I was trying this problem on leetcode,
https://leetcode.com/problems/naming-a-company/description/ .
I've observed the following
My Code :
long long distinctNames(vector<string>& ideas) {
unordered_map<string,bool> isPresent;
vector<vector<long long>> dp(26,vector<long long>(26,0));
int n = ideas.size();
long long ans = 0;
for(int i = 0; i < n; i++)
isPresent[ideas[i]] = true;
for(int i = 0; i < n; i++)
{
char x = ideas[i][0];
string ts = ideas[i];
for(int j = 0; j < 26; j++)
{
char y = 'a' + j;
ts[0] = y;
if(!isPresent[ts])
dp[x-'a'][j]++;
}
}
for(int i = 0; i < 26; i++)
{
for(int j = 0; j < 26; j++)
{
if(i==j) continue;
ans += (dp[i][j] * dp[j][i]);
}
}
return ans;
}
This code was getting TLE (85/89).
But, in the same code, if I replace
!isPresent[ts]
with
!isPresent.count(ts)
Same code runs much faster and passes.
Anyone can explain why ?
isPresent[ts] returns a reference to a map value object (so you can write isPresent[ts] = something. So if ts is not present in the map, then isPresent[ts] must default construct a map entry so that it has something to return a reference to. This is the reason that map::operator[] is not const.
isPresent.count(ts) has no such problems. If the ts key is not present then the map is unchanged.
operator[] on std::unordered_map inserts a default constructed element if none is yet present (which is also why the operator doesn't work on a const object.) Insertion potentially requires allocating memory or other somewhat slow things. Calling count does not, nor would e.g. find.
Related
I want to create a string array and then after writing lines into it I want to change one exact character into int. I already know that all the characters are going to be numbers. As my goal is to change the one character at a time, options like atoi, stoi etc. are perhaps off? The closest I got is that:
#include <iostream>
int main()
{
int n=0,suma=0,i=0;
int multiplier[11]={1,3,7,9,1,3,7,9,1,3,1};
std::cin>>n;
std::string str[n];
for (int i = 0; i < n; ++i)
{
std::cin>>str[i];
}
for (int i = 0; i < n; ++i)
{
for (int j = 0; j < 11; ++j)
{
i = str[i][j] - '0';
std::cout << i;
}
}
}
Although this is the output I get
"1-48"
I know that the string is going to be 11 characters long. Any ideas?
EDIT: It was a single typo that caused my confuse :p Yet still I'm looking forward to read and learn from your suggestions such as using different way to read n (from user input) strings. :)
In your loop:
for (int i = 0; i < n; ++i)
{
for (int j = 0; j < 11; ++j)
{
i = str[i][j] - '0';
std::cout << i;
}
}
you are modifying outer loop variable i (looks like for the purpose of printing a value).
Given an unfortunate input, you would go out-of-bounds fast.
The int winner should be set to 2 under certain conditions but it's somehow being set to a variety of higher values, most commonly 6. I have no idea how this is happening, as there is no other function in my class that affects winner, and the variable isn't even mentioned anywhere else in the program. What is most confusing to me is that I have an almost identical function (P2Move()) that is literally identical in how it sets the winner variable to P1Move(), and that function runs perfectly.
Some info: The class this is part of is called Board, which acts as a checkerboard array made up of Square class objects.
Below is the function causing the problem. Near the bottom, the statement else if((canTake.size()==0)&&(canMove.size()==0)) {Board::winner = 2;} causes the problem. Everything else seems to work when I remove the problematic part from the function, but I need that part to work in order to submit the final project.
void Board::P1Move()
{
P1pieces = 0;
std::vector <Move> canMove;
std::vector <Move> canTake;
for(int j = 0; j < bSize; j++)
{ //Start of j loop.
for(int i = 0; i < bSize; i++)
{ //Start of i loop.
