I am almost done with my code except I need help on two thing. Here is my code: Code. For the function below, I am trying to make it so that I can use the input of "n" to initialize my array, myBits, instead of a constant, which is currently 5.
My Other question is right below that. I am trying to switch all of the right most bits to "true". I wrote the for loop in "/* .....*/" but it doesn't seem to be working. Right above it, I do it long ways for C(5,4) ....(myBit[0] = myBit[1]....etc...... (I am using this to find r-combinations of strings).... and it seems to work. Any help would be appreciated!!
void nCombination(const vector<string> &Vect, int n, int r){
bool myBits[5] = { false }; // everything is false now
myBits[1] = myBits[2] = myBits[3] = myBits[4] = true;
/* for(int b = n - r - 1; b = n - 1; b++){
myBits[b] = true; // I am trying to set the r rightmost bits to true
}
*/
do // start combination generator
{
printVector(Vect, myBits, n);
} while (next_permutation(myBits, myBits + n)); // change the bit pattern
}
These are called variable length arrays (or VLAs for short) and they are not a feature of standard C++. This is because we already have arrays that can change their length how ever they want: std::vector. Use that instead of an array and it will work.
Use std::vector<bool>:
std::vector<bool> myBits(n, false);
Then you have to change your while statement:
while (next_permutation(myBits.begin(), myBits.end()));
You will also have to change your printVector function to take a vector<bool>& as the second argument (you won't need the last argument, n, since a vector knows its own size by utilizing the vector::size() function).
As to your program: If you're attempting to get the combination of n things taken r at a time, you will need to write a loop that initializes the last right r bools to true instead of hard-coding the rightmost 4 entries.
int count = 1;
for (size_t i = n-1; i >= 0 && count <= r; --i, ++count)
myBits[i] = true;
Also, you should return immediately from the function if r is 0.
Related
I have an array of values e.g. 1, 4, 7, 2.
I also have another array of values and I want to add its values to this first array, but only when they all are different from all values that are already in this array. How can I check it? I've tried many types of loops, but I always ended with an iteration problem.
Could you please tell me how to solve this problem? I code in c++.
int array1[7] = {2,3,7,1,0};
int val1 = rand() % 10;
int val2 = rand() % 10;
int array2[2] = {val1, val2};
and I am trying to put every value from array2 into array1. I tried loop
for (int x:array2)
{
while((val1 && val2) == x)
{
val1 = rand() % 10;
val2 = rand() % 10;
}
}
and many more, but still cannot figure it out. I have this problem because I may have various number of elements for array2. So it makes this "&&" solution infinite.
It is just a sample to show it more clearly, my code has much more lines.
Okay, you have a few problems here. If I understand the problem, here's what you want:
A. You have array1 already populated with several values but with space at the end.
1. How do you identify the number of entries in the array already versus the extras?
B. You have a second array you made from two random values. No problem.
You want to append the values from B to A.
2. If initial length of A plus initial length of B is greater than total space allocated for A, you have a new problem.
Now, other people will tell you to use the standard template library, but if you're having problems at this level, you should know how to do this yourself without the extra help from a confusing library. So this is one solution.
class MyArray {
public:
int * data;
int count;
int allocated;
MyArray() : data(nullptr), count(0), allocated(0) {}
~MyArray() { if (data != nullptr) free(data); }
// Appends value to the list, making more space if necessary
void add(int value) {
if (count >= allocated) {
// Not enough space, so make some.
allocated += 10;
data = (data == nullptr) malloc(allocated * sizeof(int))
: realloc)data, allocated * sizeof(int));
}
data[count++] = value;
}
// Adds value only if not already present.
void addUnique(int value) {
if (indexOf(value) < 0) {
add(value);
}
}
// Returns the index of the value, if found, else -1
int indexOf(int value) {
for (int index = 0; index < count; ++index) {
if (data[index] == value) {
return index;
}
}
return -1;
}
}
This class provides you a dynamic array of integers. It's REALLY basic, but it teaches you the basics. It helps you understand about allocation / reallocating space using old-style C-style malloc/realloc/free. It's the sort of code I was writing back in the 80s.
