I'm working on an application and my problem started when i tried to encode a model that has an enum property using NSCoding. So i had the idea to convert it to the rawValue and the way back. I looked around a bit and came around the macro NS_ENUM, so my code looks like this:
typedef NS_ENUM(NSInteger, SectionType) {
SectionTypeText = 0,
SectionTypeVideo = 1,
SectionTypeLink = 2,
SectionTypeFile = 3,
SectionTypeQuiz = 4,
SectionTypeAudio = 5,
SectionTypeGame = 6,
SectionTypeHomework = 7
};
But i could find no possible way to convert these to the associated value and the way back. How could i do it? There is a better approach than the NS_ENUM macro?
My Objective-C is a bit rusty, but I think I would just cast it:
SectionType type = (SectionType) 2;
Back works the same:
int typeNumber = type;
I wrote a function containing array as argument,
and call it by passing value of array as follows.
void arraytest(int a[])
{
// changed the array a
a[0] = a[0] + a[1];
a[1] = a[0] - a[1];
a[0] = a[0] - a[1];
}
void main()
{
int arr[] = {1, 2};
printf("%d \t %d", arr[0], arr[1]);
arraytest(arr);
printf("\n After calling fun arr contains: %d\t %d", arr[0], arr[1]);
}
What I found is though I am calling arraytest() function by passing values, the original copy of int arr[] is changed.
Can you please explain why?
When passing an array as a parameter, this
void arraytest(int a[])
means exactly the same as
void arraytest(int *a)
so you are modifying the values in main.
For historical reasons, arrays are not first class citizens and cannot be passed by value.
For passing 2D (or higher multidimensional) arrays instead, see my other answers here:
How to pass a multidimensional [C-style] array to a function in C and C++, and here:
How to pass a multidimensional array to a function in C++ only, via std::vector<std::vector<int>>&
Passing 1D arrays as function parameters in C (and C++)
1. Standard array usage in C with natural type decay (adjustment) from array to ptr
#Bo Persson correctly states in his great answer here:
When passing an array as a parameter, this
void arraytest(int a[])
means exactly the same as
void arraytest(int *a)
Let me add some comments to add clarity to those two code snippets:
// param is array of ints; the arg passed automatically "adjusts" (frequently said
// informally as "decays") from `int []` (array of ints) to `int *`
// (ptr to int)
void arraytest(int a[])
// ptr to int
void arraytest(int *a)
However, let me add also that the above two forms also:
mean exactly the same as
// array of 0 ints; automatically adjusts (decays) from `int [0]`
// (array of zero ints) to `int *` (ptr to int)
void arraytest(int a[0])
which means exactly the same as
// array of 1 int; automatically adjusts (decays) from `int [1]`
// (array of 1 int) to `int *` (ptr to int)
void arraytest(int a[1])
which means exactly the same as
// array of 2 ints; automatically adjusts (decays) from `int [2]`
// (array of 2 ints) to `int *` (ptr to int)
void arraytest(int a[2])
which means exactly the same as
// array of 1000 ints; automatically adjusts (decays) from `int [1000]`
// (array of 1000 ints) to `int *` (ptr to int)
void arraytest(int a[1000])
etc.
In every single one of the array examples above, and as shown in the example calls in the code just below, the input parameter type adjusts (decays) to an int *, and can be called with no warnings and no errors, even with build options -Wall -Wextra -Werror turned on (see my repo here for details on these 3 build options), like this:
int array1[2];
int * array2 = array1;
// works fine because `array1` automatically decays from an array type
// to a pointer type: `int *`
arraytest(array1);
// works fine because `array2` is already an `int *`
arraytest(array2);
As a matter of fact, the "size" value ([0], [1], [2], [1000], etc.) inside the array parameter here is apparently just for aesthetic/self-documentation purposes, and can be any positive integer (size_t type I think) you want!
In practice, however, you should use it to specify the minimum size of the array you expect the function to receive, so that when writing code it's easy for you to track and verify. The MISRA-C-2012 standard (buy/download the 236-pg 2012-version PDF of the standard for £15.00 here) goes so far as to state (emphasis added):
Rule 17.5 The function argument corresponding to a parameter declared to have an array type shall have an appropriate number of elements.
...
If a parameter is declared as an array with a specified size, the corresponding argument in each function call should point into an object that has at least as many elements as the array.
...
The use of an array declarator for a function parameter specifies the function interface more clearly than using a pointer. The minimum number of elements expected by the function is explicitly stated, whereas this is not possible with a pointer.
