I need to implement my map with a vector. My map is layed out like:
map<strong,double> mapName;
I need to convert over to vector so that I can do a linear search through the elements.
Thanks in advance for the time.
You can easily convert to a vector using vector's range constructor like so:
map<string,double> item_map;
// ... populate item map ...
// copy elements to a vector.
vector< pair<string,double> > item_vector(item_map.begin(), item_map.end());
However, if you only need to do a linear search, you don't need to copy the elements. Just iterator over the items like so:
typedef map<string,double>::iterator iterator;
iterator current = item_map.begin();
const iterator end = item_map.end();
for (; current != end; ++current) {
// current->first is the 'string' part.
// current->second is the 'double' part.
}
You don't have to convert to vector to do a linear search. You can use C++ iterators to grab the beginning of a map and the end, then access the key and value with first and second.
See this
For example:
for (map<strong, double>::iterator ii = mapName.begin();
ii!=mapName.end();ii++) {
cout << ii->first << endl; //returns a key
cou << ii->second << endl; //returns ii->first's current corresponding value.
}
Related
With old-style loops, I can delve into a QJsonArray and, in the example below, add element "foo" with contents of existing element "bar" for each array item. How can I do this using C++11 range-based for?
// QJsonArray oldArray contains an array of which one element is "bar"
QJsonArray newArray;
int i, b = oldArray.count();
for (i=0; i<n; ++i) {
QJsonObject element = oldArray.at(i).toObject();
element["foo"] = element["bar"];
newArray.append(element);
}
I have tried the following (admittedly as trial and error):
auto&
for (auto& element : oldArray) {
element["foo"] = element["bar];
newArray.append(element);
}
I get the error
non-const lvalue reference to type 'QJsonValueRef' cannot bind to a temporary of type 'QJsonValueRef'
const auto&
for (const auto& element : oldArray) {
...
I get a warning
loop variable 'element' is always a copy because the range of type 'QJsonArray' does not return a reference
const auto
for (const auto element : oldArray) {
element["foo"] = element["bar];
...
I get the error
no viable overloaded operator[] for type 'const QJsonValueRef'
relating to element["bar"]
The problem is that the iterator for QJsonArray returns a temporary QJsonValueRef object, and lvalue references can not bind to temporaries. We can either hold that temporary by value:
// QJsonArray oldArray contains an array of which one element is "bar"
QJsonArray newArray;
for (auto v : oldArray) {
QJsonObject element = v.toObject();
element["foo"] = element["bar"];
newArray.append(element);
}
In this case v is a QJsonValueRef object (similar to what oldArray.at(i) gives in the old-style loop). After that, we convert that QJsonValueRef to a QJsonObject using .toObject().
Or we can use forwarding references since they can bind to rvalues:
for (auto&& v : oldArray) {
...
}
In this case v is deduced to be an rvalue reference to a QJsonValueRef.
Both solutions are identical in terms of the number of objects being created/destructed (since in the former, the copy is elided under C++17 guaranteed copy elision rules or even in pre-C++17 when using any decent compiler. In the latter, the reference is bound to the temporary and this prolongs its lifetime to match the whole iteration).
Notes
This problem is similar to what happens when using range-based-for with a std::vector<bool>; since both std::vector<bool> and QJsonArrayhave iterators that return proxy objects.
If using the Clang code model, the second solution generates the warning "loop variable 'v' is always a copy because the range of type 'QJsonArray' does not return a reference". See this question for an explanation.
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 trying to reverse the words within a QStringList. Below is the code up to now, but I keep on getting a "Index out of Range" error. From the error it would seem that I am trying to use data that is out of scope, but not able to figure out my problem.
QString reversed;
QStringList reversedlist;
QStringlist list = input.split(" ");
for (int i=0;i<list.length(); ++1)
reversedlist[i] = list[list.length() -1 -i];
reversed = reversedlist.join(" ");`
Thanks
As pointed out by #ThorngardSO, the reversedlist is initially empty, and you are trying to access the invalid index in your loop code. You should add values to the list using one of the following functions:
STL-compatible function push_back() (inserts value at the end of the list)
STL-compatible function push_front() (inserts value at the beginning of the list)
Qt function append() (Qt alternative for push_back)
Qt function prepend() (Qt alternative for push_front)
As you see, prepend() inserts the element at the beginning of the list, that is why it makes the reversing of the list very simple:
for (int i = 0; i < list.length(); ++i) {
reversedlist.prepend(list[i]);
}
Also, note that there is a typo in your loop: it should be ++i instead of ++1.
Your reversedList is initially empty. You have to actually append the items, like this:
reversedlist.push_back (list[list.length () - 1 - i]);
Naturally, trying to access non-existing items via reversedList[i] does not work and throws the index-out-out-range error.
You got the index out of range as there is no sting inside the QStringList reversedlist. So when your code reach the line reversedlist[0], it throw the "index out of range" error. And you can read the value using [0] and cant assign.
if you want to assign the value to a particular index of the QStringList.
QString reversed;
QStringList reversedlist;
QString input="123 456 789 000";
QStringList list = input.split(" ");
for (int i=0;i<list.length(); ++i){
//value to particular index
reversedlist.insert(i,list[list.length() -1 -i]);
}
reversed = reversedlist.join(" ");
qDebug() << reversed;
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;
hi a have a function that reads from a text file line by line each line I do some operations on it substitute a string..etc
then I push_back that line into a vector
this is my class in Parser.h
class Parser
{// start class
public:
vector<const char*> patterns;
Parser();
~Parser();
void RuleParser(const char *TextFileName); // this is the function that takes the file name
private:
};// end class
segment from function RuleParser
std::ifstream ifs(TextFileName);
while (!ifs.eof())
{
.
.modification code
.
patterns.push_back((buildString).c_str()); //buildString is the modified line
cout << buildString << endl;
}
but when I try to check out if the data in the vector is correct it output totally different data.
I even put a cout after the push_back to check it's integrty but I found buildString is correct... thats the data each time being pushed ... what I am doing wrong.
here is the loop I use to see if my data correct.
for (int i = 0;i < patterns.size() ;i++)
{
cout << patterns.at(i) << endl;
}
Well patterns is the collection of pointers so you end up push_back'ing a pointer to the same buildString in each iteration of the loop, instead of push_back'ing the string contents. Then when buildString changes in next iteration of the loop, the pointer becomes invalid but it still remains in patterns - not good
I suggest you declare patterns as:
vector<std::string> patterns;
This way when you do:
patterns.push_back(buildString.c_str())
the contents of the string will be copied instead of the pointer, and remain valid througout.