CODEWITHSUNDEEP
cfunctionfor-loop
peo965
peo965

Reputation: 39

Manipulating triangle values using a for() loop

I'm writing a program that will change the coordinates of a triangle (one coordinate at a time) in order to create a new triangle and then measure that new perimeter (>100 triangles total).

I have the perimeter function below, but I'm confused as to how I can change only one point of the triangle at a time while still changing each of the three points at some point in time (right now I'm only changing ptA 100 times, but I would rather change ptA 35 times, ptB 35 times, etc). Also, when I run the program as below, why does it only give 5 perimeters instead of 110?

double find3Ddistance (double pt0[], double pt1[]);
double find3Dperimeter (double pt0[], double pt1[], double pt2[]);

int main(){
    double ptA[3] = {0,0,0}; //values for testing perimeter function
    double ptB[3] = {6,0,0};
    double ptC[3] = {0,8,0};

    for (i = 0; i < 110; ++i) {
      find3Dperimeter(ptA, ptB, ptC);
      ++ptA[i];
      }
      return 0;
}

double find3Ddistance (double pt0[], double pt1[]) {
    double x = pt0[0] - pt1[0];
    double y = pt0[1] - pt1[1];
    double z = pt0[2] - pt1[2];
    double dist;
    dist = pow(x,2) + pow(y,2) + pow(z,2);
    dist = sqrt(dist);
    return dist;
}

double find3Dperimeter (double pt0[], double pt1[], double pt2[]) {
    double one = find3Ddistance(pt0, pt1);
    double two = find3Ddistance(pt1, pt2);
    double three = find3Ddistance(pt2, pt0);
    double perimeter = one + two + three;
    printf("Perimeter: %lf", perimeter);
    return perimeter;
}

Upvotes: 1

Views: 72

Answers (3)

Dese
Dese

Reputation: 348

You modify ptA[i] for i between 0 and 109. That's incorrect, you should change only ptA[0], ptA[1] or ptA[2]. For example, you could change this part: 

for (i = 0; i < 110; ++i) {
    find3Dperimeter(ptA, ptB, ptC);
    ++ptA[i];
}

to something like that: 

for (i = 0; i < 35; ++i) {
    find3Dperimeter(ptA, ptB, ptC);
    ++ptA[0];
    ++ptB[1];
    ++ptC[2];
    find3Dperimeter(ptA, ptB, ptC);
    ++ptA[1];
    ++ptB[2];
    ++ptC[0];
    find3Dperimeter(ptA, ptB, ptC);
    ++ptA[2];
    ++ptB[0];
    ++ptC[1];
}

This is just an example, but you don't have to call find3Dperimeter only once per loop.

Upvotes: 1

morels
morels

Reputation: 2105

Try this code:

double find3Ddistance (double pt0[], double pt1[]);
double find3Dperimeter (double pt0[], double pt1[], double pt2[]);

int main(){
    double ptA[3] = {0,0,0}; //values for testing perimeter function
    double ptB[3] = {6,0,0};
    double ptC[3] = {0,8,0};

    for (int i = 0; i < 110; ++i) {
      int j = i%3;
      find3Dperimeter(ptA, ptB, ptC);
      if(j==0) ++ptA[j];
      else if (j==1) ++ptB[j];
      else ++ptC[j];
    }
    return 0;  
}

double find3Ddistance (double pt0[], double pt1[]) {
    /*is the same*/
}

double find3Dperimeter (double pt0[], double pt1[], double pt2[]) {
    /*is the same */
}

This way you can iterate 110 times changing only 35 times every ptX. The module operator should help you changing one array per time . It makes j to cycle the values 0,1,2 that are used as indexes of the three points.

Upvotes: 1

dbush
dbush

Reputation: 223689

Here's your problem:

++ptA[i];

The array ptA only has 3 elements, but i ranges from 0 to 109, so you're referencing the array out of bounds. This leads to undefined behavior.

Assuming you want to increase one of the three dimensions, you would do this:

for (i = 0; i < 110; ++i) {
    find3Dperimeter(ptA, ptB, ptC);
    ++ptA[0];
}

Or for all three:

for (i = 0; i < 110; ++i) {
    find3Dperimeter(ptA, ptB, ptC);
    ++ptA[0];
    ++ptA[1];
    ++ptA[2];
}

If you want to do each point 35 times, just use 3 separate loops:

for (i = 0; i < 35; ++i) {
    find3Dperimeter(ptA, ptB, ptC);
    ++ptA[0];
    ++ptA[1];
    ++ptA[2];
}
for (i = 0; i < 35; ++i) {
    find3Dperimeter(ptA, ptB, ptC);
    ++ptB[0];
    ++ptB[1];
    ++ptB[2];
}
for (i = 0; i < 35; ++i) {
    find3Dperimeter(ptA, ptB, ptC);
    ++ptC[0];
    ++ptC[1];
    ++ptC[2];
}

Upvotes: 2

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