In other words, they recommend using the explicit size format, even though the C standard technically doesn't enforce it--it at least helps clarify to you as a developer, and to others using the code, what size array the function is expecting you to pass in.
2. Forcing type safety on arrays in C
(Not recommended (correction: sometimes recommended, especially for fixed-size multi-dimensional arrays), but possible. See my brief argument against doing this at the end. Also, for my multi-dimensional-array [ex: 2D array] version of this, see my answer here.)
As #Winger Sendon points out in a comment below my answer, we can force C to treat an array type to be different based on the array size!
First, you must recognize that in my example just above, using the int array1[2]; like this: arraytest(array1); causes array1 to automatically decay into an int *. HOWEVER, if you take the address of array1 instead and call arraytest(&array1), you get completely different behavior! Now, it does NOT decay into an int *! This is because if you take the address of an array then you already have a pointer type, and pointer types do NOT adjust to other pointer types. Only array types adjust to pointer types. So instead, the type of &array1 is int (*)[2], which means "pointer to an array of size 2 of int", or "pointer to an array of size 2 of type int", or said also as "pointer to an array of 2 ints". So, you can FORCE C to check for type safety on an array by passing explicit pointers to arrays, like this:
// `a` is of type `int (*)[2]`, which means "pointer to array of 2 ints";
// since it is already a ptr, it can NOT automatically decay further
// to any other type of ptr
void arraytest(int (*a)[2])
{
// my function here
}
This syntax is hard to read, but similar to that of a function pointer. The online tool, cdecl, tells us that int (*a)[2] means: "declare a as pointer to array 2 of int" (pointer to array of 2 ints). Do NOT confuse this with the version withOUT parenthesis: int * a[2], which means: "declare a as array 2 of pointer to int" (AKA: array of 2 pointers to int, AKA: array of 2 int*s).
Now, this function REQUIRES you to call it with the address operator (&) like this, using as an input parameter a POINTER TO AN ARRAY OF THE CORRECT SIZE!:
int array1[2];
// ok, since the type of `array1` is `int (*)[2]` (ptr to array of
// 2 ints)
arraytest(&array1); // you must use the & operator here to prevent
// `array1` from otherwise automatically decaying
// into `int *`, which is the WRONG input type here!
This, however, will produce a warning:
int array1[2];
// WARNING! Wrong type since the type of `array1` decays to `int *`:
// main.c:32:15: warning: passing argument 1 of ‘arraytest’ from
// incompatible pointer type [-Wincompatible-pointer-types]
// main.c:22:6: note: expected ‘int (*)[2]’ but argument is of type ‘int *’
arraytest(array1); // (missing & operator)
You may test this code here.
To force the C compiler to turn this warning into an error, so that you MUST always call arraytest(&array1); using only an input array of the corrrect size and type (int array1[2]; in this case), add -Werror to your build options. If running the test code above on onlinegdb.com, do this by clicking the gear icon in the top-right and click on "Extra Compiler Flags" to type this option in. Now, this warning:
main.c:34:15: warning: passing argument 1 of ‘arraytest’ from incompatible pointer type [-Wincompatible-pointer-types]
main.c:24:6: note: expected ‘int (*)[2]’ but argument is of type ‘int *’
will turn into this build error:
main.c: In function ‘main’:
main.c:34:15: error: passing argument 1 of ‘arraytest’ from incompatible pointer type [-Werror=incompatible-pointer-types]
arraytest(array1); // warning!
^~~~~~
main.c:24:6: note: expected ‘int (*)[2]’ but argument is of type ‘int *’
void arraytest(int (*a)[2])
^~~~~~~~~
cc1: all warnings being treated as errors
Note that you can also create "type safe" pointers to arrays of a given size, like this:
int array[2]; // variable `array` is of type `int [2]`, or "array of 2 ints"
// `array_p` is a "type safe" ptr to array of size 2 of int; ie: its type
// is `int (*)[2]`, which can also be stated: "ptr to array of 2 ints"
int (*array_p)[2] = &array;
...but I do NOT necessarily recommend this (using these "type safe" arrays in C), as it reminds me a lot of the C++ antics used to force type safety everywhere, at the exceptionally high cost of language syntax complexity, verbosity, and difficulty architecting code, and which I dislike and have ranted about many times before (ex: see "My Thoughts on C++" here).
For additional tests and experimentation, see also the link just below.