Square sq = board[i][j];
bool cTakeL = canTakeL(i,j);
bool cTakeR = canTakeR(i,j);
bool cMoveL = canMoveL(i,j);
bool cMoveR = canMoveR(i,j);
if(board[i][j].getPl() == P1)
{
P1pieces++;
if(cTakeL)
{
Move a = Move(sq.getIndex(),board[i-2][j+2].getIndex(),board[i-1][j+1].getIndex(),0);
canTake.push_back(a);
}
if(cTakeR)
{
Move b = Move(sq.getIndex(),board[i+2][j+2].getIndex(),board[i+1][j+1].getIndex(),0);
canTake.push_back(b);
}
if(cMoveL)
{
Move c = Move(sq.getIndex(),board[i-1][j+1].getIndex(),0,0);
canMove.push_back(c);
}
if(cMoveR)
{
Move d = Move(sq.getIndex(),board[i+1][j+1].getIndex(),0,0);
setWinner(d.getSpos());
canMove.push_back(d);
}
}
} //End of i loop.
} //End of j loop.
if(canTake.size()!=0)
{
time_t t;
time(&t);
srand(t);
int moveNum = rand()%canTake.size();
std::string output = "p1 ";
Move out = canTake.at(moveNum);
int i = 0;
int j = 0;
for(int y = 0; y < bSize; y++)
{
for(int x = 0; x < bSize; x++)
{
if(board[x][y].getIndex()==out.getSpos())
{
i = x;
j = y;
}
}
}
if(board[i-2][j+2].getIndex()==out.getEndPos())
{
board[i-2][j+2].setOcc(true);
board[i-2][j+2].setPl(P1);
board[i-1][j+1].setOcc(false);
board[i-1][j+1].setPl(NA);
}
else if(board[i+2][j+2].getIndex()==out.getEndPos())
{
board[i+2][j+2].setOcc(true);
board[i+2][j+2].setPl(P1);
board[i+1][j+1].setOcc(false);
board[i+1][j+1].setPl(NA);
}
output = output + out.toString();
setCmove(output);
board[i][j].setOcc(false);
board[i][j].setPl(NA);
}
else if(canMove.size()!=0)
{
time_t t;
time(&t);
srand(t);
int moveNum = rand()%canMove.size();
std::string output = "p1 ";
Move out = canMove.at(moveNum);
int i = 0;
int j = 0;
for(int y = 0; y < bSize; y++)
{
for(int x = 0; x < bSize; x++)
{
if(board[x][y].getIndex()==out.getSpos())
{
i = x;
j = y;
}
}
}
if(board[i-1][j+1].getIndex()==out.getEndPos())
{
board[i-1][j+1].setOcc(true);
board[i-1][j+1].setPl(P1);
}
else if(board[i+1][j+1].getIndex()==out.getEndPos())
{
board[i+1][j+1].setOcc(true);
board[i+1][j+1].setPl(P1);
}
output = output + out.toString();
setCmove(output);
board[i][j].setOcc(false);
board[i][j].setPl(NA);
}
else if((canTake.size()==0)&&(canMove.size()==0))
{
Board::winner = 2;
}
P1pieces = canTake.size() + canMove.size();
}
You are working with std::vector, which is a good thing. (Too much beginner "C++" code uses C arrays.) The vector class template allows for a pretty easy way to find out if and where you might have an out-of-bounds access (as suggested in the comments):
Instead of accessing vector elements using operator[], change your code to use the .at() member function. .at() is bounds-checking, and will throw an exception if you access out-of-bounds (instead of silently breaking your program).
In production code, operator[] is usually preferred as omitting the bounds check is more efficient. But while learning, .at() can help you quite a bit.
Also, getting in the habit of using code checkers like valgrind or the assert macro to check your assumptions is a good thing, even when you got past the point where you wouldn't use .at() anymore.
C++ rookie here.
