Console plotter

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

float linspace(float minv, float maxv, int nsteps, int n){
    return (maxv - minv)/ (float) nsteps * n + minv;
}

float rlinspace(float minv, float maxv, int nsteps, float n){
    return (nsteps * (n - minv) ) / (maxv - minv);
}

int calc_index(int w, int i, int j){
    return j * w + i;
}

int main()
{
    int console_w = 79, console_h = 20;

    float xmin = -2*M_PI, xmax = 2*M_PI;
    float ymin = -1.2, ymax = 1.2;

    int i, j;

    char x_axis_s = '|';
    char y_axis_s = '-';
    char empty_space_s = ':';
    char function_s = '*';

    // allocate array
    char char_arr[console_w * console_h];
    memset(char_arr, empty_space_s, console_w * console_h*sizeof(char) );

    // fill the array with axis
    for(i = 0; i < console_w - 1; i++){
        for(j = 0; j < console_h - 1; j++){

            float x = linspace(xmin, xmax, console_w, i);
            float x1 = linspace(xmin, xmax, console_w, i + 1);

            float y = linspace(ymin, ymax, console_h, j);
            float y1 = linspace(ymin, ymax, console_h, j + 1);

            int idx = calc_index(console_w, i, console_h -1 - j);

            if( x <= 0 && x1 > 0){
                char_arr[idx] = x_axis_s;
            }

            if (y <= 0 && y1 > 0){
                char_arr[idx] = y_axis_s;
            }
        }
    }

    // formula to print
    for(i = 0; i < console_w; i++){

        // get x
        float x = linspace(xmin, xmax, console_w, i);
        // calculate y
        float y = sin(x);

        // find where y is in the matrix
        int hc = console_h - rlinspace(ymin, ymax, console_h - 1, y);

        // assign the function symbol
        if(hc > 0 && hc < console_h){
            char_arr[calc_index(console_w, i, hc)] = function_s;
        }
    }

    for(j= 0; j < console_h; j++){
        for(i = 0; i < console_w; i++){
            printf("%c", char_arr[calc_index(console_w, i, j)]);
        }
        printf("\n");
    }

    return 0;
}