while statements

[Shawn Minisall]

I just learned about while statements and get why you place them around 
inputs for validation, but I'm a little lost on exactly where to place 
it with what condition in this program where the number of fat grams 
exceeds the total number of calories so that it loops back and asks you 
the two questions again instead of just saying The calories or fat grams 
were incorrectly entered.  Any idea's?

thx

while cal <=0:
    #Prompt for calories
        cal = input("Please enter the number of calories in your food: ")
        if cal <=0:
            print "Error.  The number of calories must be positive."

    #Prompt for fat
        fat = input("Please enter the number of fat grams in your food: ")
        if fat <=0:
            print "Error.  The number of fat grams must be positive."


    #Calculate calories from fat
        calfat = float(fat) * 9
        
    #Calculate number of calories from fat
        caldel = calfat / cal
 
    #change calcent decimal to percentage
        calcent = caldel * 100

    #evaluate input
    if calfat > cal:
        print "The calories or fat grams were incorrectly entered."

    elif calcent > 0 and calfat < cal:
    
        if caldel <= .3:
            print "Your food is low in fat."
        elif caldel >= .3:
            print "Your food is high in fat."

     
    #Display percentage of calories from fat
        print "The percentage of calories from fat in your food is %", 
calcent