I have the following code:
std::vector<float> MyBuffer::readAverage(int numberOfBuffers) {
std::vector<float> result = std::vector<float>(streams.size());
for (int i = 0; i < streams.size(); ++i) {
result[i] = getAverage(streams[i], numberOfBuffers);
}
return result;
}
float MyBuffer::getAverage(std::deque<float> input, int numberOfBuffers) {
float sum = 0;
for (int i = 0; i < numberOfBuffers; ++i) {
sum += input[i];
}
return sum / numberOfBuffers;
}
This code randomly crashes at getAverage(), I am not sure why.
Strange thing (for me as a C++ rookie at least) is that when I inline the function, it does not crash:
std::vector<float> MyBuffer::readAverage(int numberOfBuffers) {
std::vector<float> result = std::vector<float>(streams.size());
for (int i = 0; i < streams.size(); ++i) {
float sum = 0;
for (int i1 = 0; i1 < numberOfBuffers; ++i1) {
sum += streams[i][i1];
}
result[i] = sum / numberOfBuffers;
}
return result;
}
I can understand that there may be many reasons why this specific code is crashing - so my question relates more to what changes when I inline it, rather than calling a function? In my mind it should be exactly the same thing, but I guess there is something about the way C++ works that I am not grasping?
The program has many potential reasons why it can cause a crash.
bufferDurationMs is not initialized in the provided code, I hope its initialized to value other than 0.
for (int i = 0; i < streams.size(); ++i) {
result[i] = getAverage(streams[i], numberOfBuffers); } use result.size() instead of streams.size() as result is lvalue. It
is better to check both of these conditions in for.
It is quite possible that numberOfBuffers can be 0 in which case code would crash(divide by zero)
Some optimizations that can be done in the code:
std::vector<float> result = std::vector<float>(streams.size()); use reserve rather than using a costly operation of creating a
vector and assigning it to lvalue.
std::vector result; result.reserve(streams.size());
float MyBuffer::getAverage(std::deque<float> input, int numberOfBuffers) prefer const reference rather than creating a copy
of an object
const std::deque& input
So I have a vector of vectors type double. I basically need to be able to set 360 numbers to cosY, and then put those 360 numbers into cosineY[0], then get another 360 numbers that are calculated with a different a now, and put them into cosineY[1].Technically my vector is going to be cosineYa I then need to be able to take out just cosY for a that I specify...
My code is saying this:
for (int a = 0; a < 8; a++)
{
for int n=0; n <= 360; n++
{
cosY[n] = cos(a*vectorOfY[n]);
}
cosineY.push_back(cosY);
}
which I hope is the correct way of actually setting it.
But then I need to take cosY for a that I specify, and calculate another another 360 vector, which will be stored in another vector again as a vector of vectors.
Right now I've got:
for (int a = 0; a < 8; a++
{
for (int n = 0; n <= 360; n++)
{
cosProductPt[n] = (VectorOfY[n]*cosY[n]);
}
CosProductY.push_back(cosProductPt);
}
The VectorOfY is besically the amplitude of an input wave. What I am doing is trying to create a cosine wave with different frequencies (a). I am then calculation the product of the input and cosine wave at each frequency. I need to be able to access these 360 points for each frequency later on in the program, and right now also I need to calculate the addition of all elements in cosProductPt, for every frequency (stored in cosProductY), and store it in a vector dotProductCos[a].
I've been trying to work it out but I don't know how to access all the elements in a vector of vectors to add them. I've been trying to do this for the whole day without any results. Right now I know so little that I don't even know how I would display or access a vector inside a vector, but I need to use that access point for the addition.
Thank you for your help.
for (int a = 0; a < 8; a++)
{
for int n=0; n < 360; n++) // note traded in <= for <. I think you had an off by one
// error here.
