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.
Related
I'm writing a smart contract and want to use Arrays to manipulate data, but looking at the AssemblyScript docs, I'm not sure the best way to proceed.
It seems fine to me to just use:
let testData:string[] = []
but when I consulted the assemblyscript docs, there are three recommended ways to create an Array:
// The Array constructor implicitly sets `.length = 10`, leading to an array of
// ten times `null` not matching the value type `string`. So, this will error:
var arr = new Array<string>(10);
// arr.length == 10 -> ERROR
// To account for this, the .create method has been introduced that initializes
// the backing capacity normally but leaves `.length = 0`. So, this will work:
var arr = Array.create<string>(10);
// arr.length == 0 -> OK
// When pushing to the latter array or subsequently inserting elements into it,
// .length will automatically grow just like one would expect, with the backing
// buffer already properly sized (no resize will occur). So, this is fine:
for (let i = 0; i < 10; ++i) arr[i] = "notnull";
// arr.length == 10 -> OK
When would I want to use one type of instantiation over another? Why wouldn't I just always use the version I presented in the beginning?
Nothing wrong with the array literal approach. It is basically equivalent to
let testData = new Array<string>();
However, sometimes you know what the length of the array should be and in such cases, preallocating the memory using Array.create is more efficient.
UPDATE
With this PR Array.create deprecated and should not be used anymore.
OLD ANSWER
let testData:string[] = []
semantically the same as
let testData = new Array<string>()
AssemblyScript doesn't support preallocated sparse arrays (arrays with holes) for reference elements which not explicitly declared as nullable like:
let data = new Array<string>(10);
data[9] = 1; // will be compile error
Instead you could use:
let data = new Array<string | null>(10);
assert(data.length == 10); // ok
assert(data[0] === null); // ok
or Array.create but in this case your length will be zero. Array.create is actually just reserve capacity for backing buffer.
let data = Array.create<string>(10);
assert(data.length == 0); // true
For plain (non-reference) types you could use usual way without care about nullabe or creating array via Array.create:
let data = new Array<i32>(10);
assert(data.length == 10); // ok
assert(data[0] == 0); // ok
In my program, I store objective-c objects in a c array, like this
va_start(list, o);
retval->objs = malloc(SIZE * count);
retval->objs[0] = (__bridge void *)o;
for (int i = 1; i < count; i++)
{
id o = va_arg(list, id);
retval->objs[i] = (__bridge void *)o;
}
va_end(list);
(count is a number containing how many objects will be added; that value is always correct)
objs is a void ** and is part of retval, which is a pointer to a struct. As of now, SIZE is defined as 100. Increasing and decreasing that had no effect.
As you can see, I bridge o to a void *, as I have to. objs, when all the objects are added, contains 3 objective-c objects. When I try to access a value like this
void *obj = CLArrayObjectAtIndex(_arr, ind);
return (__bridge id)obj;
this is the CLArrayObjectAtIndex() function
void *CLArrayObjectAtIndex(CLArrayType *arr, int ind)
{
void *o = arr->objs[ind];
if (o)
return o;
else
perror("Attempt to access NULL object or index out of bounds."), abort();
}
if the index (ind) is 0, it works. If the index is 1, the program crashes when it returns in main. If the index is 2, the program crashes as soon as I try to access it. If the index is 1, the value returned above is correct, but when the program crashes on return it is nil.
If the index is 1, the EXC_BAD_ACCESS code is 1; if the index is 2, the code is EXC_I386_GPFLT, a general protection fault. I already checked here for an explanation of this exception, although I couldn't find anything helpful. So, does anybody see why this error may be occurring?
when you store obj-c objects in C array don't just bridge cast them since that way arc doesn't know they are still used and releases them. __bridge_retain them so they stay around later, when you free the array __bridge_transfer them to give them back to ARC
also don't define size as 100.. sizeof(id) should work. You only need to store pointers
I'm using a for-loop to determine whether the long double is an int. I have it set up that the for loop loops another long double that is between 2 and final^1/2. Final is a loop I have set up that is basically 2 to the power of 2-10 minus 1. I am then checking if final is an integer. My question is how can I get only the final values that are integers?
