SDL questions

I was following a tutorial and I created this simple program that displays some graphics. I have a question about it.

Here is the code :

Code:

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#include "SDL/SDL.h"
#include "SDL/SDL_image.h"
#include "SDL/SDL_ttf.h"
#include <string>
using namespace std;

const int SCREEN_WIDTH = 640;
const int SCREEN_HEIGHT = 480;
const int SCREEN_BPP = 32;

SDL_Surface *background = NULL;
SDL_Surface *board = NULL;
SDL_Surface *circle = NULL;
SDL_Surface *x = NULL;
SDL_Surface *title = NULL;
SDL_Surface *screen = NULL;

SDL_Surface *arrowKey = NULL;

SDL_Surface *upMessage = NULL;
SDL_Surface *downMessage = NULL;
SDL_Surface *leftMessage = NULL;
SDL_Surface *rightMessage = NULL;

SDL_Event event;

TTF_Font *font = NULL;
SDL_Color textColor = {255, 255, 255};


SDL_Surface *load_image(string filename)
{
        //Temporary storage for the image that's loaded.
        SDL_Surface *loadedImage = NULL;

        //The optimized image that will be used.
        SDL_Surface *optimizedImage = NULL;

        //Load the image.
        loadedImage = IMG_Load(filename.c_str());

        //If image loaded
        if(loadedImage != NULL)
        {
                //Create an optimized image.
                optimizedImage = SDL_DisplayFormat(loadedImage);

                //Free the old image.
                SDL_FreeSurface(loadedImage);

                /*
                if(optimizedImage != NULL)
                {
                        //Map the color key
                        Uint32 colorkey = SDL_MapRGB(optimizedImage->format, 0xFF, 0xFF, 0xFF);

                        //Set all pixels of that color to be transparent.
                        SDL_SetColorKey(optimizedImage, SDL_SRCCOLORKEY, colorkey);
                }       
                */               
        }
        return optimizedImage;
}
void ApplySurface(int x, int y, SDL_Surface *source, SDL_Surface *destination)
{
        //Make a temporary rectange to hold the offsets.
        SDL_Rect offset;

        //Give the offsets to the rectangle.
        offset.x = x;
        offset.y = y;

        //Blit the surface.
        SDL_BlitSurface(source, NULL, destination, &offset);
}
bool init()
{
        //Initialize all SDL subsystems.
        if(SDL_Init(SDL_INIT_EVERYTHING) == -1)
        {
                return false;
        }

        //Set up screen.
        screen = SDL_SetVideoMode(SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP, SDL_SWSURFACE);

        //If there was an error.
        if(screen == NULL)
        {
                return 1;
        }

        //Initialize SDL_ttf
        if(TTF_Init() == -1 )
        {
                return false;
        }

        //Set caption.
        SDL_WM_SetCaption("Event Test", NULL);

        //If everything initialized fine.
        return true;
}
bool load_files()
{
        //Load the images.
        background = load_image("blueBackground.png");
        board = load_image("tBoard.png");
        x = load_image("xMark.png");
        circle = load_image("oMark.png");

        //Open the font.
        font = TTF_OpenFont("arial.ttf", 24);
       
        if(font == NULL)
        {
                return false;
        }       
       
        //If there was an error.
        if(background == NULL)
        {
                return false;
        }

        //If everything loaded fine.
        return true;
}
void clean_up()
{
        //Free the surfaces.
        SDL_FreeSurface(background);
        SDL_FreeSurface(board);
        SDL_FreeSurface(x);
        SDL_FreeSurface(circle);
       
        //Quit SDL.
        SDL_Quit();
}
int main(int argc, char *args[])
{
        //Make sure program waits for a quit.
        bool quit = false;

        //Initialize.
        if(init() == false)
        {
                return 1;
        }

        //Load the files.
        if(load_files() == false)
        {
                return 1;
        }

        //Render text.
        title = TTF_RenderText_Solid(font, "This is the Title.", textColor);

        upMessage = TTF_RenderText_Solid(font, "The up button was pressed.", textColor);
        downMessage = TTF_RenderText_Solid(font, "The down button was pressed." , textColor);
        leftMessage = TTF_RenderText_Solid(font, "The left button was pressed." , textColor);
        rightMessage = TTF_RenderText_Solid(font, "The right button was pressed." , textColor);

        if(title == NULL)
        {
                return 1;
        }       

        ApplySurface(0,0,background, screen);
        ApplySurface(500,25,title,screen);

        //Update screen.
        if(SDL_Flip(screen) == -1)
        {
                return 1;
        }

        while(quit == false)
        {
                //While there is an event to handle.
                while(SDL_PollEvent(&event))
                {
                        //If a key was pressed.
                        if(event.type == SDL_KEYDOWN)
                        {
                                switch(event.key.keysym.sym)
                                {
                                        case SDLK_UP: arrowKey = upMessage;
                                                break;
                                        case SDLK_DOWN: arrowKey = downMessage;
                                                break;
                                        case SDLK_LEFT: arrowKey = leftMessage;
                                                break;
                                        case SDLK_RIGHT: arrowKey = rightMessage;
                                                break;
                                }
                        }
                        //If the user has Xed out of the window.
                        else if(event.type == SDL_QUIT)
                        {
                                //Quit the program.
                                quit = true;
                        }
                }
                //If a message needs to be displayed.
                if(arrowKey != NULL)
                {
                        ApplySurface(0,0,background, screen);
                        ApplySurface((SCREEN_WIDTH - arrowKey->w)/2, (SCREEN_HEIGHT - arrowKey->h)/2, arrowKey, screen);

                        arrowKey = NULL;
                }

                if(SDL_Flip(screen) == -1)
                {
                        return 1;
                }
        }

        clean_up();

        return 0;
}

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The title is displayed when the program is first started. Then, when I press a key, a new message is displayed. It appears as if the new message, along with a new copy of the background, are being placed over the title and original background. So, each time a key is pressed, a new copy of the background and message are applied. Is this the correct way to do this? It seems after a long time, this would be very inefficient. I was thinking whenever I would want to display a new background or message, that I would replace the old with the new. But, i'm not sure how I would do this here.

Any help would be appreciated.
Thanks.

PS: i just started SDL a few days ago.

The short answer is you have to clear the screen surface between button presses to attain the desired effect.

There are a few ways of doing it, the most straight forward way is to use

Code:

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int SDL_FillRect  (SDL_Surface *dst, SDL_Rect *dstrect, Uint32 color);

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To fill the the screen's rectangle ( if you don't have one make one or use the surfaces rectangle ).