"cout<<count<<endl;" isn't printing anything - c++

cout<<count<<endl; sould provide an output according to the conditions , but it isn't printing anything, what is the fault/error/defects in the code that are causing such results ?
it is my first question , sorry if i am not completely understandable.
i used the following code , i can't understand whats happening here.this is a simple input output question. the output provides us info about matching two team's uniform.
#include <stdio.h>
#include <iostream>
using namespace std;
main(){
int a;
cin>>a;
int **b;
b=new int*[a];
for (int i = 0; i < a; i++)
{
b[i]=new int[2];
for (int j = 0; j <2 ; j++)
{
cin>>b[i][j];
}
}
int count=0;
for (int i = 0; i < a*(a-1); i++)
{ for (int j = 0; j < a; j++)
if (b[i][0]==b[j][1])
count=count+1;
}
cout<<count<<endl;
for (size_t i = 0; i < a; i++)
{
delete b[i];
}
delete b;
}
input:
3
1 2
2 4
3 4
output does not contain anything

You use the array out of bounds and delete when you should delete[]. Comments in code:
#include <iostream> // use the correct headers
#include <cstddef>
// not recommended: https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice
using namespace std;
int main() { // main must return int
size_t a; // better type for an array size
cin >> a;
int** b;
b = new int*[a];
for(size_t i = 0; i < a; i++) {
b[i] = new int[2];
for(size_t j = 0; j < 2; j++) {
cin >> b[i][j];
}
}
int count = 0;
std::cout << a * (a - 1) << "\n"; // will print 6 for the given input
for(size_t i = 0; i < a * (a - 1); i++) {
// i will be in the range [0, 5]
for(size_t j = 0; j < a; j++)
if(b[i][0] == b[j][1]) count = count + 1;
// ^ undefined behaviour
}
cout << count << endl;
for(size_t i = 0; i < a; i++) {
delete[] b[i]; // use delete[] when you've used new[]
}
delete[] b; // use delete[] when you've used new[]
}

Related

Heap corruption detected error in C++

Here is my code:
#include<iostream>
#include<cstdlib>
using namespace std;
int main() {
int** arr=NULL;
int num=0;
cin >> num;
int* big=NULL;
arr = new int*[num];
for (int i = 0; i < num; i++) {
arr[i] = new int[5];
}
big = new int[num];
for (int i = 0; i < num; i++) {
for (int j = 0; j < 5; j++) {
while (1) {
cin >> arr[i][j];
if (arr[i][j] >= 0 && arr[i][j] < 100)
break;
}
}
}
for (int i = 0; i < 5; i++) {
big[i] = 0;
}
for (int i = 0; i < num; i++) {
for (int j = 0; j < 5; j++) {
if (big[i] < arr[i][j]) {
big[i] = arr[i][j];
}
}
}
for (int i = 0; i < num; i++) {
cout << "Case #" << i + 1 << ": " << big[i] << endl;
}
delete[]big;
for (int i = num-1; i>=0; i--) {
delete[]arr[i];
}
delete[]arr;
return 0;
}
When I run this code, it says that there are heap corruption error (heap corruption detected). I think it means that there are some errors at 'new' or 'delete' parts in my codes, but I cannot find them. I hope someone to answer. Thanks.
Error is here:
big = new int[num];
...
for (int i = 0; i < 5; i++) {
big[i] = 0;
}
So when you have num less than 5 you are writing outside the array.
Anyway you are using C++ so use vector for such tasks.
#include<iostream>
#include<cstdlib>
#include<vector>
using namespace std;
int main() {
vector<vector<int>> arr;
int num=0;
cin >> num;
arr.resize(num, vector<int>(5));
for (auto &row : arr) {
for (auto &cell : row) {
while (1) {
cin >> cell ;
if (cell >= 0 && cell < 100)
break;
}
}
}
vector<int> big(arr.size());
for (int i = 0; i < arr.size(); i++) {
for (auto &cell : arr[i]) {
if (big[i] < cell) {
big[i] = cell;
}
}
}
for (int i = 0; i < num; i++) {
cout << "Case #" << i + 1 << ": " << big[i] << endl;
}
return 0;
}
In many places in your code, you're indexing your big array using indexes from 0 to 5, while the array is allocated using user input, if user input was 4 for example, your code is undefined behavior.
If you're using c++, you shouldn't be manually allocating the arrays, use std::vector instead, it will take care of managing memory for you, so you don't have to new and delete memory yourself.
With std::vector, your code would look somewhat like this.
std::vector<std::vector<int>> arr;
std::vector<int> big;
cin>>num;
arr.resize(num, std::vector<int>(5));
big.resize(5);
You will also be able to use at method to access elements while bound-checking, and size method to get the number of elements of the array.

