sigma in c programming

[ko2dy]

i got an equation composed of sigma : b = E (xi-x)*(yi-y) for i=1 and the last i is n

does anyone know how to write such program in C?
since it is a bit boring if i write :
b = (x1-x)*(y1-y) + (x2-x)*(y2-y) + ... + (xn-x)*(yn-y)

[shacktar]

//you may be using another data type such as int
float b = 0;

//look into arrays to see how to instantiate them and fill in the values for your case
float xArray[] = {...};
float yArray[] = {...};

float x = 5; //or whatever value it has
float y = 10; //or whatever value it has

int i = 0; //loop counter
int n = /*whatever*/; //number of values

for(i = 0; i < n; i++) //arrays of size n are indexed 0..n-1
{
     b += (xArray[i] - x) * (yArray[i] - y);
}

http://www.cplusplus.com/doc/tutorial/control/
http://www.cplusplus.com/doc/tutorial/arrays/

[chipp]

@ko2dy: you just need a little understanding of looping (and arrays) :D
@shacktar: what do you mean by this:

int i = 0;
for (i = 0; /*...blablabla*/) {

:D

[shacktar]

@chipp ko2dy specifically asked for C code. In C, all variable declarations need to be at the beginning of the block (I initialized i for good practice). Doing for(int i = 0; /*foo*/) is illegal in C.

[ko2dy]

@shacktar i tried your program recommendation, but there is an error said "pointer can only be subtracted by pointer"
i think it's incorrect to subtract x[i] with only x

[D80DSL3A]

Yes x[i] - x would be a problem. You need a different name for the array. Notice that shacktar used xArray[i] - x

[ko2dy]

oh thank you for all the helps... i've solved my problem :)

[chipp]

@shacktar: i mean, it should be like this right?

int i;
for (i = 0; /*blablabla*) */

btw, do you mean that you intentionally initialized i twice?

[1vRz3TCk]

shacktar wrote:

Doing for(int i = 0; /*foo*/) is illegal in C.

It is legal in C99 (declarations and statements are also allowed to appear in any order within a block in C99).

Chipp,
int i = 0; is initialization ( an initial value is assigned at the point of declaration)
i = 0 is just assignment.

Initialization is good practice.

[shacktar]

It is legal in C99

Heh, what do you know. Of course, this isn't the first time C99 has surprised me (I didn't know it supported variable length arrays until recently).

[1vRz3TCk]

I must admit that I tend not to program to C99 standard due to the patchiness of compliant compilers.