How to declare values of x and y to display it correctly (scaling also)?

Question

Im writing a program in C (using allegro lib) which display points (i could say some sequence) from txt file(two columns X & Y). These are floating points and they can be less than zero. Size of a window is 800x600. As we know values of x increase from left to right, but y ones increase from up to down. I have a big problem with declaring variables and scaling then. Here is a piece of code I have a problem with:

Array col1 consists of X values, col2 consists of Y values. I sort them with qsort and then get min and max. Then i declare height and width of the window and the scale..

float x_max, x_min, y_max, y_min;
        x_max=(col1[(n-1)]);
        x_min=(col1[0]);
        y_max=(col2[(n-1)]);
        y_min=(col2[0]);

        int height, width;
        height=600;
        width=800;

        float x_scale, y_scale;
        y_scale = height/(fabsf(y_max)+ fabsf(y_min));
        x_scale = width/(fabsf(x_max)+ fabsf(x_min));

Now i reopen txt file and create arrays not sorted arrays col1 and col2. Now i declare xpn and ypn to get the value of x or y from the array, then i mutliply it by scale and eventually i want to display it, but.. not all are displayed and they are they all are displayed wrong. I use abs and fabsf, its bad I know..

Could you help me with this code, declarationa of values of x and y and scaling them? I've tried almost everything and it doesnt work... Thx in advance :)

float xpn, ypn;
int xpoint, ypoint;

hline( screen, 5, 300, 795, makecol( 0, 0, 0 ) );
vline( screen, 400, 5, 595, makecol( 0, 0, 0 ) );

        for(i=0;i<n;i++)
        {
        float xpn, ypn;
        xpn=fabsf(col1[i]);
        ypn=fabsf(col2[i]);

        int xpoint, ypoint;
        xpoint=abs(xpn*x_scale);
        ypoint=abs(ypn*y_scale);

putpixel(screen,xpoint,ypoint,makecol(0,0,0));

Answer 1

Sounds like from your last comment you just need to add +300 and +400?

xpn=fabsf(col1[i]);
ypn=fabsf(col2[i]);

change to

xpn=col1[i]+400;
ypn=col2[i]+300;

and you would not need to get the abs value that way it should be positive.

for the y values being swapped from were you want them try

ypn=-col2[i]+300;

this will switch the negative to positive and vise versa

not exactly sure about the scaling.. not sure how you are trying to scale...