Deleting char matrix

I'm having trouble deleting a char matrix ,
I'm using visual studio and I'm getting an error once the code reaches this line :
delete[] AlloBoard[i];
this is the full code:
#include <iostream>
using namespace std;
char** buildMat(int h, int w)
{
char**mat;
mat = new char*[w];
for (int i = 0; i < w; i++)
mat[i] = new char[h];
return mat;
}
int main()
{
char** AlloBoard=buildMat(4,5);
for (int i = 0; i < 4; i++)
for (int j = 0; j < 5; j++)
AlloBoard[i][j] = 'x';
for (int i = 0; i < 4; i++)
{
for (int j = 0; j < 5; j++)
cout << AlloBoard[i][j];
cout << endl;
}
for (int i = 0; i < 5; i++)
delete[] AlloBoard[i];
delete[] AlloBoard;
cout << "DONE" << endl;
}
appreciate the help!
You initially create 5 arrays of 4 chars each, but then you treat it like 4 arrays of 5 chars each. If your intent is to have matrix like this:
xxxxx
xxxxx
xxxxx
xxxxx
You need to change
buildMat(4,5);
to
buildMat(5,4);
And when deleting, do the loop to 4 not 5
for (int i = 0; i < 4; i++)
delete[] AlloBoard[i];
delete[] AlloBoard;
https://ideone.com/4cgCt3

C++ multiplying matrices using dynamically allocated memory

I am trying to write a function in C++ that multiplies two matrices A, B which have been dynamically allocated. I am currently trying to get the multiplication code working, then I will attempt to turn it into a function. Right now I am getting an error of sorts; "segmentation fault (core dumped)". I have narrowed it down to the multiplication part of my code but I have no idea what is wrong with it. Can someone help me please? My code is shown below.
#include <iostream>
#include <cassert>
int main()
{
int rowsA = 5; // number of rows
int colsA= 3; // number of coloumns
// dynamically allocating A
double** A;
A = new double* [rowsA];
A[0] = new double [rowsA*colsA];
for (int i = 1; i < rowsA; i++)
{
A[i] = A[i-1] + colsA;
}
// Storing elements of matrix A
for(int i = 0; i < rowsA; ++i)
{
for(int j = 0; j < colsA; ++j)
{
std::cout << "Enter element A" << i + 1 << j + 1 << " : ";
std::cin >> A[i][j];
}
}
int rowsB = 3; // number of rows
int colsB = 5; // number of coloumns
// dynamically allocating B
double** B;
B = new double* [rowsB];
B[0] = new double [rowsB*colsB];
for (int i = 1; i < rowsB; i++)
{
B[i] = B[i-1] + colsB;
}
// Storing elements of matrix B
for(int i = 0; i < rowsB; ++i)
{
for(int j = 0; j < colsB; ++j)
{
std::cout << "Enter element B" << i + 1 << j + 1 << " : ";
std::cin >> B[i][j];
}
}
// checking matrix multiplication qualification
assert(colsA == rowsB);
// dynamically allocating C
double** C;
C = new double* [rowsA];
C[0] = new double [rowsA*colsB];
for (int i = 1; i < rowsA; i++)
{
C[i] = C[i-1] + colsB;
}
// Initializing elements of matrix C to 0
for(int i = 0; i < rowsA; ++i)
{
for(int j = 0; j < colsB; ++j)
{
C[i][j]=0;
}
}
// multiplication
for(int i = 0; i < rowsA; ++i)
{
for(int j = 0; j < colsB; ++j)
{
for(int k = 0; k < colsB; ++k)
{
C[i][j] += A[i][k] * B[k][j];
}
}
}
// Displaying the multiplication of matrices A, B
std::cout<< "Matrix C: " << std::endl;
for(int i = 0; i < rowsA; ++i)
{
for(int j = 0; j < colsB; ++j)
{
std::cout << " " << C[i][j];
if(j == colsB-1)
{
std::cout << std::endl;
}
}
}
// deallocation
delete[] C[0];
delete[] C;
delete[] B[0];
delete[] B;
delete[] A[0];
delete[] A;
}
First, you say you are allocating the matrix properly but I'm not seeing any proof of that. You are allocating an array of pointers and only initializing the first index.. A[0] for example. This is incorrect. You need to allocate EACH ROW.
You have:
double** A;
A = new double* [rowsA];
A[0] = new double [rowsA*colsA]; //Incorrect. You only allocated A[0].
You need to allocate A[0] through A[rowsA - 1]..
Next, your multiplication loop (the inner most loop) is incorrect. It should be:
Iterate ColsA for that inner loop.
You have:
for(int i = 0; i < rowsA; ++i)
{
for(int j = 0; j < colsB; ++j)
{
for(int k = 0; k < colsB; ++k) //ColsB is incorrect. Should be colsA..
{
C[i][j] += A[i][k] * B[k][j]; //Segfaults here due to bad iteration..
}
}
}
The following would be correct:
#include <iostream>
#include <cassert>
double** create_matrix(int rows, int cols)
{
double** mat = new double* [rows]; //Allocate rows.
for (int i = 0; i < rows; ++i)
{
mat[i] = new double[cols](); //Allocate each row and zero initialize..
}
return mat;
}
void destroy_matrix(double** &mat, int rows)
{
if (mat)
{
for (int i = 0; i < rows; ++i)
{
delete[] mat[i]; //delete each row..
}
delete[] mat; //delete the rows..
mat = nullptr;
}
}
int main()
{
int rowsA = 5; // number of rows
int colsA= 3; // number of coloumns
double** matA = create_matrix(rowsA, colsA);
int rowsB = 3; // number of rows
int colsB = 5; // number of coloumns
double** matB = create_matrix(rowsB, colsB);
//Checking matrix multiplication qualification
assert(colsA == rowsB);
double** matC = create_matrix(rowsA, colsB);
//Multiplication
for(int i = 0; i < rowsA; ++i)
{
for(int j = 0; j < colsB; ++j)
{
for(int k = 0; k < colsA; ++k) //ColsA..
{
matC[i][j] += matA[i][k] * matB[k][j];
}
}
}
//Clean up..
destroy_matrix(matA, rowsA);
destroy_matrix(matB, rowsB);
destroy_matrix(matC, rowsA);
}