My explanation may have been a bit confusing so here is my entire loop code. BTW I am using long doubles because I plan on increasing these numbers very largely.
for (long double ld = 1; ld<10; ld++) {
long double final = powl(2, ld) - 1;
//Would return e.g. 1, 3, 7, 15, 31, 63...etc.
for (long double pD = 2; pD <= powl(final, 0.5); pD++) {
//Create new long double
long double newFinal = final / pD;
//Check if new long double is int
long int intPart = (long int)newFinal;
long double newLong = newFinal - intPart;
if (newLong == 0) {
NSLog(#"Integer");
//Return only the final ints?
}
}
}
Just cast it to an int and subtract it from itself?
long double d;
//assign a value to d
int i = (int)d;
if((double)(d - i) == 0) {
//d has no fractional part
}
As a note... because of the way floating point math works in programming, this == check isn't necessarily the best thing to do. Better would be to decide on a certain level of tolerance, and check whether d was within that tolerance.
For example:
if(fabs((double)(d - i)) < 0.000001) {
//d's fractional part is close enough to 0 for your purposes
}
You can also use long long int and long double to accomplish the same thing. Just be sure you're using the right absolute value function for whatever type you're using:
fabsf(float)
fabs(double)
fabsl(long double)
EDIT... Based on clarification of the actual problem... it seems you're just trying to figure out how to return a collection from a method.
-(NSMutableArray*)yourMethodName {
NSMutableArray *retnArr = [NSMutableArray array];
for(/*some loop logic*/) {
// logic to determine if the number is an int
if(/*number is an int*/) {
[retnArr addObject:[NSNumber numberWithInt:/*current number*/]];
}
}
return retnArr;
}
Stick your logic into this method. Once you've found a number you want to return, stick it into the array using the [retnArr addObject:[NSNumber numberWithInt:]]; method I put up there.
Once you've returned the array, access the numbers like this:
[[arrReturnedFromMethod objectAtIndex:someIndex] intValue];
Optionally, you might want to throw them into the NSNumber object as different types.
You can also use:
[NSNumber numberWithDouble:]
[NSNumber numberWithLongLong:]
And there are matching getters (doubleValue,longLongValue) to extract the number. There are lots of other methods for NSNumber, but these seem the most likely you'd want to be using.
When declaring a block what's the rationale behind using this syntax (i.e. surrounding brackets and caret on the left)?
(^myBlock)
For example:
int (^myBlock)(int) = ^(int num) {
return num * multiplier;
};
C BLOCKS: Syntax and Usage
Variables pointing to blocks take on the exact same syntax as variables pointing to functions, except * is substituted for ^. For example, this is a function pointer to a function taking an int and returning a float:
float (*myfuncptr)(int);
and this is a block pointer to a block taking an int and returning a float:
float (^myblockptr)(int);
As with function pointers, you'll likely want to typedef those types, as it can get relatively hairy otherwise. For example, a pointer to a block returning a block taking a block would be something like void (^(^myblockptr)(void (^)()))();, which is nigh impossible to read. A simple typedef later, and it's much simpler:
typedef void (^Block)();
Block (^myblockptr)(Block);
Declaring blocks themselves is where we get into the unknown, as it doesn't really look like C, although they resemble function declarations. Let's start with the basics:
myvar1 = ^ returntype (type arg1, type arg2, and so on) {
block contents;
like in a function;
return returnvalue;
};
This defines a block literal (from after = to and including }), explicitly mentions its return type, an argument list, the block body, a return statement, and assigns this literal to the variable myvar1.
A literal is a value that can be built at compile-time. An integer literal (The 3 in int a = 3;) and a string literal (The "foobar" in const char *b = "foobar";) are other examples of literals. The fact that a block declaration is a literal is important later when we get into memory management.
Finding a return statement in a block like this is vexing to some. Does it return from the enclosing function, you may ask? No, it returns a value that can be used by the caller of the block. See 'Calling blocks'. Note: If the block has multiple return statements, they must return the same type.