{
cosY[n] = cos(a*vectorOfY[n]);
}
cosineY.push_back(cosY);
}
Is sound so long as cosY has been pre-allocated to contain at least 360 elements. You could
std::vector<std::vector<double>> cosineY;
std::vector<double> cosY(360); // strongly consider replacing the 360 with a well-named
// constant
for (int a = 0; a < 8; a++) // same with that 8
{
for int n=0; n < 360; n++)
{
cosY[n] = cos(a*vectorOfY[n]);
}
cosineY.push_back(cosY);
}
for example, but this hangs on to cosY longer than you need to and could cause problems later, so I'd probably scope cosY by throwing the above code into a function.
std::vector<std::vector<double>> buildStageOne(std::vector<double> &vectorOfY)
{
std::vector<std::vector<double>> cosineY;
std::vector<double> cosY(NumDegrees);
for (int a = 0; a < NumVectors; a++)
{
for int n=0; n < NumDegrees; n++)
{
cosY[n] = cos(a*vectorOfY[n]); // take radians into account if needed.
}
cosineY.push_back(cosY);
}
return cosineY;
}
This looks horrible, returning the vector by value, but the vast majority of compilers will take advantage of Copy Elision or some other sneaky optimization to eliminate the copying.
Then I'd do almost the exact same thing for the second step.
std::vector<std::vector<double>> buildStageTwo(std::vector<double> &vectorOfY,
std::vector<std::vector<double>> &cosineY)
{
std::vector<std::vector<double>> CosProductY;
for (int a = 0; a < numVectors; a++)
{
for (int n = 0; n < NumDegrees; n++)
{
cosProductPt[n] = (VectorOfY[n]*cosineY[a][n]);
}
CosProductY.push_back(cosProductPt);
}
return CosProductY;
}
But we can make a couple optimizations
std::vector<std::vector<double>> buildStageTwo(std::vector<double> &vectorOfY,
std::vector<std::vector<double>> &cosineY)
{
std::vector<std::vector<double>> CosProductY;
for (int a = 0; a < numVectors; a++)
{
// why risk constantly looking up cosineY[a]? grab it once and cache it
std::vector<double> & cosY = cosineY[a]; // note the reference
for (int n = 0; n < numDegrees; n++)
{
cosProductPt[n] = (VectorOfY[n]*cosY[n]);
}
CosProductY.push_back(cosProductPt);
}
return CosProductY;
}
And the next is kind of an extension of the first:
std::vector<std::vector<double>> buildStageTwo(std::vector<double> &vectorOfY,
std::vector<std::vector<double>> &cosineY)
{
std::vector<std::vector<double>> CosProductY;
std::vector<double> cosProductPt(360);
for (std::vector<double> & cosY: cosineY) // range based for. Gets rid of
{
for (int n = 0; n < NumDegrees; n++)
{
cosProductPt[n] = (VectorOfY[n]*cosY[n]);
}
CosProductY.push_back(cosProductPt);
}
return CosProductY;
}
We could do the same range-based for trick for the for (int n = 0; n < NumDegrees; n++), but since we are iterating multiple arrays here it's not all that helpful.
I am trying to add values to a map variable that takes int keys and char values. The map is to contain the positions of letters in the alphabet, and the corresponding letter at that position. For some reason, I am getting an error from the .insert() portion inside the for loop.
map<int, char> cipher;
for (int i = 0; i < 26; i++)
{
cipher.insert(i, char(97 + i));
}
Here is the correct syntax when using a map:
for (int i = 0; i < 26; i++)
{
cipher[i] = char(97 + i);
}
//To use it
std::cout << cipher[letterindex] << std::endl;
I would do it slightly different:
I will use map::emplace instead as it inserts a new element into the container which is constructed in-place with the given args(if it's not already there) and avoid unnecessary copy or move operations if possible.
for (int i = 0; i < 26; i++)
{
cipher.emplace(i, char(97 + i));
}
try it -> https://ideone.com/pKf99l
http://en.cppreference.com/w/cpp/container/map/emplace
You should use std::make_pair:
for (int i = 0; i < 26; i++)
{
cipher.insert(std::make_pair(i, char(97 + i)));
}
More details here:
http://en.cppreference.com/w/cpp/container/map/insert
http://en.cppreference.com/w/cpp/utility/pair/make_pair
The problem is there is no overload of the insert functions that takes separate key/value arguments.