about arrays and a logical error

I expect the code below to print the numbers in ascending order or descending but when I give cout statement all I get is the same array back. I am not able to make out where I am making mistake (and I am using this method because this program is needed for school and I don't want any function to do this for me)
#include<iostream>
#include<algorithm> //<utility> for C++11
using namespace std;
int main()
{
int array[5] = { 1,4,6,9,5 };
for (int i = 0; i<5; i++)
{
for (int j = 0; j<5; j++)
{
if (array[i]<array[j])
{
swap(array[i], array[j]);
}
}
}
for (int k = 0; k<5; k++)
cout << array[k] << " ";
return 0;
}
I think the 4th line should be:
for (int j = i + 1; j < 5; j++)
Also the swap function will need to pass by reference:
void swap(int &a, int &b)
{
int temp = a;
a = b;
b = a;
}

print stars as much as the values in the array

I'm trying to make a C++ program start creating an array and takes the values from the user , then print every value + star as much the value is .. Example : the user had entered 5 then the output must be like this
5*****
Input
1
2
3
4
5
6
output
1*
2**
3***
4****
and so on
.. help :(
#include <iostream>
using namespace std;
void main()
{
int arr[10];
for (int i = 0; i < 10; i++)
{
cin >> arr[i];
int x = arr[i];
for (int j = 0; x <= arr[i]; j++)
{
cout<< "*";
}
}
}
And another help please can you give me some useful link to practice on programming to be professional
Your code is wrong. Use the following code:
#include <iostream>
using namespace std;
int main() {
int arr[10];
for (int i = 0; i < 10; i++)
{
cin >> arr[i];
int x = arr[i];
for (int j = 0; j < x; j++){ // your condition was wrong
cout<< "*";
}
cout<<endl; // for better formatting
}
return 0;
}
For edited question
int main() {
int arr[10];
for (int i = 0; i < 10; i++)
{
cin >> arr[i];
}
for (int i = 0; i < 10; i++)
{
int x = arr[i];
cout << x;
for (int j = 0; j < x; j++){ // your condition was wrong
cout << "*";
}
cout << endl;
}
return 0;
}
#include <iostream>
using namespace std;
void main()
{
int nbValues = 10;
int arr[nbValues];
// First recover the values
for (int i = 0; i < nbValues; i++)
{
cin >> arr[i];
}
// Then print the output
for (int i = 0; i < nbValues; i++)
{
int x = arr[i];
cout << x;// Print the number
for (int j = 0; j < x; j++)
{
cout<< "*";// Then print the stars
}
cout << endl;// Then new line
}
}