Now, your main code:
MyArray array;
array.add(2);
array.add(3);
array.add(7);
// etc. Yes, you could write a better initializer, but this is easy to understand
MyArray newValues;
newValues.add(rand() % 10);
newValues.add(rand() % 10);
for (int index = 0; index < newValues.count; ++index) {
array.addUnique(newValues.data[index]);
}
Done.
The key part of this is the addUnique function, which simply checks first whether the value you're adding already is in the array. If not, it appends the value to the array and keeps track of the new count.
Ultimately, when using integer arrays like this instead of the fancier classes available in C++, you HAVE TO keep track of the size of the array yourself. There is no magic .length method on int[]. You can use some magic value that indicates the end of the list, if you want. Or you can do what I did and keep two values, one that holds the current length and one that holds the amount of space you've allocated.
With programming, there are always multiple ways to do this.
Now, this is a lot of code. Using standard libraries, you can reduce all of this to about 4 or 5 lines of code. But you're not ready for that, and you need to understand what's going on under the hood. Don't use the fancy libraries until you can do it manually. That's my belief.
I need to put the first value of a loop = 0, and then use a range to start the loop.
In MatLab this is possible : x = [0 -range:range] (range is a integer)
This will give a value of [0, -range, -range+1, -range+2, .... , range-1, range]
The problem is I need to do this in C++, I tried to do by an array and then put in like the value on the loop without success.
//After loading 2 images, put it into matrix values and then trying to compare each one.
for r=1:bRows
for c=1:bCols
rb=r*blockSize;
cb=c*blockSize;
%%for each block search in the near position(1.5 block size)
search=blockSize*1.5;
for dr= [0 -search:search] //Here's the problem.
for dc= [0 -search:search]
%%check if it is inside the image
if(rb+dr-blockSize+1>0 && rb+dr<=rows && cb+dc-blockSize+1>0 && cb+dc<=cols)
%compute the error and check if it is lower then the previous or not
block=I1(rb+dr-blockSize+1:rb+dr,cb+dc-blockSize+1:cb+dc,1);
TE=sum( sum( abs( block - cell2mat(B2(r,c)) ) ) );
if(TE<E)
M(r,c,:)=[dr dc]; %store the motion vector
Err(r,c,:)=TE; %store th error
E=TE;
end
end
end
end
%reset the error for the next search
E=255*blockSize^2;
end
end
C++ doesn't natively support ranges of the kind you know from MatLab, although external solutions are available, if somewhat of an overkill for your use case. However, C++ allows you to implement them easily (and efficiently) using the primitives provided by the language, such as for loops and resizable arrays. For example:
// Return a vector consisting of
// {0, -limit, -limit+1, ..., limit-1, limit}.
std::vector<int> build_range0(int limit)
{
std::vector<int> ret{0};
for (auto i = -limit; i <= limit; i++)
ret.push_back(i);
return ret;
}
The resulting vector can be easily used for iteration:
for (int dr: build_range0(search)) {
for (int dc: build_range0(search)) {
if (rb + dr - blockSize + 1 > 0 && ...)
...
}
}
The above of course wastes some space to create a temporary vector, only to throw it away (which I suspect happens in your MatLab example as well). If you want to just iterate over the values, you will need to incorporate the loop such as the one in build_range0 directly in your function. This has the potential to reduce readability and introduce repetition. To keep the code maintainable, you can abstract the loop into a generic function that accepts a callback with the loop body:
// Call fn(0), fn(-limit), fn(-limit+1), ..., fn(limit-1), and fn(limit)
template<typename F>
void for_range0(int limit, F fn) {
fn(0);
for (auto i = -limit; i <= limit; i++)
fn(i);
}
The above function can be used to implement iteration by providing the loop body as an anonymous function:
for_range0(search, [&](int dr) {
for_range0(search, [&](int dc) {
if (rb + dr - blockSize + 1 > 0 && ...)
