C++ if 语句总是返回真

Question

I have a bunch of maths manipulations that have thresholds, yet no matter what I change, the if statements always return true. No compile errors, can't get the debugger to work. This is a function, the XY and Z arrays are all correct (I printed them to check earlier), the maths is right at least for the blade distance check, yet that always returns true. I ran the same code (rewritten obviously) through matlab and that returns true or false depending on my data, so clearly its something with the way I've written this that's wrong. Also is there any way to slimline this?

bool Device::_SafeD(char _Type, float _Data[20][3]) {
    bool S;
    double x[20], y[20], z[20];
    for (int i=0; i<20; i++) {
        x[i] = _Data[i][0];
        y[i] = _Data[i][1];
        z[i] = _Data[i][2];
    }

    // Check angles for needle
    if (_Type == 'n') {
        for (int i=0; i<20; i++) {
            float dot, moda, modb, c, angle;
            dot = ((x[i]*x[i+1]) + (y[i]*y[i+1]) + (z[i]*z[i+1]));
            moda = sqrt(pow(x[i],2)+pow(y[i],2)+pow(z[i],2));
            modb = sqrt(pow(x[i+1],2)+(y[i+1],2)+(z[i+1],2));
            c = dot/(moda*modb);
            angle = acos(c);

            if (angle > 45){
                S = 0;
            } else {
                S = 1;
            }
        }
    }

    // Check distance for blade
    if (_Type == 'b'){
        for (int i=0; i<19; i++) {
            float distance = (x[i+1]-x[i]) + (y[i+1]-y[i]) + (z[i+1]-z[i]);
            cout << "distance " << distance << endl;

            if (distance > 5.0) {
                S  = 0;
            } else {
                S =  1;
            }
        }
    }
    if (S == 0) {
        return 0;
    }
    if(S == 1) {
        return 1;
    }
}

Cheers

Answer 1

The most likely error is that you are comparing angle to an angle in degree while the return value of acos is in radians.

        if (angle > 45){

Convert the angle to degrees before comparing to 45 and you should be OK.

        if (radians_to_degrees(angle) > 45){

where

double radians_to_degrees(double in)
{
    return (in*180/M_PI);
}

Another option is to compute the equivalent of 45 degrees in radians and compare it with angle .

double const radian_45 = M_PI*45.0/180;

and use

        if (angle > radian_45){

The other error is spelled out clearly in a comment by @Bob__:

OP uses two loops to check the angle and the distance, inside those loops a flag ( S ) is set to 0 or 1 , but it's done for every index in the array, so it's overwritten. The return value of the entire function (in the provided code) depends only by the last two elements in the array.