How to debug why the compiler crashes on my C++ program? - c++

Question

Code 1: supposed to take a matrix (m by n) size and then find the min value in each row. Doesn't show any errors, runs but the black screen (devc++) for the compiler simply crashes without doing anything and has a ridiculously high return value (3221225725 to be exact).
I'm not sure how to fix or improve it, also it works when the size of the matrix is constant, like instead of cin to get size a simple number makes it work. I am not sure why; I'm new to programming.
#include <iostream>
#include <stdlib.h>
using namespace std;
int main()
{
int m,n;
int B[m][n];
int A[m+1][n+1] = {0};
cin>>m;
cin>>n;
for (int x = 0; x < m; ++x)
{
for (int y = 0; y < n; ++y)
{
cout<< "Enter in value for row " << x << ", column " << y << ".\n";
cin>> A[x][y];
}
}
cout << "Input:" <<endl;
for (int x = 0; x < m; ++x)
{
for (int y = 0; y < n; ++y)
{
cout<< A[x][y] << "\t";
}
cout << "\n";
}
for (int x = 0; x < m; ++x)
{
for (int y = 0; y < n; ++y)
{
A[x][4] = A[x][1];
if (A[x][4] > A[x][y])
A[x][4] = A[x][y];
}
}
cout <<"Output:"<<endl;
for (int x = 0; x < m+1; ++x)
{
for (int y = 0; y < n+1; ++y)
{
cout << A[x][y] << "\t";
}
cout<<"\n";
}
getchar ();
return 0;
}
And this is code 2:
#include <iostream>
#include <math.h>
using namespace std;
int main ()
{
int i,j,R,C,Too;
double a[i][j];
float f;
Too=0;
cin>>R;
cin>>C;
for (int i=0; i<R; i++)
for (int j=0; j<C; j++)
{
f=i+j/2;
a[i][j]=sin(f);
if (a[i][j]>0)
{
Too=Too+1;
}
cout << "a[" << i << "][" << j << "]: ";
cout << a[i][j]<< endl;
}
cout<<Too<<" Shirheg eyreg element bn"<<endl;
}
the elements in this matrix is generated by the formula f=i+j/2; a[i][j]=sin(f);
and simply outputs how many positive elements are there. for some odd reason the elements are always double like the output is something like:
0
0
0.841471
0.841471
and one other number then gets getting double and then a number and then double.
how to fix?

Answer 1

Strange return values and crashes when working in C++ are often a sign of uninitialized values. For example, consider the following:
int main() {
int m,n; // <-- declare 'm' and 'n', but we don't initialize them to have a value
int B[m][n]; // <-- use the values in 'm' and 'n' to allocate memory for 'B'
...
cin >> m; // <-- only now are you setting m to a value, but you already used it.
cin >> n; // <-- same thing with n.
...
}
The above code is wrong because it uses the variables identified by 'm' and 'n' before it sets them to have a particular value, so until you set their value (with cin) they just have whatever value happened to be sitting in memory at the time they were created.
Assigning a value to them is called "initialization" or initializing them the first time you do it. Before that they just hold that value from memory.
Ultimately, each line of code is using some variables and may be making assignments to other variables. So make sure each variable that is being used has already been initialized before you reach that line of code.
You will have to debug your own code in order to learn effectively, and the above example is not the only error or issue here, but a few basic tips to help you along the way:
Always look out for anything using uninitialized values like m and n
If it makes sense, initialize values as soon as they are created
i and j and k as variable names are usually declared only in a loop, because you usually only need them to exist inside the loop.
If you declare int i in a function, you should not declare int i in a loop within that function. The two i's now mean different things, which is confusing.
put spaces around your << and >> operators. It will make the code easier to read.
If you're having trouble, try printing out the value of a variable with cout and see if it makes sense. If not, there is a problem before that point in the program.
If you're having trouble, try working on one section of the code at a time until it behaves the way you expect.