initialize vectors

**violatro** · 06-07-2010

Hi guys...
I do not understand why the hell have I to initialize a vector. For example it is sensible, up to me, to initialize a scalar in this situation

Code:

int foo(int a)
{
     int i, res=0; //ok, great idea!!! otherwise bad random result
     for(i=0; i<a;i++) res+=i;
     return(res);
}

but in this one I not agree

Code:

void foo2(int *res, int a)
{
     int i, res=0; //ok, great idea!!! otherwise bad random result
     for(i=0; i<a;i++) res[i]=i;
}

int main()
{
     int *res, ini_res[10], i;
     res=&ini_res[0];
     foo2(res, 10);
    for(i=0; i<10;i++) printf("%d\n",res[i]); 
}

but I am getting bad result without initializing ini_res...
Has somebody a clue?

**MK27** · 06-07-2010

Why do you think an array of ints would be any different than a single int? "Vector" is not a datatype in C, and "scalar" is not particularly meaningful either, since all types in C are "scalar". Use the correct nomenclature if you want to be properly understood. Importing terms from some other setting is pointless.

You can use this to initialize everything to zero:

Code:

int ray[10] = {0};

However, by the standard zero is the only value that will work that way. You can initialize this way too:

Code:

int ray[4] = {1,2,3,4};

If you do that, any elements you leave of the end (eg, if it were ray[10]) will be zero.

**laserlight** · 06-07-2010

Originally Posted by MK27

"scalar" is not particularly meaningful either, since all types in C are "scalar".

Correction:

Originally Posted by C99 Section 6.2.5 Paragraph 21

Arithmetic types and pointer types are collectively called scalar types. Array and structure types are collectively called aggregate types.

Originally Posted by C99 Section 6.2.5 Note 37

Note that aggregate type does not include union type because an object with union type can only contain one member at a time.

But yes, you should "use the correct nomenclature", i.e., aggregate type, or simply array, instead of "vector".

**violatro** · 06-07-2010

what I am trying to say is that I am initializing the vector with the final values without wasting time with loops to fill up the vector with "temporary" values...
I did not know that in C it is sufficient to initialize a vector to zero in the way you showed me. Now I am happy because I can initialize a vector without a loop!!!

**MK27** · 06-07-2010

Originally Posted by laserlight

Correction:
But yes, you should "use the correct nomenclature", i.e., aggregate type, or simply array, instead of "vector".

Thanks laserlight. I was not aware of the term "aggregate" or that an array qualifies as a type distinct from it's "scalar" base.

Originally Posted by violatro

what I am trying to say is that I am initializing the vector with the final values

IT IS NOT A VECTOR! GRRRR!!! You might as well use the term "bunch 'o" or "collection" or "set" or whatever.

**KBriggs** · 06-07-2010

Code:

void foo2(int *res, int a)
{
     int i, res=0; //ok, great idea!!! otherwise bad random result
     for(i=0; i<a;i++) res[i]=i;
}

You are passing foo2 an int * called res, and then in the first line you are declaring an int called res. This is BAD. You are declaring a variable with the same name but different type from what it was originally.

I think what you think you are doing is this:

Code:

void foo2(int *res, int a)
{
     int i;
     res=0;
     for(i=0; i<a;i++) res[i]=i;
}

And I think you think that this is going to set all of the values in the array res to 0. It won't. res is a pointer, telling res=0 probably won't get you anywhere. If you want to make all of the values in the int array 0, you have to use a loop in this context.

Of course, since you are setting the elements of res equal to i in your loop there is no need to initialize here anyway, but there would be if you were doing +=i instead of =i, in which case your function would have to look like:

Code:

void foo2(int *res, int a)
{
     int i;
     res=0;
     for(i=0; i<a;i++)
     {
          res[i]=0;
          res[i]+=i;
     }
}

And stop calling it a vector. It is an array of ints.

**violatro** · 06-07-2010

your suggestion is not working with variable-sized arrays...(bad new for me)

**violatro** · 06-07-2010

Originally Posted by MK27

Thanks laserlight. I was not aware of the term "aggregate" or that an array qualifies as a type distinct from it's "scalar" base.

IT IS NOT A VECTOR! GRRRR!!! You might as well use the term "bunch" or "collection" or "set" or whatever.

I did not see laser's reply....sorry! array!

**violatro** · 06-07-2010

Originally Posted by KBriggs

Code:

void foo2(int *res, int a)
{
     int i, res=0; //ok, great idea!!! otherwise bad random result
     for(i=0; i<a;i++) res[i]=i;
}

You are passing foo2 an int * called res, and then in the first line you are declaring an int called res. This is BAD. You are declaring a variable with the same name but different type from what it was originally.

I think what you think you are doing is this:

Code:

void foo2(int *res, int a)
{
     int i;
     res=0;
     for(i=0; i<a;i++) res[i]=i;
}

And I think you think that this is going to set all of the values in the array res to 0. It won't. res is a pointer, telling res=0 probably won't get you anywhere. If you want to make all of the values in the int array 0, you have to use a loop in this context.

And stop calling it a vector. It is an array of ints.

yes, it was a mistake. The second version is the right one.
doing a loop to fill the vector with an iterative procedure is very time wasting.
All of you are telling me to call a vector as "array"...ok, forgive me...but nobody, up to now, makes clear why is not the same to fill the array once, with the right values

**KBriggs** · 06-07-2010

It is the same, if you are putting the right values in using =. You just have to be careful when yo uuse the += operator that the value that is already in the slot you want is correct before you add anything to it.

Your mistake was the double declaration though, there was nothing wrong with how you were filling the array.

**laserlight** · 06-07-2010

Originally Posted by violatro

your suggestion is not working with variable-sized arrays...(bad new for me)

Yes, but then there are functions such as memset() from <string.h> that could help, depending on what you want. But if you need to use a loop, just use a loop.

**violatro** · 06-07-2010

Originally Posted by KBriggs

It is the same, if you are putting the right values in using =. You just have to be careful when yo uuse the += operator that the value that is already in the slot you want is correct before you add anything to it.

Your mistake was the double declaration though, there was nothing wrong with how you were filling the array.

I thought so. but if I do not initialize the vector I get bad results...I really do not understand why. Maybe it is because I am using C code inside R...

**violatro** · 06-07-2010

Originally Posted by laserlight

Yes, but then there are functions such as memset() from <string.h> that could help, depending on what you want. But if you need to use a loop, just use a loop.

Ok...but in this way I loose time. Maybe there is something wrong in the code because I used to do not initialize and all worked well...

**KBriggs** · 06-07-2010

. but if I do not initialize the vector I get bad results...I really do not understand why

Post the smallest compilable program that demonstrates the problem. I think your problem is not coming from where you think it is.

**laserlight** · 06-07-2010

Originally Posted by violatro

Ok...but in this way I loose time.

Have you actually measured the difference and found that it was significant with respect to the rest of your program?

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