Attempting to set a specific pixel in SDL2.0 fails - c

Question

I am attempting to write a chip-8 emulator, and already have the processor all wrapped up. I am attempting to write a small program to experiment with rendering a chip-8 ROM in SDL. I have ran into a snag however. In the below program:
#include <stdio.h>
#include <SDL2/SDL.h>
const int SCREEN_HEIGHT = 320;
const int SCREEN_WIDTH = 640;
void put_pixel_xy(SDL_Renderer *renderer, int x, int y, unsigned char r, unsigned char g, unsigned char b) {
SDL_SetRenderDrawColor(renderer, r, g, b, 255);
SDL_RenderDrawPoint(renderer, x, y);
}
int main() {
if(SDL_Init(SDL_INIT_EVERYTHING) < 0) {
fprintf(stderr, "error: SDL could not initialize: %s\n", SDL_GetError());
return 1;
}
SDL_Window *window = SDL_CreateWindow("Chip-8 Emulator", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, SCREEN_WIDTH, SCREEN_HEIGHT, SDL_WINDOW_SHOWN);
if(window == NULL) {
fprintf(stderr, "error: could not create SDL window: %s\n", SDL_GetError());
return 1;
}
SDL_Renderer* renderer = SDL_CreateRenderer(window, -1, 0);
SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
SDL_RenderClear(renderer);
SDL_RenderPresent(renderer);
SDL_Delay(2000);
for(int x = 0; x < SCREEN_WIDTH; ++x) {
for(int y = 0; y < SCREEN_HEIGHT/2; ++y) {
put_pixel_xy(renderer, x, y, 255, 255, 255);
}
}
SDL_RenderPresent(renderer);
SDL_Delay(5000);
SDL_DestroyWindow(window);
SDL_Quit();
return 0;
}
I am attempting to make half of the screen white, and half black. However, whenever I run this program I get the following result:
Any help as to why this is happening would be much appreciated. Thank you!