Help Applying SDL_Surface on an array of SDL_Surface?

Question

I'm trying to make a 4x4 blocks (50x50 px) with SDL Surface. Added another block (50x50 px, I wrote the variable as 'flag') that can only move within the 4x4 blocks, so I can select one of the blocks with this one. The problem is when I move it (say I move it to flag_x + 50), the previous surface (flag_x) is still there, even though I already use blit surface. It's like taking a multiple snapshots of walking person. How can I move 'flag' normally within the 4x4 blocks? Or do I use the wrong method?

Below is the code

#include "SDL/SDL.h"
#include "SDL/SDL_image.h"
#include "SDL/SDL_ttf.h"
#include <string>
#include <sstream>

using namespace std;

//Screen attributes
const int SCREEN_WIDTH = 640;
const int SCREEN_HEIGHT = 480;
const int SCREEN_BPP = 32;

//The frame rate
const int FRAMES_PER_SECOND = 20;

//The surfaces
SDL_Surface *screen = NULL;
SDL_Surface *matchbox[4][4];
SDL_Surface *flag;    

....
....
....

bool load_files()
{    
    flag = load_image("matchbox2.png");
    if( flag == NULL )
   {
       return false;
   }

   for(int i=0;i<4;i++){
      for(int j=0;j<4;j++){
         matchbox[i][j] = load_image("matchbox.png");                  
         if( matchbox[i][j] == NULL )
            return false;
      }
   }
   return true;
}

void clean_up()
{
//Free the surfaces
SDL_FreeSurface( flag );

for(int i=0;i<4;i++){
   for(int j=0;j<4;j++){ 
      SDL_FreeSurface(matchbox[i][j]); 
      matchbox[i][j] = NULL; 
   }
} 
//Quit SDL
SDL_Quit();
}

int main( int argc, char* args[] )
{    
   for(int i=0;i<4;i++){
      for(int j=0;i<4;i++) { 
         matchbox[i][j] = NULL; 
      } 
}

   Timer fps;

   bool quit = false;

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

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

   int matchbox_x = 50;
   int matchbox_y = 50;
   int flag_x = 50;
   int flag_y = 50;

   //Fill the screen white
   SDL_FillRect( screen, &screen->clip_rect, SDL_MapRGB( screen->format, 0xFF, 0xFF, 0xFF ) );

   //Apply matchbox to the screen
   for(int i=0;i<4;i++){
      for(int j=0;j<4;j++){ 
         apply_surface( matchbox_x , matchbox_y , matchbox[i][j], screen );
         matchbox_x += 50;
      }
      matchbox_x = 50;
      matchbox_y += 50;
   }

   //key event
   while( quit == false )
   {
      fps.start();   

      //While there's events to handle
      while( SDL_PollEvent( &event ) )
      {
         //If a key was pressed
         if( event.type == SDL_KEYDOWN )
         { 
            switch( event.key.keysym.sym )
            {
                case SDLK_UP: flag_y -= 50; break;
                case SDLK_DOWN: flag_y += 50; break;
                case SDLK_LEFT: flag_x -= 50; break;
                case SDLK_RIGHT: flag_x += 50; break;
            }
         }
         else if( event.type == SDL_QUIT )
         {
            quit = true;
         }
     }

     //collision detection
     if(flag_x < 50)
        flag_x += 50;
     if(flag_x > 200)
        flag_x -= 50;

     if(flag_y < 50)
        flag_y += 50;
     if(flag_y > 200)
        flag_y -= 50;

     apply_surface(flag_x, flag_y, flag, screen);

     //Update the screen
     if( SDL_Flip( screen ) == -1 )
     {
        return 1;
     }
     //Cap the frame rate
     if( fps.get_ticks() < 1000 / FRAMES_PER_SECOND )
     {
        SDL_Delay( ( 1000 / FRAMES_PER_SECOND ) - fps.get_ticks() );
     }

  }


  //Free the surfaces and quit SDL
  clean_up();

return 0;
}

function examples taken from http://lazyfoo.net/SDL_tutorials/index.php

Answer 1

You need to add in a Draw( ) method that will redraw your entire map every time a player had issue an event. This is typical for game programming, because when graphics are placed onto the screen, it stays there until you instruct it to be erase or "redraw" over the existing graphics.

Within your main, add a Draw( ) method at the end of it, but before the fps.get_ticks( ) calculation kicks in.

This allows the computer to accurately calculate your fps putting into consideration of all the game logic and draw cycle.

I hope this helps.