About the random number in snake games - c++

Question

#include <iostream>
#include <string>
#include <windows.h>
#include <conio.h>
#include <time.h>
using namespace std;
int map_height=22;
int map_width=22;
string border="ùþ";
int snakeheadX=map_width/2-1;
int snakeheadY=map_height/2;
string head_symbol="¡·";
int snakesize;
bool gameover=false;
enum action{up, down, right1, left1, stop};
action moving=stop;
int foodx;
int foody;
void food();
void draw(){
system("cls");
food();
for(int i=0;i<map_width;i++){ // Game frame
cout<<border;
}
cout<<endl;
for(int i=0;i<map_height;i++){
cout<<border;
for(int j=0;j<map_width-2;j++){
if(i==snakeheadY && j==snakeheadX){
cout<<head_symbol;
}
else if(i==foody && j==foodx){
cout<<"¡¯";
}
else
cout<<" ";
}
cout<<border;
cout<<endl;
}
for(int i=0;i<map_width;i++){
cout<<border;
}
}
void running(int moving){
if(moving==up){ //Up
snakeheadY--;
}
if(moving==down){ //Down
snakeheadY++;
}
if(moving==left1){ //Left
snakeheadX--;
}
if(moving==right1){ //Right
snakeheadX++;
}
if(moving==stop){
snakeheadX=snakeheadX;
snakeheadY=snakeheadY;
}
}
void food(){
int prev_foodx=foodx;
int prev_foody=foody;
int a=rand()%(map_width-3)+1;
int b=rand()%(map_height-1)+1;
if(foodx==snakeheadX && foody==snakeheadY){
if(prev_foodx==a && prev_foody==b){
foodx=rand()%(map_width-3)+1;
foody=rand()%(map_height-1)+1;
}
else{
foodx=a;
foody=b;
}
}
}
void move_logic(int getkeyboarddown){
if(getkeyboarddown=='w'){ //Up
snakeheadY--;
moving=up;
}
if(getkeyboarddown=='s'){ //Down
snakeheadY++;
moving=down;
}
if(getkeyboarddown=='a'){ //Left
snakeheadX--;
moving=left1;
}
if(getkeyboarddown=='d'){ //Right
snakeheadX++;
moving=right1;
}
}
void game_logic(){
if (snakeheadX>=map_width || snakeheadX<=0 || snakeheadY<=0 || snakeheadY>=map_height){
cout<<endl<<"Game over!";
gameover=true;
}
}
int main()
{
srand(time(0));
foodx=rand()%(map_width-3)+1;
foody=rand()%(map_height-1)+1;
while(!gameover){
Sleep(50);
cout<<foodx<<" "<<foody;
running(moving);
draw();
game_logic();
if(kbhit()){
move_logic(getch());
}
}
return 0;
}
The code above is my code of snake game in C++. Although the game is not finished, I found the food may sometimes appear in same location after the snake ate. Therefore, in the function food(), I add a script of code below to prevent this state.
if(foodx==snakeheadX && foody==snakeheadY){
if(prev_foodx==a && prev_foody==b){
foodx=rand()%(map_width-3)+1;
foody=rand()%(map_height-1)+1;
cout<<"ok";
}
else{
foodx=a;
foody=b;
}
But I still found the food sometimes appear in same location after the snake ate that means the code I wrote is useless. How can I solve this problem?

Answer 1

The problem is in your move_logic and running functions. You aren't generating the food in the same position twice in a row by chance, what is actually happening is that when the player changes direction the snake actually moves twice, causing it to jump over a position. If you are passing by the food and change direction to get the food, you will actually jump over the food instead.
E.g. Starting with snakeheadX = 2, snakeheadY = 5 and moving = left1, say the player presses s to change direction. move_logic() increments snakeheadY to 6, then running() increments snakeheadY to 7, and only after that are draw() and food() called, so if the food was in position (2,6), then it was missed, and it looks like the snake went right through it, or like new food was generated in the same position.
To fix this, function move_logic() should not modify snakeheadX or snakeheadY. Only the running() function should modify them.
Aside from that, it looks like it should work, but there's a lot that could be cleaned up. E.g. You should use chained else if statements in functions running() and move_logic(). In function food(), prev_foodx and prev_foody aren't actually doing anything useful, because foodx and foody haven't changed at the time when prev_foodx and prev_foody are checked. food() also generated the random positions every time it is called, rather than only when necessary, and does allow food to be generated twice in the same position, albeit only with a chance of 1/22^4. So the randomness could be better implemented using a loop, like this:
void food() {
if (foodx == snakeheadX && foody == snakeheadY) {
int new_foodx;
int new_foody;
do
{
new_foodx = rand() % (map_width - 3) + 1;
new_foody = rand() % (map_height - 1) + 1;
} while (new_foodx == foodx && new_foody == foody);
foodx = new_foodx;
foody = new_foody;
}
}

Answer 2

I don't see what's causing you're issue, but one change you can make to prevent the code from randomly generating the same x and y coordinates two times in a row is to replace the code
if(prev_foodx==a && prev_foody==b){
foodx=rand()%(map_width-3)+1;
foody=rand()%(map_height-1)+1;
}
with
if(prev_foodx==a && prev_foody==b){
food();
}
The food function would then keep calling itself until it comes up with new x and y coordinates for the food. You can also optimize the function so it is not generating random numbers every time the screen is drawn by moving the if statement above the variables you're declaring like so.
void food(){
if(foodx==snakeheadX && foody==snakeheadY){
int prev_foodx=foodx;
int prev_foody=foody;
int a=rand()%(map_width-3)+1;
int b=rand()%(map_height-1)+1;
if(prev_foodx==a && prev_foody==b){
food();
} else{
foodx=a;
foody=b;
}
}
}
Also the name food does not tell me what is going on at all. Perhaps in the draw function you could say
if(foodx==snakeheadX && foody==snakeheadY){
changeFoodPos();
}`
and then instead of your food function, you have a changeFoodPos function. It's always better to have functions that are for one specific purpose like changing the position of food, than to you have a function that could be doing anything with food. Also, you don't need the prev_foodx and prev_foody variables. You could just do
void changeFoodPos(){
if(foodx==snakeheadX && foody==snakeheadY){
int newX = rand()%(map_width-3)+1;
int newY = rand()%(map_height-1)+1;
if(foodx==newX && foody==newY){
changeFoodPos();
} else{
foodx = newX;
foody = newY;
}
}
}
Anyhow, I know this doesn't directly answer you're question. It looks like David Scarlett was able to figure out what the issue was, but I hope this helps you out as you're learning to code.