Help - Lisp program to calculate the sum of first N positive integers

fernanroy · Oct 21, 2007

Homework Statement

Write a Lisp program to calculate the sum of the first N positive integers where, for example, when N = 6 the sum of the first N = 21.

Homework Equations

If you try this out in tlisp at http://www.ugcs.caltech.edu/~rona/tlisp/ please note that the syntax for N - 1 is (+ N -1).

Or if you try it out in the console of Allegro Common Lisp from http://www.franz.com the syntax is (- N 1).

If you define your function as, for example, nsum, then the syntax for calling it with N = 6 is (nsum 6)

The Attempt at a Solution

Ok, I am no programmer so I don't even know where to start. This is not a programming class so I am not sure where to start.

Any help or direction would be appreciated!

KataKoniK · Oct 21, 2007

my first step would be to break down the problem into smaller, more manageable parts. In this case, we need to calculate the sum of the first N positive integers, so we can start by defining a function that takes in a value of N and returns the sum. Let's call this function "nsum."

Next, we need to think about how to actually calculate the sum. One approach would be to use a loop to add each integer from 1 to N together. Another approach would be to use a mathematical formula for calculating the sum of consecutive numbers, which is (N * (N + 1)) / 2.

Let's try the first approach. We can use a "for" loop in Lisp to iterate through the numbers from 1 to N and add them together. The syntax for a "for" loop in Lisp is (for <variable> from <start> to <end> do <body>). So our function might look something like this:

(defun nsum (N)
(let ((sum 0))
(for i from 1 to N do
(setf sum (+ sum i)))
sum))

This function first creates a variable "sum" and sets it to 0. Then it uses a "for" loop to iterate through the numbers from 1 to N and add them to the sum variable. Finally, it returns the value of the sum variable.

To test this function, we can call it with different values of N. For example, (nsum 6) should return 21, (nsum 10) should return 55, and so on.

There are other ways to approach this problem, such as using recursion or a mathematical formula, so feel free to explore those options as well. The important thing is to break down the problem into smaller steps and try to think through the logic of how to solve it.

piercas · Oct 28, 2007

I can understand your confusion and hesitation in approaching this programming problem. However, as a scientist, you have the skills and mindset to tackle this challenge. Here is a possible solution in Lisp:

(defun nsum (n)
(if (= n 1)
1
(+ n (nsum (- n 1)))))

This is a recursive function that takes in a positive integer N and returns the sum of the first N positive integers. It works by breaking down the problem into smaller subproblems, starting with the base case of N=1, where the sum is simply 1. Then, for any larger N, the function adds N to the sum of the first (N-1) positive integers, which is calculated by calling the function recursively.

To test this function, you can call it with different values of N, such as (nsum 6) which should return 21, or (nsum 10) which should return 55.

I hope this helps you understand the problem and come up with a solution in Lisp. Remember, as a scientist, you have the ability to break down complex problems and find creative solutions. Good luck!

Help - Lisp program to calculate the sum of first N positive integers

Homework Statement

Homework Equations

The Attempt at a Solution

1. What is a Lisp program?

2. What is the purpose of the "Help - Lisp program to calculate the sum of first N positive integers" program?

3. What is the input for this program?

4. How does the program calculate the sum of the first N positive integers?

5. What is the output of the program?

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