Getting NAN when only dealing with integer and float - c++

Question

I am working on an opengl assignment where I have to make a creature (I chose a snowman) move around some terrain. I am trying to make it move around, and I am getting the strangest errors. After printing the numbers out, I frequently get "-1.#QNAN0" as a number. I don't even know what that means. Below is the snowman's update function, constructor, and the header file. I am trying to get 2 numbers to use as velocity and add them to the position while it is set to animate (randomly changing), but I don't understand what errors are causing me to not get numbers out of rand().
Each time that the probability check succeeds, it prints out:
DEBUG: probability check succeeded
-1.#QNAN0 0.000000
or
DEBUG: probability check succeeded
0.000000 0.000000
with about 50% chance of each.
From Snowman.cpp
void Snowman::update(canvas_t texture){
//randomly toggle the walking variable
int probability = rand() % 100;
//printf("DEBUG: probability = %d\n", probability);
if(probability <= 10){
printf("DEBUG: probability check succeeded\n");
walking = !walking;
dx = static_cast<float>(( (rand() % 10) - 5));
dy = static_cast<float>(( (rand() % 10) - 5));
printf("%f %f\n", dx, dy);
}
//code to control movement
if(walking){
animate = true;
x += dx;
y += dy;
constrain(x, 0, texture.width);
constrain(y, 0, texture.height);
}else{
animate = false;
}
//set the height after x and y are resolved
z = getHeight(texture);
}
Snowman::Snowman(canvas_t terrain)
{
wireFrame = false;
animate = false;
armSegments = 2;
animationFrameNumber = 0;
manualUserOffset = 0;
//set its initial position
x = rand() % terrain.width;
y = rand() % terrain.height;
dx = 0;
dy = 0;
}
From Snowman.h
class Snowman
{
public:
Snowman(canvas_t);
~Snowman(void);
void setWireframe(bool);
void toggleWireframe(void);
void setAnimate(bool);
void toggleAnimate(void);
void setArmSegments(int);
void addArmSegment(void);
void subtractArmSegment(void);
void update(canvas_t);
void draw(void);
private:
bool wireFrame;
bool animate;
bool walking;
int armSegments;
int animationFrameNumber;
float manualUserOffset;
float x, y, z;
int dx, dy;
inline float f(void);
inline void drawMouth(int headRadius);
inline void drawFace(int headRadius);
void drawArm(int remainingSegments);
inline void drawBody();
inline float getHeight(canvas_t);
};

Answer 1

dx and dy are ints, but your format specifier %f requires a double or a float. So you have undefined behaviour.