...
});
});
(Note that both anonymous functions capture enclosing variables by reference in order to be able to mutate them.)
Reading your comment, you could do something like this
for (int i = 0, bool zero = false; i < 5; i++)
{
cout << "hi" << endl;
if (zero)
{
i = 3;
zero = false;
}
}
This would start at it 0, then after doing what I want it to do, assign i the value 3, and then continue adding to it each iteration.
newbie here. Even newer to recursion. I'm writing a function for my C++ program, and as you'll be able to tell, I'm a bit clueless when it comes to recursive algorithms. I'd appreciate it greatly if someone could fix my function so I can get it working and perhaps have a better idea how to handle recursion afterward.
My function takes a two-dimensional square array of booleans, and integer i, and an integer array_size as parameters. The function returns a boolean value.
The array is an adjacency matrix that I use to represent a set of conditionals. For example, if the value at [0][3] is true, then 0 -> 3 (if 0, then 3). If [3][7] is true, then 3 -> 7 (if 3, then 7). By the transitive property, 0 -> 7 (if 0, then 7).
The integer i is a particular element in the set of conditionals. The function will return true if this element is transitively connected to the last element in the array. The last element in the array is the integer (array_size - 1),
The integer array_size is the size of each dimension of the square array. If array_size is 20, then the array is 20x20.
The idea of this function is to determine if there is any logical "path" from the first integer element to the last integer element by the transitive property. When the path exists, the function returns true, otherwise, it returns false. The recursive call should allow it to traverse all possible paths, returning true once it finally reaches the last element and false if all paths fail.
For example, if i = 0 and array_size = 10, then the function will return whether or not 0 -> 9 is valid according to the conditionals provided by the matrix and the transitive property.
This is my code so far:
bool checkTransitivity(bool **relations, int i, int array_size){
bool isTransitive = false;
if (i == array_size - 1)
{
isTransitive = true;
}
else
{
for (int j = i; j < array_size; j++){
if (relations[i][j])
{
isTransitive = checkTransitivity(relations, j, array_size);
}
}
}
return isTransitive;
Currently, the function returns true for all input.
Any help at all is appreciated. Thanks in advance!
EDIT: This first part is unnecessary because of your if-else statement. Move on to END OF EDIT.
Let's start with what a base case in a recursive function is:
if (i == array_size - 1)
{
isTransitive = true;
}
Well you do have a base case, but nothing is being returned. You are just setting a flag to true. What you want to do is:
if (i == array_size - 1) {
return true;
}
Now the function will work its way up the recursive stack to return true. END OF EDIT.
But we still need to fix the recursive case:
else {
for (int j = i; j < array_size; j++) {
if (relations[i][j]) {
isTransitive = isTransitive || checkTransitivity(relations, j, array_size);
}
}
}
return isTransitive;
The || means binary OR. So you have the logic right. You want to check each possible path to see if it can get there, but by setting isTransitive to the result of each check, isTransitive is only going to be set to the last call. By doing isTransitive = isTransitive || recursive call, isTransitive will be true as long as one of the calls results in a true value.
The last thing I want to say is a caution: if relations[i][j] == true and relations[j][i] == true, your code will still be in an infinite loop. You must find a way to eliminate the potential backtracking. One way to do this is to create another array that stores which paths you have already checked so you do not infinitely loop.
