有人知道为什么我的变量(x)在这个程序中将其值从3更改为0(x++后的1)吗?我试图创建一个注册系统,但它会覆盖

就像标题所说的,我试图创建一个注册系统,但由于某种原因,它覆盖了应该是defaultuser[0]=“12291955”的数组,它变成了我输入的内容,除了我想让它转到defaultuser[3](用x=3表示)。所以基本上,注册系统应该添加第四个用户。(以下是我的main.c文件的头文件)

#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
#include <string.h>

void loginsystem(){
    char defaultuser[][100] = {"12291995", "12221999", "12241344"};
    char defaultpass[][100] = {"test1", "test2", "test3"};
  

    int i = 0, indicator = 1, x = 3;
    char userinput[20], passinput[20], choicest[20], prog[20]; 
    const char* choicecmp;
    system("cls");
    printf("\nSystem");
    printf("\nWould you like to create an account or log into an existing account? (Type Signup or Login) ");
    scanf(" %[^\n]s", choicest);
    printf("%d", x); //x=3
    choicecmp = strlwr(choicest);
    if(strcmp(choicecmp, "signup")==0 || strcmp(choicecmp, "sign up") == 0 )
    {
        //system("cls"); 
        printf("\nEnter your ID Number: ");
        scanf(" %s", defaultuser[x]);
        printf("\nEnter your password: ");
        scanf(" %s", defaultpass[x]);
   
        getch();
        choicecmp = "login";
        x++; 
        printf("%d", x); //x becomes 1 for some reason
        getch();
    }
    if(strcmp(choicecmp, "login")==0 || strcmp(choicecmp, "log in") == 0 ){
    do{
    system("cls");
    printf("\nPlease login!");
    printf("\n\nUsername (ID Number): ");
    scanf(" %s", &userinput);
    printf("\nPassword: ");
    do{
            passinput[i] = getch();
            if(passinput[i] != '\r')
            printf("*");
            i++;
        }while(passinput[i-1] != '\r');
        passinput[i-1] = '\0';

    for(int i = 0; i < x; i++){ 
    if(strcmp(userinput, defaultuser[i]) == 0 && strcmp(passinput, defaultpass[i]) == 0)
    {
        system("cls");
        indicator = 0;
        printf("\nWelcome back User");
    }}
    return ;
}

我试着调试代码,结果发现int x=3在注册系统中变成了x=0,在x++之后变成了1。