I have written the code below on Qt,when I put values in it it program.exe stops working.
struct aim
{
int i : 1;
int j : 1;
};
int main()
{
aim missed;
printf("Enter value of i :: ");
scanf("%u",missed.i);
printf("Enter value of j :: ");
scanf("%u",missed.j);
}
can anyone help me out with this problem?
There are a few problems with your code:
A 1-bit signed integer isn't very useful, it can only hold the values -1 and 0.
You can't have a pointer to a bit-field, that's not what pointers mean.
Also, there's nothing in the %d specifier that tells the scanf() function that the target value is a bit field (nor is there any other % specifier that can do this, see 2).
The solution is to scanf() to a temporary variable, range-check the received value, then store it in the bit field.
Because the C/C++ standard does not allow to access the members of a bitfield via a pointer and you have to pass scanf a pointer.
Related
I've got a bit of Arduino code that is driving me nuts at the moment and I'm hoping you helpful people can help.
What I want to do is initialise a pointer to one of two arrays of unsigned ints.
A switch is read each pass and the pointer is assigned accordingly, only the compiler has a hissy fit over the assignments.
unsigned int spl_add_tbl[4096]; // arrary table no zero crossing points.
unsigned int zxd_add_tbl[4096]; // array table for zero crossing points.
unsigned int *tbl_ptr; // init table pointer
// ZXD switch D64. H = off, L = on.
if (digitalRead(64) == 1) {
// Zero Crossing off, assign Table Pointer to sample table
*tbl_ptr = &spl_add_tbl[0];
} else {
// Zero Crossing on, assign Table pointer to Zero Crossing Table and set A/D read mapping.
*tbl_ptr = &zxd_add_tbl[0];
}
The compiler gives the error: invalid conversion from 'unsigned int*' to 'unsigned int' [-fpermissive]
tbl_ptr is a pointer - you should not dereference it in order to assign an array to it:
if(digitalRead(64) == 1)
{
// Zero Crossing off, assign Table Pointer to sample table
tbl_ptr = &spl_add_tbl[0];
}
else
{
// Zero Crossing on, assign Table pointer to Zero Crossing Table and set A/D read mapping.
tbl_ptr = &zxd_add_tbl[0];
}
There is an example of shuffle of OpenCL during the document.
//Examples that are not valid are:
uint8 mask;
short16 a;
short8 b;
b = shuffle(a, mask); // invalid
But I can not understand why. I test this during Android with AndroidStudio, and the result said:build program failed:BC-src-code:9:9:{9:9-9:16}: error: no matching builtin function for call to 'shuffle'. Then, I change the short to int, like this:
uint8 mask;
int16 a;
int8 b;
b = shuffle(a, mask);
and it is ok. I can not find any reason from the document, can anybody help me?
Thanks!
I think the critical part of the description in the spec is this:
The size of each element in the mask must match the size of each element in the result.
I take that to mean that if you want to shuffle a vector of shorts, your mask must be a vector of ushort; a mask of uint8 would only be valid for shuffling vectors with elements of 4 bytes - in other words, int, uint, and float.
So the following should be valid again:
ushort8 mask; // <-- changed
short16 a;
short8 b;
b = shuffle(a, mask); // now valid
I have got this code:
import std.stdio;
import std.string;
void main()
{
char [] str = "aaa".dup;
char [] *str_ptr;
writeln(str_ptr);
str_ptr = &str;
*(str_ptr[0].ptr) = 'f';
writeln(*str_ptr);
writeln(str_ptr[0][1]);
}
I thought that I am creating an array of pointers char [] *str_ptr so every single pointer will point to a single char. But it looks like str_ptr points to the start of the string str. I have to make a decision because if I am trying to give access to (for example) writeln(str_ptr[1]); I am getting a lot of information on console output. That means that I am linking to an element outside the boundary.
Could anybody explain if it's an array of pointers and if yes, how an array of pointers works in this case?
What you're trying to achieve is far more easily done: just index the char array itself. No need to go through explicit pointers.
import std.stdio;
import std.string;
void main()
{
char [] str = "aaa".dup;
str[0] = 'f';
writeln(str[0]); // str[x] points to individual char
writeln(str); // faa
}
An array in D already is a pointer on the inside - it consists of a pointer to its elements, and indexing it gets you to those individual elements. str[1] leads to the second char (remember, it starts at zero), exactly the same as *(str.ptr + 1). Indeed, the compiler generates that very code (though plus range bounds checking in D by default, so it aborts instead of giving you gibberish). The only note is that the array must access sequential elements in memory. This is T[] in D.
An array of pointers might be used if they all the pointers go to various places, that are not necessarily in sequence. Maybe you want the first pointer to go to the last element, and the second pointer to to the first element. Or perhaps they are all allocated elements, like pointers to objects. The correct syntax for this in D is T*[] - read from right to left, "an array of pointers to T".