More information can be found here: Depth First Search
I think all you need is a break condition to stop continuing the loop when you encounter a non-transitive item. See below (haven't tested)
bool checkTransitivity(bool **relations, int i, int array_size){
bool isTransitive = false;
if (i == array_size - 1)
{
isTransitive = true;
}
else
{
for (int j = i; j < array_size; j++){
isTransitive = relations[i][j] && checkTransitivity(relations, j, array_size);
if (!isTransitive)
break;
}
}
return isTransitive;
}
I'm making a C++ game which requires me to initialize 36 numbers into a vector. You can't initialize a vector with an initializer list, so I've created a while loop to initialize it faster. I want to make it push back 4 of each number from 2 to 10, so I'm using an int named fourth to check if the number of the loop is a multiple of 4. If it is, it changes the number pushed back to the next number up. When I run it, though, I get SIGABRT. It must be a problem with fourth, though, because when I took it out, it didn't give the signal.
Here's the program:
for (int i; i < 36;) {
int fourth = 0;
fourth++;
fourth%=4;
vec.push_back(i);
if (fourth == 0) {
i++;
}
}
Please help!
You do not initialize i. Use for (int i = 0; i<36;). Also, a new variable forth is allocated on each iteration of the loop body. Thus the test fourth==0 will always yield false.
I want to make it push back 4 of each number from 2 to 10
I would use the most straight forward approach:
for (int value = 2; value <= 10; ++value)
{
for (int count = 0; count < 4; ++count)
{
vec.push_back(value);
}
}
The only optimization I would do is making sure that the capacity of the vector is sufficient before entering the loop. I would leave other optimizations to the compiler. My guess is, what you gain by omitting the inner loop, you lose by frequent modulo division.
You did not initialize i, and you are resetting fourth in every iteration. Also, with your for loop condition, I do not think it will do what you want.
I think this should work:
int fourth = 0;
for (int i = 2; i<=10;) {
fourth++;
fourth%=4;
vec.push_back(i);
if (fourth==0) {
i++;
}
}
I've been able to create a static array declaration and pass that array into the vector at initialization without issue. Pretty clean too:
const int initialValues[36] = {0,1,2...,35};
std::vector foo(initialValues);
Works with constants, but haven't tried it with non const arrays.
Hey all, I'm trying to write a sort function but am having trouble figuring out how to initialize a value, and making this function work as a generic template. The sort works by:
Find a pair =(ii,jj)= with a minimum value = ii+jj = such at A[ii]>A[jj]
If such a pair exists, then
swap A[ii] and A[jj] else
break;
The function I have written is as follows:
template <typename T>
void sort(T *A, int size)
{
T min =453;
T temp=0;
bool swapper = false;
int index1 = 0, index2 = 0;
for (int ii = 0; ii < size-1; ii++){
for (int jj = ii + 1; jj < size; jj++){
if((min >= (A[ii]+A[jj])) && (A[ii] > A[jj])){
min = (A[ii]+A[jj]);
index1 = ii;
index2 = jj;
swapper = true;
}
}
}
if (!swapper)
return;
else
{
temp = A[index1];
A[index1] = A[index2];
A[index2] = temp;
sort(A,size);
}
}
This function will successfully sort an array of integers, but not an array of chars. I do not know how to properly initialize the min value for the start of the comparison. I tried initializing the value by simply adding the first two elements of the array together (min = A[0] + A[1]), but it looks to me like for this algorithm it will fail. I know this is sort of a strange type of sort, but it is practice for a test, so thanks for any input.
most likely reason it fails, is because char = 453 does not produce 453 but rather different number, depending what char is (signed versus unsigned). your immediate solution would be to use numerical_limits, http://www.cplusplus.com/reference/std/limits/numeric_limits/
you may also need to think about design, because char has small range, you are likely to overflow often when adding two chars.
The maximum value of any type is std::numeric_limits<T>::max(). It's defined in <limits>.
Also, consider a redesign. This is not a good algorithm. And I would make sure I knew what I was doing before calling my sort function recursively.
I haven't put too much time reading your algorithm, but as an alternative to std::numeric_limits, you can use the initial element in your array as the initial minimum value. Then you don't have to worry about what happens if you call the function with a class that doesn't specialize std::numeric_limits, and thus can't report a maximum value.