C++ Generate random float values between a range

Question

In my current situation I have a game world, in that game world I want to place loads of tree models. It is a waste of time for me to actually explicitly define the x y and z values of each tree model, so I want to do this with the help of random logic, so that it can just set the values for me. I don't want true randomness, so I do not need to seed the random logic, I want to generate the positions of all the trees randomly so that I don't need to type each one myself, but each time I run the game the trees are in the same position, so it is not truly random but that is what I want.

My current attempt is as follows:

for (int i = 0; i < numOfTrees; i++) {
        treeObjects.push_back(
        (new Entity(Vector3f(//x, y and z position value of each tree
                             rand() % 200 + (-200), //random value for x  
                             0.0f,                  //y value kept as 0.0f
                             rand() % 200 + (-200)) //random value for z
//above code positions the trees, the rest of the code is irrelevant
}

The issue with this is that all the trees are very tightly packed together, so I am very skeptical that they are getting a value between -200 and 200, because if they were they would be much more spread out. Does someone know of a better solution to produce what i'm after? maybe there is an issue with the way I am doing it?

Answer 1

rand() % 200 yields a random number between 0 & 200.

rand() % 200 + (-200) takes the range above and shifts; think of it as adding -200 to both the min possible outcome & the max possible outcome.

Thus, 0 to 200 becomes -200 to 0.

To get a -200 to 200, first, note the size of the range is 400 units. So you start with rand() % 400. Next shift it so that half the outcomes are negative: (rand() % 400) - 200

Answer 2

Thanks to anyone that took the time to answer my question, but I wanted to answer it myself for future people interested in a potential solution - I replaced each instance of rand() % 200 + (-200) In my question code (for the randomized x and z values of the Vector3f object) with the following:

-200 + static_cast <float> (rand()) /( static_cast <float> (RAND_MAX/(200-(-200))))

And now this code correctly generates floats between -200 and 200.