I have trouble with two dimensional vector. Example:
vector < vector<int> > data;
int i = 0;
int int_value;
while (i < 10 )
{
cin >> int_value;
data[i].push_back (int_value);
}
I want using push_back for back insert and then I want use data [i][j]. Where is the problem?
You need to initialize vector data before using data[i]. Otherwise, the vector is empty and accessing data[i] is out of range. Also, you need to increment i inside the while loop:
vector < vector<int> > data(10); // creates a vector of size 10,
// each element being an empty vector of int's
int int_value;
for (int i=0; i < 10; i++)
{
cin >> int_value;
data[i].push_back (int_value); // add int_value to the ith vector
}
After the loop, each vector contains one int value entered by the user.
data[i] does not exist, because the empty constructor of vector creates a vector of size 0. So when you call data[i] this will be out of bounds. Just like a one dimensional vector first allocate enough elements for data. In your case it seems you need to have data of size 10:
vector < vector<int> > data(10);
Also you never increase i in the while loop which it seems will lead to infinite loop.
You have an empty vector of vectors, and you are trying to access its elements (data[i]). Obviously, you can't do that before you add some (via push_back, resize or some other method of data itself).
The problem is that data[i] does not exist. First you need to push a vector<int> into the vector< vector<int> > vector, then you can add integers to it.
vector < vector<int> > data;
data is an empty vector which holds vector<int>.
data[i].push_back (int_value);
data is empty. So, doing data[i] leads to undefined behavior.
Related
This question already has answers here:
Why does my vector print out all zeroes?
(4 answers)
Closed 1 year ago.
I was trying to access vector elements today, so when I used vector<vector<int>> vec and then added elements to it. I was able to access those elements like vec[1][2].
But when I use vector<vector<int>> vec(n) and then added elements, I was not able to access the elements using vec[1][2]. I keep getting a segmentation error. Does anyone know what am I missing here?
I am adding the elements to the vector through with the help of the below code snippet.
int n;
cin >> n;
vector<vector<int>> vh;
int size, input;
for (int i = 0; i < n; i++)
{
vector<int> temp;
cin >> size;
for (int j = 0; j < size; j++)
{
cin >> input;
temp.push_back(input);
}
vh.push_back(temp);
}
I think I can guess what the problems is...
When you use the constructor with an argument:
vector<vector<int>> vh(n);
you create a vector with the size n, it means it will already have n elements, where each element will be a default-constructed vector<int>. Which means that each vector will be empty.
Then you push back a new vector:
vh.push_back(temp);
This will increase the size of the vector. After one such push_back call the size of vh will be n + 1. The new vector you add will be at index n, i.e. vh[n] is the new vector.
If you set the size when you define the vector, then you need to use indexing and assignment to set the sub-vectors:
vh[i] = temp;
To summarize:
Either you create an empty vector and push back new elements:
vector<vector<int>> vh;
and
vh.push_back(temp);
Or you create a vector with a size, and use indexing and assignment:
vector<vector<int>> vh(n);
and
v[i] = temp;
Don't mix these ways.
Now when you got your current code working (hopefully) and understand how these things work a little better, it's time to show a way how to do your code in a more "C++-ish" way... :)
// The first part is much like your current code
size_t n;
std::cin >> n;
std::vector<std::vector<int>> vh(n);
// Now iterate over all the elements in the vector
for (auto& v : vh)
{
// Get the size of the current sub-vector
size_t size;
std::cin >> size;
// Create the vector with size elements
v = std::vector<int>(size);
// Read size integers into the vector
std::copy_n(std::istream_iterator<int>(std::cin), size, begin(v));
}
I am a newbie in cpp field.
Below is my cpp code. When I use the online c++ compiler, and
why does it happened to exited, segmentation fault, and what's wrong with my code.
Thanks guys.
Problem description:
Given an array of integers arr, you’re asked to calculate for each index i the product of all integers except the integer at that index (i.e. except arr[i]). Implement a function arrayOfArrayProducts that takes an array of integers and returns an array of the products.
Solve without using division and analyze your solution’s time and space complexities.
#include <iostream>
#include <vector>
using namespace std;
vector<long> arrayOfArrayProducts(const vector<int>& arr)
{
vector<long> res = {};
int n = arr.size();
// handles edge cases as well
if(n==0 || n==1){
return res;
}
int product;
// your code goes here
for(int i=0; i<n; i++){
product = 1;
for(int j=0; j<n; j++){
if(i!=j){
product = arr[j]*product;
}
res[i]=product;
}
}
return res;
}
int main() {
// vector initiallize
//vector<int> arr{8, 10, 2};
const vector<int> arr;
int n = arr.size();
vector<long> ans(n,0);
ans = arrayOfArrayProducts(arr);
for(int i=0; i<n; i++){
cout<< ans[i] <<' ';
}
return 0;
}
Here is the line where the failure comes:
res[i] = product;
and the reason is because you declared res like this:
vector<long> res = {};
so you are trying to access the long element located at 'cell' i of an empty vector... that is not allowed in C++ and the result is undefined behavior
fix it:
the same way as in the main function
std::vector<long> arrayOfArrayProducts(const std::vector<int>& arr)
{
int n = arr.size();
std::vector<long> res(n, 0);
....
Segmentation faults typically occur when you try to read past the bounds of an array. This fault in your code means that you're trying to read/ write something from an array which is not allocated. If you take a closer look, res[i]=product means that your accessing elements from the vector res. But you haven't reallocated or inserted data into it. This is the reason for the segmentation fault.
