图灵语言中的计数循环

Question

% calculates the population of a city from year 2000 to 2020

var popstart : int := 80000
var popgrowth : real
var popend : real
var growthrate : real := 0.03

% popgrowth := popstart * growthrate
    for i : 2000..2020 by 1
popgrowth := popstart * growthrate
end for

put "year  population"
put "====  =========="
put  i, "  ", popgrowth

when I run the program, i get the error variable "i has not been declared" when I declare i as a variable, I get the error "i has already been declared"

the output should look something like this:

year population
==== ==========
2000 xxxxxxxxxx ~ 2020 XXXXXXXXXX

here's a similar, but much simpler program, where i was successful in doing what i'm attempting to do in the program above.

for i : 4 .. 19 by 3
    put i
end for

HELP PLEASE! and thanks in advance!

Answer 1

You have to declare i. i is a variable that starts from 2000 and goes up to 2020.

var i: int should do the trick.

Answer 2

Another answer, knowing even less about Turing.... I'm guessing that i is only in scope during the loop since that is where it is declared, so then it is throwing the error when you try to use i outside the loop at the end. Which explains why the smaller program you posted works but the larger one doesn't. And also why it would throw an error when you try to declare i first outside the loop and then declare it again in the loop statement.

I believe that is the same thing that Wolever was getting at, but didn't really explain why. (His answer should fix it if we're right about i only being valid in the loop)

Answer 3

Your main problem is that i is out of scope. The i can only be used within the for loop. Put the put statements within the for loop. Like this:

% calculates the population of a city from year 2000 to 2020

var popstart : real := 80000
var popgrowth : real
var popend : real
var growthrate : real := 0.03

% popgrowth := popstart * growthrate
for i : 2000..2020
    popgrowth := popstart * growthrate
    popstart += popgrowth
    put "year  population"
    put "====  =========="
    put  i, "  ", popstart
end for

@DavidWolever's Solution would only produce last year's population growth. Your program also has a logical error, in that it would only give the population growth rate, and not the total population.

I assume that the problem has been resolved with your teachers, because Turing is very much an obsolete language. It lost support years ago. I still remember some from ninth grade, but it may not be a good idea to post Turing related questions here, as nobody really understands the language and it's conventions.

Resources for help : http://touque.ca/EC/programming/Turing/

A bit more nitpicky detail that is unrelated: the for statement is not properly indented, and the by 1 is unnecessary.

Answer 4

I don't know much about Turing, but I suspect that the for i ... is an implicit declaration of i .

So, I don't know how you could fix it, but you could get around it by doing this:

var last_year: int
for i : 2000..2020 by 1
    popgrowth := popstart * growthrate
    last_year = i
end for

put "year  population"
put "====  =========="
put  last_year, "  ", popgrowth

Answer 5

% calculates population growth for city of Whitby between 2000 and 2020.

var popstart : int := 80000
var popgrowth : real
var growthrate : real := 0.03

put "year  population"
put "====  =========="
put "2000  80000"

popgrowth := popstart
for i : 2001 .. 2020 by 1
    popgrowth := popgrowth + (popgrowth * growthrate)
    put i, "  ", popgrowth:0:2
end for

Answer 6

I don't how to explain,but you can figure out whether this is right or not.(I make some change in your content.)

var popstart : int := 80000
var popgrowth : real
var popend : real
var growthrate : real := 0.03
var number:real
% popgrowth := popstart * growthrate
    for i : 2000..2020 by 1
    number:=i
popgrowth := popstart * growthrate
end for

put "year  population"
put "====  =========="
put  number, "  ", popgrowth