References
See links above. Also:
My code experimentation online: https://onlinegdb.com/B1RsrBDFD
See also:
My answer on multi-dimensional arrays (ex: 2D arrays) which expounds upon the above, and uses the "type safety" approach for multi-dimensional arrays where it makes sense: How to pass a multidimensional array to a function in C and C++
If you want to pass a single-dimension array as an argument in a function, you would have to declare a formal parameter in one of following three ways and all three declaration methods produce similar results because each tells the compiler that an integer pointer is going to be received.
int func(int arr[], ...){
.
.
.
}
int func(int arr[SIZE], ...){
.
.
.
}
int func(int* arr, ...){
.
.
.
}
So, you are modifying the original values.
Thanks !!!
Passing a multidimensional array as argument to a function.
Passing an one dim array as argument is more or less trivial.
Let's take a look on more interesting case of passing a 2 dim array.
In C you can't use a pointer to pointer construct (int **) instead of 2 dim array.
Let's make an example:
void assignZeros(int(*arr)[5], const int rows) {
for (int i = 0; i < rows; i++) {
for (int j = 0; j < 5; j++) {
*(*(arr + i) + j) = 0;
// or equivalent assignment
arr[i][j] = 0;
}
}
Here I have specified a function that takes as first argument a pointer to an array of 5 integers.
I can pass as argument any 2 dim array that has 5 columns:
int arr1[1][5]
int arr1[2][5]
...
int arr1[20][5]
...
You may come to an idea to define a more general function that can accept any 2 dim array and change the function signature as follows:
void assignZeros(int ** arr, const int rows, const int cols) {
for (int i = 0; i < rows; i++) {
for (int j = 0; j < cols; j++) {
*(*(arr + i) + j) = 0;
}
}
}
This code would compile but you will get a runtime error when trying to assign the values in the same way as in the first function.
So in C a multidimensional arrays are not the same as pointers to pointers ... to pointers. An int(*arr)[5] is a pointer to array of 5 elements,
an int(*arr)[6] is a pointer to array of 6 elements, and they are a pointers to different types!
Well, how to define functions arguments for higher dimensions? Simple, we just follow the pattern!
Here is the same function adjusted to take an array of 3 dimensions:
void assignZeros2(int(*arr)[4][5], const int dim1, const int dim2, const int dim3) {
for (int i = 0; i < dim1; i++) {
for (int j = 0; j < dim2; j++) {
for (int k = 0; k < dim3; k++) {
*(*(*(arr + i) + j) + k) = 0;
// or equivalent assignment
arr[i][j][k] = 0;
}
}
}
}
How you would expect, it can take as argument any 3 dim arrays that have in the second dimensions 4 elements and in the third dimension 5 elements. Anything like this would be OK:
arr[1][4][5]
arr[2][4][5]
...
arr[10][4][5]
...
But we have to specify all dimensions sizes up to the first one.
You are not passing the array as copy. It is only a pointer pointing to the address where the first element of the array is in memory.
You are passing the address of the first element of the array
You are passing the value of the memory location of the first member of the array.
Therefore when you start modifying the array inside the function, you are modifying the original array.
Remember that a[1] is *(a+1).
Arrays in C are converted, in most of the cases, to a pointer to the first element of the array itself. And more in detail arrays passed into functions are always converted into pointers.
Here a quote from K&R2nd:
When an array name is passed to a function, what is passed is the
location of the initial element. Within the called function, this
argument is a local variable, and so an array name parameter is a
pointer, that is, a variable containing an address.
Writing:
void arraytest(int a[])
has the same meaning as writing:
void arraytest(int *a)
So despite you are not writing it explicitly it is as you are passing a pointer and so you are modifying the values in the main.
For more I really suggest reading this.
Moreover, you can find other answers on SO here
In C, except for a few special cases, an array reference always "decays" to a pointer to the first element of the array. Therefore, it isn't possible to pass an array "by value". An array in a function call will be passed to the function as a pointer, which is analogous to passing the array by reference.
EDIT: There are three such special cases where an array does not decay to a pointer to it's first element:
sizeof a is not the same as sizeof (&a[0]).
&a is not the same as &(&a[0]) (and not quite the same as &a[0]).
char b[] = "foo" is not the same as char b[] = &("foo").
Arrays are always passed by reference if you use a[] or *a:
int* printSquares(int a[], int size, int e[]) {
for(int i = 0; i < size; i++) {
e[i] = i * i;
}
return e;
}
int* printSquares(int *a, int size, int e[]) {
for(int i = 0; i < size; i++) {
e[i] = i * i;
}
return e;
}
An array can also be called as a decay pointer.
Usually when we put a variable name in the printf statement the value gets printed in case of an array it decays to the address of the first element, Therefore calling it as a decay pointer.
