File content as of revision 0:f58f38896cfa:

// code for part 1, 2, & 3
#include "mbed.h"
#include "MCP23S17.h"

// 1
DigitalIn dip_switch8(p21);
DigitalOut green_led(p22);
// 2
DigitalIn dip_switch1(p26);
DigitalIn dip_switch2(p25);
PwmOut blue_led(p23);
// 3
SPI spi(p5, p6, p7); // SPI bus
char Opcode = 0x40; // 8-bit address for writes
MCP23S17 chip = MCP23S17(spi, p20, Opcode); // create an MCP23S17

int switch_state;

int main()
{
    // 2
    dip_switch1.mode(PullUp); // configure internal pull up resistor
    dip_switch2.mode(PullUp);
    
    blue_led.period(0.016f); // ~ 60 Hz
    blue_led.write(0.50f); // 50 % duty cycle
    
    // 3
    chip.direction(PORT_A, 0x00); // set all Port A bits to output
    chip.direction(PORT_B, 0xFF); // set all B to input
    switch_state = 0;
    
    while(1) {
        // 1
        green_led = dip_switch8;
        
        // 2
        if (!dip_switch1) {
            blue_led.write(0.25f); // half start brightness, half duty cycle
        }
        else if (!dip_switch2) {
            blue_led.write(0.125f); // 1/4 starting brightness
        } else {
            blue_led.write(0.50f); // initial
        }
        
        // 3
        // get the state of the switch (0 or 1) and flip it, 0 when off
        switch_state = !chip.read(PORT_B) & 0x01;
        // write the saved value to MCP23S17 Port A
        chip.write(PORT_A, switch_state);
        wait(.2);
        // yellow led should change opposite the green led
    }
}