Finally, some parts of a block declaration are optional. These are:
The argument list. If the block takes no arguments, the argument list can be skipped entirely.
Examples:
myblock1 = ^ int (void) { return 3; }; // may be written as:
myblock2 = ^ int { return 3; }
The return type. If the block has no return statement, void is assumed. If the block has a return statement, the return type is inferred from it. This means you can almost always just skip the return type from the declaration, except in cases where it might be ambiguous.
Examples:
myblock3 = ^ void { printf("Hello.\n"); }; // may be written as:
myblock4 = ^ { printf("Hello.\n"); };
// Both succeed ONLY if myblock5 and myblock6 are of type int(^)(void)
myblock5 = ^ int { return 3; }; // can be written as:
myblock6 = ^ { return 3; };
source: http://thirdcog.eu/pwcblocks/
I think the rationale is that it looks like a function pointer:
void (*foo)(int);
Which should be familiar to any C programmer.
I have a function that returns a variable and I want to know how to return an array the issue is it isn't an NSArray it is just an average C array like this...
-(b2Fixture*) addFixturesToBody:(b2Body*)body forShapeName:(NSString*)shape
{
BodyDef *so = [shapeObjects objectForKey:shape];
assert(so);
FixtureDef *fix = so->fixtures;
int count = -1;
b2Fixture *Fixi[4];
while(fix)
{
count++;
NSLog(#"count = %d",count);
Fixi[count]= body->CreateFixture(&fix->fixture);
if (Fixi[count]!=0) {
NSLog(#"Fixi %d is not 0",count);
}
if (body->CreateFixture(&fix->fixture)!=0) {
NSLog(#"body %d is not 0",count);
}
fix = fix->next;
}
return *Fixi;
}
If you see some variable types you don't know it's because I'm using cocos2d framework to make a game but I'm returning a variable of b2Fixture... This code compiles however only saves the value of the first block of the array "fixi[0]" not the whole array like I want to pass
anyhelp :) thankyou
You can't return a local array. You'll need to do some kind of dynamic allocation or pull a trick like having the array inside a structure.
Here is a link to an in-depth article that should help you out.
In general returning C arrays by value is a bad idea, as arrays can be very large. Objective-C arrays are by-reference types - they are dynamically allocated and a reference, which is small, is what is passed around. You can dynamically allocate C arrays as well, using one of the malloc family for allocation and free for deallocation.
You can pass C structures around by value, and this is common, as in general structures tend to be small (or smallish anyway).
Now in your case you are using a small array, it has just 4 elements. If you consider passing these 4 values around by value is reasonable and a good fit for your design then you can do so simply by embedding the C array in a C structure:
typedef struct
{
b2Fixture *elements[4];
} b2FixtureArray;
...
-(b2FixtureArray) addFixturesToBody:(b2Body*)body forShapeName:(NSString*)shape
{
BodyDef *so = [shapeObjects objectForKey:shape];
assert(so);
FixtureDef *fix = so->fixtures;
int count = -1;
b2FixtureArray Fixi;
while(fix)
{
count++;
NSLog(#"count = %d", count);
Fixi.elements[count]= body->CreateFixture(&fix->fixture);
if (Fixi.elements[count] != 0)
{
NSLog(#"Fixi %d is not 0",count);
}
if (body->CreateFixture(&fix->fixture) != 0)
{
NSLog(#"body %d is not 0", count);
}
fix = fix->next;
}
return Fixi;
}
...
// sample call outline
b2FixtureArray result = [self addFixturesToBody...]
Whether this standard C "trick" for passing arrays by value is appropriate for your case you'll have to decide.
Note: If b2fixture is an Objective-C object make sure you understand the memory management implications of having a C array of objects references depending on the memory management model (MRC, ARC, GC) you are using.
If you need to design function or method that has to return a fixed or limited size array, one possibility is to pass a pointer to the result array to the function or method as a parameter. Then the caller can take care of allocating space, or just use a local or instance variable array. You might want the called function to sanity check that the array parameter isn't NULL before using the array.