A pointer to an array is pretty rare in D, it is T[]*, but you might use it when you need to update the length of some other array held by another function. For example
int[] arr;
int[]* ptr = &arr;
(*ptr) ~= 1;
assert(arr.length == 1);
If ptr wasn't a pointer, the arr length would not be updated:
int[] arr;
int[] ptr = arr;
ptr ~= 1;
assert(arr.length == 1); // NOPE! fails, arr is still empty
But pointers to arrays are about modifying the length of the array, or maybe pointing it to something entirely new and updating the original. It isn't necessary to share individual elements inside it.
I am making a simple scheduler that executes functions contained in a FIFO queue.
Those functions have a same return type int, but have different number of int arguments.
I tried to implement it this way, but it does not seem to work. The compiler forbids conversion between int(*)() , int(*)(int), int(*)(int, int), or to any of those sort. (Arduino Sketch compiler)
Is there a way to solve this problem, or could you recommend a better way around? Thanks!
My code:
typedef int (*fnptr)(); // Tried this!
int foo(int var) {
return 0;
}
int main() {
fnptr fp = &foo; // error: invalid conversion from
// 'int (*)(int)' to 'int (*)()'
// [-fpermissive]
return 0;
}
You can cast:
fnptr fp = reinterpret_cast<fnptr>(foo);
The ()s are the "function call operator", adding them makes no sense at all in this situation, it changes the expression from "take the address of this function" to "take the address of this function's return value".
Note that aboev I don't even include the &, this is because the name of a function acts pretty much like a function pointer so it's already an address.
I am doing one project in which I define a data types like below
typedef QVector<double> QFilterDataMap1D;
typedef QMap<double, QFilterDataMap1D> QFilterDataMap2D;
Then there is one class with the name of mono_data in which i have define this variable
QFilterMap2D valid_filters;
mono_data Scan_data // Class
Now i am reading one variable from a .mat file and trying to save it in to above "valid_filters" QMap.
Qt Code: Switch view
for(int i=0;i<1;i++)
{
for(int j=0;j<1;j++)
{
Scan_Data.valid_filters[i][j]=valid_filters[i][j];
printf("\nValid_filters=%f",Scan_Data.valid_filters[i][j]);
}
}
The transferring is done successfully but then it gives run-time error
Windows has triggered a breakpoint in SpectralDataCollector.exe.
This may be due to a corruption of the heap, and indicates a bug in
SpectralDataCollector.exe or any of the DLLs it has loaded.
The output window may have more diagnostic information
Can anyone help in solving this problem. It will be of great help to me.
Thanks
Different issues here:
1. Using double as key type for a QMap
Using a QMap<double, Foo> is a very bad idea. the reason is that this is a container that let you access a Foo given a double. For instance:
map[0.45] = foo1;
map[15.74] = foo2;
This is problematic, because then, to retrieve the data contained in map[key], you have to test if key is either equal, smaller or greater than other keys in the maps. In your case, the key is a double, and testing if two doubles are equals is not a "safe" operation.
2. Using an int as key while you defined it was double
Here:
Scan_Data.valid_filters[i][j]=valid_filters[i][j];
i is an integer, and you said it should be a double.
3. Your loop only test for (i,j) = (0,0)
Are you aware that
for(int i=0;i<1;i++)
{
for(int j=0;j<1;j++)
{
Scan_Data.valid_filters[i][j]=valid_filters[i][j];
printf("\nValid_filters=%f",Scan_Data.valid_filters[i][j]);
}
}
is equivalent to:
Scan_Data.valid_filters[0][0]=valid_filters[0][0];
printf("\nValid_filters=%f",Scan_Data.valid_filters[0][0]);
?
4. Accessing a vector with operator[] is not safe
When you do:
Scan_Data.valid_filters[i][j]
You in fact do:
QFilterDataMap1D & v = Scan_Data.valid_filters[i]; // call QMap::operator[](double)
double d = v[j]; // call QVector::operator[](int)
The first one is safe, and create the entry if it doesn't exist. The second one is not safe, the jth element in you vector must already exist otherwise it would crash.
Solution
It seems you in fact want a 2D array of double (i.e., a matrix). To do this, use:
typedef QVector<double> QFilterDataMap1D;
typedef QVector<QFilterDataMap1D> QFilterDataMap2D;
Then, when you want to transfer one in another, simply use:
Scan_Data.valid_filters = valid_filters;
Or if you want to do it yourself:
Scan_Data.valid_filters.clear();
for(int i=0;i<n;i++)
{
Scan_Data.valid_filters << QFilterDataMap1D();
for(int j=0;j<m;j++)
{
Scan_Data.valid_filters[i] << valid_filters[i][j];
printf("\nValid_filters=%f",Scan_Data.valid_filters[i][j]);
}
}
If you want a 3D matrix, you would use:
typedef QVector<QFilterDataMap2D> QFilterDataMap3D;