Thread: basic maths

Hi, I am trying to work out how to write an equation in C, i know it sounds stupid, but it when i set a variable, a equal to an equation, it says there is bad sytax before the =

i.e

Code:

int trapezium()//this part copes with calculating in value of f(x) and then outputting the estimated area under the graph
{
float fx[11],y, a,b,c;

    for(i=0;i<11;i++)
    {
     fx[i] = (1 - v[i])/U;

     printf("function of x at speed %5.2f, is %5.2f\n", v[i], fx[i]);
    }

    a =dx/2 ;

    y = (2*(((1-v[1])/U) + ((1-v[2])/U) + ((1-v[3])/U)+ ((1-v[4])/U) + ((1-v[5])/U)+ ((1-v[6])/U)+ ((1-v[7])/U)+ ((1-v[8])/U)+ ((1-v[9])/U))));
    printf("y = %f", y);
}

where it says a =dx/2 ; the error here is

E:\xxxxxxxxxxxxxxxxx\Boundary_Layer_Displacement\m ain.c|35|error: syntax error before '=' token|

i think there are a few brackets wrong with the y= but that can be sorted...

cheers guys

Also, where are v and U declared?

Cheers guys that was enough to get me through that problem, and yeah have now done that bit in a loop, but it gives out RANDOM answers, the function is:

Code:

int trapezium()//this part copes with calculating in value of f(x) and then outputting the estimated area under the graph
{
float fx[11],y,a,b;

U = v[10];

        for(i=0;i<11;i++)
        {
            fx[i] = ((i - v[i])/U);

                for (n=1;n<10;n++)
                    {
                        a = a +fx[n]; //a is a section of the trapezium rule
                        printf("a = %5.2f\n", a);
                    }
        }
    float dx = 0.125;

    b= fx[0] + fx[10];

    y = (dx/2) * (b + (2*a));

    printf("y = %5.2f", y);
}

the answer comes out as y =1#R

have i done something like multiply a foat by an integer or something stupid like that?

In the code you show, you still haven't declared the U and v[] variables. What type are they? Are they global variables?

OK so the entire program is below:

Code:

#include <stdio.h>
//#include <math.h>

const float dx = 0.125; //change in x displacement, i.e measurment interval.

float v[11], U;//My global floating varables
int i,n;//My global integers, counts for loops etc

int collectdata() //retrieves data from text file
{
    int i;

    FILE *fp=NULL; //create a pointer, fp to point at the file the data points are stored at
    fp = fopen("data.txt", "r"); //open the data file from the saved location and read from it

            for (i=0; i<11; i++) //for loop scans each data point from the file and stores it in the velocity array, v.
            {
             fscanf (fp, "%f\n", &v[i]);   //scans for integers in data file
            }

    fclose(fp); //close data file

}

int trapezium()//this part copes with calculating each value of f(x), i.e f(x1), f(x2) etc and then outputting the estimated area under the graph
{
float fx[11],y,a,b;
//a and b are variables that help towards the final value for y, y being the displacement thickness
//fx is the function value, in this case the displacement thickness


        for(i=0;i<11;i++)
        {
            fx[i] = ((1 - v[i])/U);

        }
        for (n=1;n<10;n++)
                    {
                        a = a +fx[n]; //a is a section of the trapezium rule
                        //printf("a = %5.2f\n", a);
                    }


    b= fx[0] + fx[10]; //more trapezium rule

    y = (dx/2) * (b + (2*a)); // final trapezium rule

    printf("y = %5.2f", y); //print the estimated area under the graph
}



int main()
{
    collectdata();//runt he function to collect the data from the text file

   for(i=0;i<11;i++)//for loop to print all data points to the screen
    {
     printf("v%d= %5.2f\n", i+1, v[i]); //print the speeds retireved from the file
    }

    U = v[10]; //set U to the free stream veolcity - the greatest recorded speed above the wing

    trapezium(); // run the function to calculate the displacement thickness

    return 0;
}

Like it is it outputs an answer y=-0.72

But it prints all the values for the speed above, from the line

printf("v%d= %5.2f\n", i+1, v[i]); //print the speeds retireved from the file in the main function,

if you remove that line you get y=#R