And we can only pass the decay pointer to a function.
Array as a formal parameter like Mr.Bo said int arr[] or int arr[10] is equivalent to the int *arr;
They will have there own 4 bytes of memory space and storing the decay pointer received.and we do pointer arithmetic on them.
I would like to make sure the value of the pointer myFunction() returns is available, when it's not an Obj-C object.
double * vectorComponents (); //Just an example
double * vectorComponents ()
{
double componentSet[] = {1, 2, 3};
return componentSet;
}
How can I dynamically allocate these variables an then how to dealloc them. If I don't do anything it won't work. Thanks everyone.
NSLog(#":)");
You can use the C standard library functions malloc() and free():
double *vectorComponents()
{
double *componentSet = malloc(sizeof(*componentSet) * 3);
componentSet[0] = 1;
componentSet[1] = 2;
componentSet[2] = 3;
return componentSet;
}
double *comps = vectorComponents();
// do something with them, then
free(comps);
(Documentation)
Also:
If I don't do anything it won't work.
Perhaps it's worth mentioning that it didn't work because it invokes undefined behavior. componentSet in your code was a local auto-array - it's invalidated at the end of its scope (i. e. it's deallocated at the time the function returns - exactly what you wanted not to happen.)
If you return a pointer that you dynamically allocate in the function then the caller will have ownership of the object and will be required to free the value.
/**
* Returns ownership, use free to release the value when done.
*/
double * vectorComponents()
{
double *componentSet = malloc(sizeof(double) * 3);
componentSet[0] = 1.0;
componentSet[1] = 2.0;
componentSet[2] = 3.0;
return componentSet;
}
void example()
{
double *components = vectorComponents();
//use components
free(components);
}
Given your example, first question is do you really need dynamic allocation? If you just want to return the address of an array initialized inside a function you can use a static variable:
double * vectorComponents ()
{
static double componentSet[] = {1, 2, 3};
return componentSet;
}
If you do need a dynamic array then there are many ways to do it. If you compute the array you can malloc() the storage to be free()'ed later. If you wish to initialize a dynamic array, then maybe change the values, and return it you can use a static array to do that. For example:
double * vectorComponents2 ()
{
static double componentSet[] = {1, 2, 3};
double *dynamic = malloc(sizeof(componentSet));
memcpy(dynamic, componentSet, sizeof(componentSet)); // copy values
// modify contents of dynamic here if needed
return dynamic;
}
Using memcpy and a static array is shorter than setting individual values and allows the contents and size of the array to be changed easily.
I'm getting confused by pointers in objective-c.
Basically I have a bunch of static data in my code.
static int dataSet0[2][2] = {{0, 1}, {2, 3}};
static int dataSet1[2][2] = {{4, 5}, {6, 7}};
And I want to have an array to index it all.
dataSets[0]; //Would give me dataSet0...
What should the type of dataSets be, and how would I initialize it?
While it's laid out the same in memory, a pointer to a multidimensional array is different than a pointer to a flat array. The compiler has to convert the [][] index to a flat array index for any multidimensional arrays. Can't mix the two or the distinction for the compiler is lost. You can either use all flat arrays:
static int dataSet00[2] = {0,1};
static int dataSet01[2] = {2,3};
static int * dataSet0[2] = {dataSet00, dataSet01};
static int dataSet10[2] = {4, 5};
static int dataSet11[2] = {6, 7};
static int * dataSet1[2] = {dataSet10, dataSet11};
static int ** dataSets[2] = {dataSet0, dataSet1};
or one big multidimensional array:
static int dataSets[2][2][2] = {{{0,1},{2,3}},{{4,5},{6,7}}};
but not a combination of the two unless you clue the compiler in by declaring a special datatype per Jon's suggestion.
Pointers to multi-dimensional arrays can be tricky. A typedef can help:
typedef int (*DataSetType)[2];
DataSetType dataSets[] = { dataSet0, dataSet1 /* and so on*/ };
You could use an NSPointerArray.
Your index array would be an array of pointers to pointers to int.
So the declaration would look like:
int ** dataset[numOfDataSets] = {dataSet0, dataSet1, ...}
Remember that objective-c is a proper superset of ansi-c, and this question in particular is about the c language, really.
Edit: It's important to remember that in C, arrays are essentially just pointers, and two-dimensional arrays are pointers to pointers.
