SDL2: Why a figure "blinks"?

Question

I'm trying to move a square in a rectangle in SDL, but when i launch it, the rectangle and the square "blinks" until I move the square with up, left, right or down. When the square moves, the rectangle stop blinking and only the square keeps blinking. How can i move it without the blink?

Edit: The problem was that i put SDL_RenderPresent twice after the main loop. Thanks for your help.

#include <SDL2/SDL.h>
#include <stdio.h>
#include <stdbool.h>
#include <string.h>
#include <SDL2/SDL_image.h>

const int SCREEN_WIDTH = 640;
const int SCREEN_HEIGHT = 500;


int main() {
    SDL_Init(SDL_INIT_VIDEO);
    SDL_Window* window = SDL_CreateWindow("", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, SCREEN_WIDTH, SCREEN_HEIGHT, SDL_WINDOW_OPENGL);
    SDL_Renderer* renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC);

    bool out = false;
    SDL_Event event;

    SDL_Rect grid;
    grid.x = 25;
    grid.y = 25;
    grid.w = 300;
    grid.h = 450;

    SDL_Rect square;
    square.x = 175;
    square.y = 25;
    square.w = 25;
    square.h = 25;

    //Main loop
   
    while (out==false){
        while (SDL_PollEvent(&event)){
            if (event.type==SDL_QUIT){out=true;}
            else if (event.type==SDL_KEYDOWN){
              // Keys
                switch (event.key.keysym.sym){
                    case SDLK_m:
                        out=true;
                        break;
                    case SDLK_LEFT:
                        square.x-=25;
                        SDL_RenderPresent(renderer);
                        break;
                    case SDLK_RIGHT:
                        square.x+=25;
                        SDL_RenderPresent(renderer);
                        break;
                    default:
                        break;
                        }
                    }
                }

        SDL_SetRenderDrawColor(renderer,0,255,0,0);
        SDL_RenderFillRect(renderer, &grid);
        SDL_RenderPresent(renderer);

        SDL_SetRenderDrawColor(renderer,255,0,0,255);
        SDL_RenderFillRect(renderer,&square);
        SDL_RenderPresent(renderer); 
        }
    SDL_DestroyRenderer(renderer);
    SDL_DestroyWindow(window);
    SDL_Quit();
    return 0;
}

Answer 1

As @HolyBlackCat pointed out, you should only call SDL_RenderPresent() once per frame, not once per shape or keypress:

#include <SDL.h>
#include <SDL_image.h>
#include <stdbool.h>

int main()
{
    SDL_Init( SDL_INIT_VIDEO );
    SDL_Window* window = SDL_CreateWindow(
        "",
        SDL_WINDOWPOS_UNDEFINED,
        SDL_WINDOWPOS_UNDEFINED,
        640,
        500,
        SDL_WINDOW_OPENGL );
    SDL_Renderer* renderer = SDL_CreateRenderer(
        window,
        -1,
        SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC );

    bool out = false;
    SDL_Event event;

    SDL_Rect grid;
    grid.x = 25;
    grid.y = 25;
    grid.w = 300;
    grid.h = 450;

    SDL_Rect square;
    square.x = 175;
    square.y = 25;
    square.w = 25;
    square.h = 25;

    //Main loop
    while( out == false )
    {
        while( SDL_PollEvent( &event ) )
        {
            if( event.type == SDL_QUIT )
            {
                out = true;
            }
            else if( event.type == SDL_KEYDOWN )
            {
                // Keys
                switch( event.key.keysym.sym )
                {
                case SDLK_m:
                    out = true;
                    break;
                case SDLK_LEFT:
                    square.x -= 25;
                    break;
                case SDLK_RIGHT:
                    square.x += 25;
                    break;
                default:
                    break;
                }
            }
        }

        SDL_SetRenderDrawColor( renderer, 0, 255, 0, 0 );
        SDL_RenderFillRect( renderer, &grid );

        SDL_SetRenderDrawColor( renderer, 255, 0, 0, 255 );
        SDL_RenderFillRect( renderer, &square );
        
        SDL_RenderPresent( renderer );
    }

    SDL_DestroyRenderer( renderer );
    SDL_DestroyWindow( window );
    SDL_Quit();
    return 0;
}