type: job != jon

Dmitri · Accepted Answer · 2011-11-07 22:40:16Z

0

Your Person type is an array of three structures, each of which is similar to your struct internalStruct. So, you can't just assign a struct internalStruct to a Person, though you can (with some help) assign it to one of the elements of a Person.

Also, copying arrays in C requires copying element by element, or copying the block of memory with a function like memcpy().

So the simplest way to do this would be to define struct internalStruct before Person, and define Person in terms of struct internalStruct:

struct internalStruct {
  int age;
  int height;
};

typedef struct internalStruct Person[3];

Doing it this way makes it possible to assign a struct internalStruct to an element of Person without type mismatches. For example:

struct internalStruct s1 = {4,32},
                      s2 = {2,4},
                      s3 = {2,4};
Person jon;
jon[0] = s1;
jon[1] = s2;
jon[2] = s3;

If you have an array of three struct internalStructs, you could copy it with a loop:

struct internalStruct st[3] = { {4,32}, {2,4}, {2,4} };
Person jon;
for (int i = 0; i < 3; i++)
  jon[i] = st[i];

If you don't want to define Person in terms of struct internalStruct, then you have to make some assumptions, like that the layout of the two structures will be identical. If so, you could copy with memcpy():

struct internalStruct intr[3] = { {4,32}, {2,4}, {2,4} };
Person jon;
memcpy(jon, intr, sizeof(Person));

answered Nov 7, 2011 at 22:40

Dmitri

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2 Comments

Anonymous Over a year ago

Is there a way to get a memory map of of different structures (unions/enum/typedef) for particular compiler.

Dmitri Over a year ago

@Anonymous Carefully sift through the documentation for the compiler? Otherwise, in code, you can check that sizeof gives the same size for both structures, and that offsetof gives the same values for the corresponding members in each structure. Usually if they're defined the same way and with the same attributes, they'll match -- but it's better not to depend on it, and to just use the same type instead of two types that are probably identical.

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