Moving a character in a table

I just started learning programming languages and I want to make a character (point '*') in a table move.

This is my code

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

int main() {
    int v;
    int x = 50, y = 10;
    int i, j;
    char screen[80][25];

    // declare and initialize table
    for (i = 0; i < 80; i++)
        for (j = 0; j < 25; j++)
            screen[i][j] = ' ';

    // coordinate system
    for (i = 0; i < 80; i++) screen[i][12] =  '-';
    for (j = 0; j < 25; j++) screen[40][j] =  '|';

    // point, position
    screen[x][y] = '*';

    // print result
    for (j = 0; j < 25; j++) {
        for (i = 0; i < 80; i++)
            printf("%c", screen[i][j]);
        printf("\n");
    }
}

This prints a table of size 80x25 and a coordinate system with a centre in (40,12). I set the position of the character '*' on coordinates (x,y). I defined x and y as 50 and 10.

Now I want to move my star by changing x and y. How do I change x and y (position of the star)? Maybe with scanf function? I tried to use scanf like this:

int v;
...
scanf("%d", &v);
if(v == 1)
{
    y--;
}

but then everything (table, coordinate system and the character) disappeared. Please help.

Thanks.