Saya membuat program sederhana untuk menghitung total penjualan yang melibatkan if/else dan terus mendapatkan pesan kesalahan

Program ini menghitung diskon (jika pelanggan adalah seorang guru) dan pajak penjualan dan menemukan total penjualan. Saya terus mendapatkan kesalahan:

[Peringatan] perbandingan antara pointer dan integer [diaktifkan secara default]

#include <stdio.h>
#include <math.h>

#define SALES_TAX .05
#define DISCOUNT_LOW .10
#define DISCOUNT_HIGH .12
#define DISCOUNT_LIMIT .100

int main(void)
{
    double purchase_total;
    double discount;
    double discounted_total;
    double sales_tax;
    double total;
    int teacher;
    FILE* output_file;

    /* request inputs */
    printf("Is the customer a teacher (y/n)?");
    scanf("%d", &teacher);
    printf("Enter total purchases.");
    scanf("%lf", &purchase_total);

    /* calculations for teacher */
    if (teacher == "y");
    {/*calculate discount (10% or 12%) and 5% sales tax */
        /* purchase total less than 100 */
        if (purchase_total < 100)
        {
            /* calculate 10% discount */
            discount = purchase_total * DISCOUNT_LOW;
            discounted_total = purchase_total - discount;
        }

        /*purchase total greater than 100 */
        else
        {   /* calculate 12% discount */
            discount = purchase_total * DISCOUNT_HIGH;
            discounted_total = purchase_total - discount;
        }

        printf("Total purchases    $%f\n", purchase_total);
        printf("Teacher's discount (12%%)    %fs\n", discount);
        printf("Discounted total     %f\n", discounted_total);
        printf("Sales tax (5%%)    %f\n", sales_tax);
        printf("Total     $%f\n", total);
    }


    /* calculation for nonteacher */
    if (teacher =="n");
    {
        /* calculate only 5% sales tax */
        sales_tax = purchase_total *  sales_tax;
        total = purchase_total + sales_tax;

        printf("Total purchases    $%f\n", purchase_total);
        printf("Sales tax (5%%)    %f\n", sales_tax);
        printf("Total     $%f\n", total);
    }

    return (0);
}