Edit 2: I think act actually I muffed operator precedence. Should be:
int (** dataset)[numOfDataSets] = {dataSet0, dataSet1, ...}
I'm experimenting with Obj-C blocks and trying to have a struct with two blocks in it where one block is to change what the other block does.
this is a really roundabout way to do something simple... and there may be better ways to do it, but the point of the exercise is for me to understand blocks. here's the code , it doesn't work, so what am I missing/not understanding and/or doing wrong?
//enumerate my math operation options so i can have something more understandable
//than 0, 1, 2, etc... also makes it easier to add operations, as opTypeTotal
//will be 1 plus the index of the operation before it.
typedef enum
{
opTypeAdd = 0,
opTypeSubtract = 1,
opTypeTotal
} opType;
//not sure if (struct someMathStruct)* is correct, probably is wrong
//the intent is to pass a pointer to someMathStruct, but the compiler
//won't know about its existance until a few lines later...
typedef (void)(^changeBlock)(opType,(struct someMathStruct)*);
typedef (void)(^mathBlock)(int,int,int*);
//hold two blocks, to be defined later at runtime
typedef struct someMathStruct{
mathBlock doMath;
changeBlock changeOperation;
} SomeMath;
//i want to declare an array of blocks of type mathBlock
//the intent is to have the array index to correspond with the opTypes enumerated above
//almost certain i'm doing this wrong
mathBlock *m[opTypeTotal] = malloc(sizeof(mathBlock *)*opTypeTotal);
//just two simple math operations as blocks
m[opTypeAdd] = ^(void)(int a,int b,int *result){*result = a+b;};
m[opTypeSubtract] = ^(void)(int a,int b,int *result){*result = a-b;};
//this block is what's supposed to change the other block in the struct
//it takes an opType, and a pointer to the SomeMath struct
//is this the right way to access the member variables of the struct?
changeBlock changeMe = ^(void)(opType a, SomeMath *b) {
//should make adding operations as easy as just adding cases
switch (a)
{
case opTypeAdd: *b.doMath=m[a]; break;
case opTypeSubtract:
default: *b.doMath=m[a]; //catch-all goes to subtracting
}
}
...
SomeMath mathFun;
int theTotal = 0; //a test int to work with
//do i need to copy the changeMe block?
//or can i just do what i'm doing here as the block itself isn't unique
mathFun.changeOperation = changeMe;
mathFun->changeOperation(opTypeAdd, &mathFun);
mathFun->doMath(theTotal,11,&theTotal); //result should be 11
mathFun->changeOperation(opTypeSubtract, &mathFun);
mathFun->doMath(theTotal,3,&theTotal); //result should be 8
NSLog(#"the result: %d",theTotal); //should output "the result: 8"
The code seems to work as you expect (the result is 8) once you fix the compilation errors:
Compile with: gcc -o test test.m -framework Foundation
#import <Foundation/Foundation.h>
//enumerate my math operation options so i can have something more understandable
//than 0, 1, 2, etc... also makes it easier to add operations, as opTypeTotal
//will be 1 plus the index of the operation before it.
typedef enum
{
opTypeAdd = 0,
opTypeSubtract = 1,
opTypeTotal
} opType;
struct someMathStruct; // Forward declare this as a type so we can use it in the
// changeBlock typedef
typedef void (^changeBlock) (opType,struct someMathStruct*);
typedef void (^mathBlock) (int,int,int*);
//hold two blocks, to be defined later at runtime
typedef struct someMathStruct{
mathBlock doMath;
changeBlock changeOperation;
} SomeMath;
int main()
{
//i want to declare an array of blocks of type mathBlock
//the intent is to have the array index to correspond with the opTypes
// enumerated above
mathBlock *m = calloc(opTypeTotal, sizeof(mathBlock *));
//just two simple math operations as blocks
m[opTypeAdd] = ^(int a,int b,int *result){*result = a+b;};
m[opTypeSubtract] = ^(int a,int b,int *result){*result = a-b;};
changeBlock changeMe = ^(opType a, SomeMath *b) {
//should make adding operations as easy as just adding cases
switch (a)
{
case opTypeAdd: b->doMath = m[a]; break;
case opTypeSubtract:
default: b->doMath = m[a]; //catch-all goes to subtracting
}
};
SomeMath mathFun;
int theTotal = 0; //a test int to work with
mathFun.changeOperation = changeMe;
mathFun.changeOperation(opTypeAdd, &mathFun);
mathFun.doMath(theTotal,11,&theTotal); //result should be 11
mathFun.changeOperation(opTypeSubtract, &mathFun);
mathFun.doMath(theTotal,3,&theTotal); //result should be 8
NSLog(#"the result: %d",theTotal); //should output "the result: 8"
}