Thanks for your attention... I am really confused.. Why I cannot convert char type to uppercase with toupper when I gave the char an array... like this "char drh[1]"?

Here is my code.. This code is not finished yet.. I'm stuck in the midway.

```
#include <stdio.h>
#include <conio.h>
#include <stdlib.h>
#include <ctype.h>

using namespace std;
int main()
{
    char nama[50], gender[6], drh[1];
    int tahun_l, usia;
    float berat, tinggi, berat_i;

    printf("Masukkan Data Anda\n\n");
    // INPUT ZONE

    //First
    printf("Nama\t\t\t\t: ");
    scanf("%[^\n]", &nama);

    printf("Tahun Lahir (yyyy)\t\t: ");
    scanf("%d", &tahun_l);

    printf("Gol Darah (A/B/C)\t\t: ");
    scanf("%s", &drh);

    system("cls");

    //Second
    printf("Masukkan Data Anda\n\n");

    printf("Nama\t\t\t\t: %s\n", nama);
    fflush(stdin);

    printf("Tahun Lahir (yyyy)\t\t: %d\n", tahun_l);
    fflush(stdin);

    printf("Gol Darah (A/B/O)\t\t: %s\n", drh);
    drh = toupper(drh); //not working
    fflush(stdin);

    printf("Jenis Kelamin (Pria/Wanita)\t: ");
    scanf("%s", &gender);

    printf("Berat Badan (kg)\t\t: ");
    scanf("%f", &berat);

    printf("Tinggi Badan (cm)\t\t: ");
    scanf("%f", &tinggi);

    //FORMULA ZONE
    usia    = 2008 - tahun_l;
    berat_i = tinggi - 100 - (0.1 * (tinggi - 100));
    //OUTPUT ZONE
    printf("\n%s, berdasarkan data yang Anda masukkan,\n", nama);
    printf("Anda berjenis kelamin %s, saat ini Anda berusia %d tahun,\n", gender, usia);
    printf("Berat badan = %.2f kg, tinggi badan = %.2f cm, golongan darah = %s \n", berat, tinggi, drh);
    printf("\nBerat badan ideal adalah %.2f", berat_i);

    getche();
    return 0; 
}

```