Arrays not filling in as expected in C++ [closed] - c++

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I just started trying to learn C++ yesterday, and one of my goals for the near future is to make a matrix multiplication function.
Before trying that, I wanted to get a feel of how arrays work, so I made a simple program that is supposed to make two arrays that represent (mathematical) vectors in 3D space and take their dot product. Below is the code that I wrote.
/*Initializing two vectors, v1 and v2*/
int v1[3] = { 0 };
int v2[3] = { 0 };
int sum = 0; //This will become the sum for the dot product
int i=0; //counter for the following loop
for(; i<3; ++i)
v1[i] = v2[i] = i+1; //This should make both vectors equal {1,2,3} at the end of the loop
sum += v1[i]*v2[i]; //Component-wise, performing the dot product operation
std::cout<< sum <<std::endl;
return 0;
When the code is done running, the output is supposed to come out to be 1*1 +2*2 +3*3 = 14
However, the output is actually coming out as 647194768, which doesn't seem to make any sense. I heard from some friends that in C++, if you aren't careful about initializing the arrays, then some crazy stuff happens, but I'm completely dumbfounded how something this simple could mess up so badly.
Could you provide some deeper insight into why this is happening, and what about C++'s logic causes this?

Answer 1

You are accidentally putting the sum += line outside the for loop. This is because you have no braces on the loop. So only the first line v1[i]... is included in the for loop. Change it like this:
/*Initializing two vectors, v1 and v2*/
int v1[3] = { 0 };
int v2[3] = { 0 };
int sum = 0; //This will become the sum for the dot product
for(int i=0; i<3; ++i)
{
v1[i] = v2[i] = i+1; //This should make both vectors equal {1,2,3} at the end of the loop
sum += v1[i]*v2[i]; //Component-wise, performing the dot product operation
}
std::cout<< sum <<std::endl;
return 0;
Note the use of braces { .. } around the for loop statements. Now this gives the correct answer: 14.

Answer 2

As you ask deeper insight here i am writing some explanation of your code
int v1[3] = { 0 };
int v2[3] = { 0 };
Initializing v1 and v2 with 0,
int sum = 0; //This will become the sum for the dot product
Here you Initializing sum with 0 (GOOD)
int i=0; //counter for the following loop
for(; i<3; ++i)
v1[i] = v2[i] = i+1; //This should make both vectors equal {1,2,3} at the end of the loop
sum += v1[i]*v2[i]; //Component-wise, performing the dot product operation
Here you make mistake as you did not provide {..} compiler will consider first list in loop statement. like below
v1[0] = v2[0] = 0+1
v1[1] = v2[1] = 1+1
v1[2] = v2[2] = 2+1
Now second line will come in picture,
Note that now i is become 3.
sum += v1[i]*v2[i];
Here, you try to access invalid location of an array (4th one)
that's the reason why you got 647194768
You got junk value nothing else
Correct code is already given by metacubed, so I am not writing here,
best of luck for you further study.