Setting a pin on PORTA in the PIC18F452 high: Compilation of code failed
For setting an output you should use  LATAbits.RA0 = x instead of  PORTAbits.RA0 = x.

Also, try adding some brackets. Try this:

#include &lt;p18f452.h&gt;

void main (void)
{
    // array containing vital sign values 
    unsigned char pulseR = 0x32;  // syntax error 
    unsigned char sysP = 0x64;
    unsigned char diasP = 0x26;

    // acceptable limits for vital signs according to relavant heath officials
    unsigned char pulseR_ULimit = 0xA0;
    unsigned char pulseR_LLimit = 0x28;

    unsigned char diastolic_ULimit = 0x5A;
    unsigned char diastolic_LLimit = 0x3C;

    unsigned char systolic_ULimit = 0x8C;
    unsigned char systolic_LLimit = 0x5A;

    TRISAbits.TRISA0 = 0;

    //Comparing obtained values with acceptable limits
    if ((sysP &lt; systolic_LLimit)||(sysP &gt; systolic_ULimit))
    {
        LATAbits.RA0 = 1;
    }
    else if ((diasP &lt; diastolic_LLimit)||(diasP &gt; diastolic_ULimit)) 
    {
        LATAbits.RA0 = 1 ;    
    }
    else if ((pulseR &lt; pulseR_LLimit)||(pulseR &gt; pulseR_ULimit)) 
    {
        LATAbits.RA0 = 1;
    }
    else 
    {                
        LATAbits.RA0 = 0;
    }
}