while statements

[martyw]

Shawn Minisall wrote:
> 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
> 
you could change to something like this


while True: # don test in the loop
    #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."
         continue ### here you don need to go any further in this loop

    #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."
         continue ### here you don need to go any further in this loop


    #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:
     else: # calcent will be bigger than zero now by construction,
           # and I guess you dont want to test for equality of
           # calfat and 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 %f%%" % calcent
         break # we got a satisfactory result, leave this loop