Variable not getting updated in code running on mbed board - mbed

Question

#include "mbed.h"
DigitalOut myled1(LED1);
DigitalOut myled2(LED2);
DigitalOut myled3(LED3);
DigitalOut myled4(LED4);
Serial pc(USBTX, USBRX);
int main()
{ int c;
while(1)
{
pc.scanf("%d",&c);
pc.printf("hey i received %d",c);
if (c==1050)
{myled1=1;
myled2=0;
myled3=0;
myled4=0;
}
else if (c==2050)
{ myled1=0;
myled4=0;
myled3=0;
myled2=1;
}
else if (c==3050)
{myled3=1;
myled4=0;
myled2=0;
myled1=0;
}
else if (c==4050)
{myled4=1;
myled3=0;
myled2=0;
myled1=0;
}
}
}
The above code receives value of 'c' from terminal and takes decision based on the same. The issue I am facing is that when I use the printf, variabl c gets updated and the code runs properly otherwise not. Please help.

Answer 1

Serial data is transmitted one byte at a time. When you send a multiple digit integer by writing 1050 to the terminal, each number is sent individually. The function getNumber() below collects all bytes until carriage return is received. By subtracting the ASCII value of 48 (which is 0) the received character is converted to integer. The number is shifted left by multiplying with 10 before the received integer is added to it.
Best approach would be to use interrupts. See the example code below:
#include "mbed.h"
DigitalOut myled1(LED1);
DigitalOut myled2(LED2);
DigitalOut myled3(LED3);
DigitalOut myled4(LED4);
Serial pc(USBTX, USBRX);
unsigned int getNumber()
{
unsigned int val = 0; // Max value 65535
while(1)
{
char c = pc.getc();
pc.putc(c); // echo back to terminal
if (c == '\r') // enter pressed
break;
val = ((c - 48) + (val * 10));
}
return val;
}
void callback()
{
unsigned int n = getNumber();
pc.printf("Number = %d \r\n", n); // Write number to terminal
if (n==1050)
{
myled1=1;
myled2=0;
myled3=0;
myled4=0;
}
else if (n==2050)
{
myled1=0;
myled2=1;
myled3=0;
myled4=0;
}
else if (n==3050)
{
myled1=0;
myled2=0;
myled3=1;
myled4=0;
}
else if (n==4050)
{
myled1=0;
myled2=0;
myled3=0;
myled4=1;
}
}
int main() {
pc.attach(&callback);
while (1)
{
wait(0.5);
}
}