To resolve this, resize the res vector to the size of arr so that the vector will have the elements in memory to be accessed,
vector<long> arrayOfArrayProducts(const vector<int>& arr)
{
vector<long> res(srr.size());
...
You are getting this error because you are trying to access the index i of the vector<int> res that you never initialized. on this line:
res[i] = product;
For that you have to understand how Vector works, Vector is dynamic container which can shrink and expand based on its usage, in your case, You Initialized it with default constructor, which did not allocated any memory to hold int. So when you try to access that you get undefined behavior.
In case of default construction of vector. You cannot assign to a particular index. but you have can put data in it via this:
res.push_back(data);
You can fix it by using these constructors:
std::vector<int> res(arr.size(), 0); // In second parameter you can pass default values for all the elements in the vector.
std::vector<int> res(arr.size()); //This will simply initialize elements with default constructor
I'm a newbie in C++. I've just learnt about vector in STL.
However, when I tried to input an integer into my vector:
vector<int> v;
cin>>v[i]
The program returned segmentation fault. Please help me out.
Your vector doesn't have any elements in it, so the internal array is null. When you try to read something into it, you're trying to deference a null pointer (resulting in the segfault). Add elements to the vector first:
vector<int> v(100); //Create vector with 100 elements
for(int i = 0; i < 100; i++) {
cin >> v[i];
}
Alternatively, you could read elements into a local variable, then add them into the vector:
vector<int> v;
for(int i = 0; i < 100; i++) {
int new_val;
cin >> new_val;
v.push_back(new_val);
}
So I have a vector container, and I generate a random string (as of right now it's
just asterisks for the sake of me being able to easily see the differences in size) to assign through the vector. All is fine and dandy, got everything working with that..
When I pass the vector to a function that is supposed to iterate through and assign the size
of each element in the vector to a new vector, I always get an error. What is the best way to do this?
The code below is not my original attempt, but I'm late for class and have to run. So as of right now, here's how I'm trying to do it.
vector<int> siz;
int length = contain.size();
for (vector<string>::iterator i = contain.begin(); i != contain.end(); ++i){
siz.push_back(contain.size());
}
for (vector<int>::iterator j = siz.begin(); j != siz.end(); ++j){
cout << *j << endl;
}
You need to get the size of each individual element, rather than the length of the container vector.
Your i variable is an iterator on the container vector, which operates essentially as a pointer to each element. In the body of your loop, access the size of the object i points to, to get the size of that object.
vector<int> siz;
for (vector<string>::iterator i = contain.begin(); i != contain.end(); ++i){
siz.push_back(i->size());
}
If I understand your intention, you want a vector siz that contains the length of each string in the vector contain, correct? If so, you do not want to do contain.size() because that is just the length of your string vector. To get the size of each string, see below.
vector<int> siz;
for (vector<string>::iterator i = contain.begin(); i != contain.end(); ++i){
string currentString = *i;
siz.push_back(currentString.size());
}
i have created a map called select_p and vector of this map is called pts. i have stored data in a array and i want to pushbcak these data into my vector of map. i tried this by inserting value of array into new vector and then pushback into my map.but it is not working please help me to correct these codes? thanks
#include<iostream>
#include<cstdlib>
#include <map>
#include <vector>
using namespace std;
int main()
{
int M=7;
int N=6;
int i=0;
int * temp;
map<int,vector<int> > select_p;
vector<int>pts;
for (int m=0; m<M; m++)
{
for (int n=0; n<N; n++)
{
vector<int>id;
if (n==0 && m==5)
{
temp = new int[3,i+N,i+N+1,i+1];
unsigned ArraySize = sizeof(temp) / sizeof(int);
id.insert(id.begin(),temp[0], temp[ArraySize]);
select_p[i].push_back(id);
}
i++;
}
}
delete[] temp;
system("PAUSE");
return 0;
}
for (int m=0; m<M; m++) {
for (int n=0; n<N; n++) {
if (n==0 && m==5) {
Why are you looping when you only actually do anything for a single pair of values of m and n? The loops are completely useless here; you would get the same effect by just setting n = 0 and m = 5.
temp = new int[3,i+N,i+N+1,i+1];
Whatever you think this does, that's not what it does. This is equivalent to temp = new int[i+1];. The rest of the expression inside of the [] has no effect.
That said, you should not use new to create arrays in your program. Use std::vector; it is far easier to use correctly.
unsigned ArraySize = sizeof(temp) / sizeof(int);
This does not work. When you dynamically allocate an array, you are responsible for keeping track of how many elements are in it. Given a pointer to a dynamically allocated array (like temp here) there is no way to determine the number of elements in the array.
What you have is equivalent to sizeof(int*) / sizeof(int), which is not going to do what you expect.
id.insert(id.begin(),temp[0], temp[ArraySize]);
std::vector::insert takes a range of iterators: you have provided it with two values. Presumably you want to use temp, which points to the initial element of the dynamically allocated array, and temp + i + 1, which points one past the end of the array. That said, since you haven't set the values of the elements in the array, you are copying uninitialized memory, which probably isn't what you mean to do.
select_p[i].push_back(id);
select_p[i] is a std::vector<int>. std::vector<int>::push_back() takes a single int that is appended to the sequence. Presumably you just mean to use assignment to assign id to select_p[i].
You should get a good introductory C++ book if you want to learn to program in C++. I am sorry to say that your program is nonsensical.