How to move a rectangle in SDL2 using C

Question

I'm unable to move the rectangle I made in the program. There is no error message in the compiler when I run the program. Can you please tell me what I missed out in the keyboard event. Other event that I assigned to the window works fine. Instead of writing instructions down can you please show me the solution. I have been on this forever. Thanks

#include <SDL.h>
#include <stdlib.h>

int main()
{
    SDL_Window *o;
    SDL_Renderer *r;
    SDL_Event e;
    SDL_Rect q;
    int i =1;

    SDL_Init(SDL_INIT_VIDEO);

    o = SDL_CreateWindow("Game test",
                            SDL_WINDOWPOS_UNDEFINED,
                            SDL_WINDOWPOS_UNDEFINED,
                            1024,
                            800,
                            SDL_WINDOW_SHOWN | SDL_WINDOW_RESIZABLE);

    r = SDL_CreateRenderer(o, -1,SDL_RENDERER_ACCELERATED);

    while(i)
    {
        while(SDL_PollEvent(&e) !=0)
        {
            if(e.type == SDL_QUIT)
                i=0;
            else if(e.type == SDL_KEYDOWN)
            {
                switch(e.key.keysym.sym)
                {
                case SDLK_ESCAPE:
                case SDLK_q:
                    i=0;
                break;
                case SDLK_UP:
                    q.y -=10;
                break;
                case SDLK_DOWN:
                    q.y +=10;
                break;
                case SDLK_RIGHT:
                    q.x +=10;
                break;
                case SDLK_LEFT:
                    q.x -=10;
                break;
                }
            }
        }
        SDL_SetRenderDrawColor(r,0,0,255,255);
        SDL_RenderClear(r);
        // Creating the rectangle
        q.x=475;
        q.y=700;
        q.h=50;
        q.w=50;

        SDL_SetRenderDrawColor(r,0,0,0,255);

        SDL_RenderFillRect(r,&q);

        SDL_RenderPresent(r);
    }

    SDL_DestroyWindow(o);
    SDL_DestroyRenderer(r);
    SDL_Quit();

    return 0;
}

2501 · Answer 1 · 2016-01-29T12:24:44.360

2

You didn't initialize the rectangle object:

SDL_Rect q;

before potentially modifying its values in the event loop, which will result in undefined behavior:

case SDLK_UP:
    q.y -=10;
break;

Then any change made to the object is invalidated when you set constant values to it:

q.x=475;
q.y=700;
q.h=50;
q.w=50;

So the solution is clear, set the constant values to the object only when you define it.

edited Jan 29 '16 at 12:24

answered Jan 29 '16 at 12:16

2501

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Are you suggesting to use struct?? – user5771881 Jan 29 '16 at 12:21
@user5771881 There was a typo in the answer, maybe that confused you. – 2501 Jan 29 '16 at 12:25
I don't understand by initializing the rectangle object? Can you insert you solution to my code so I can see what you mean..... – user5771881 Jan 29 '16 at 12:31
@user5771881 You have to set some values to the rectangle before you use it. – 2501 Jan 29 '16 at 12:38
Are you talking about this: a =475, b=700 q.x=a; q.y=b; since a and b variables can be altered would the rectangle move? – user5771881 Jan 29 '16 at 12:42
@user5771881 I think you need to read a book or a tutorial about C. – 2501 Jan 29 '16 at 12:48
@user5771881 https://stackoverflow.com/questions/562303/the-definitive-c-book-guide-and-list – 2501 Jan 29 '16 at 12:50
Hey, I read the tutorial on the webpage you showed me and I solved it. Thanks for being patient with me. How can I give you points? – user5771881 Jan 29 '16 at 13:07
1

@user5771881 Great. If you found my answer to be helpful, please accept it (The green checkmark next to the answer on the left side), or upvote it. – 2501 Jan 29 '16 at 13:09

How to move a rectangle in SDL2 using C

1 Answers1