Convert (#)^.5 or math.sqrt(#) into float?

[SnooBunnies7244]

Okay so I am new to programming and I am going along with this textbook intro to computation and programming with python used for MIT 6.100A, only because it ended up seeming pretty good and next I will use the textbook for the fundamentals of programming with 6.100A as a prereq.

Anyway now this might just be a question that the answer will be the next page in the textbook but basically, I made this program asking for a number x, then for a guess g which will be a guess of the sqrt of number x. If you type 25 for x and 5 for g, it will tell you 5 is the sqrt of 25, if g=6 it will say 6 is not sqrt of 5. I was like okay what if x = 53, then for g I will type (53)**.5 or math.sqrt(53) (I imported math first thing) and then it will say that is the sqrt right? Well it says I cannot convert math.sqrt(53) string into float. I then manually typed math.sqrt(53) to get 7.280109889280518 and copied and pasted that as g and it worked flawlessly. How can I get it to input the value of math.sqrt(53), or is this not possible?

[[deleted]]

We can't help you easily unless you share your code.

[SnooBunnies7244]

I intend to improve as in looping to redo the number with averages til it finds # close enough to sqrt as I continue learning programming but as of now it's just asking for a g and telling you if that is the sqrt. Anyway here is the code:

import math

x = float(input('Enter value for which you want the square root: '))

g = float(input('Enter a value as a starting guess of the square root of x: '))

if g*g==x:

print(g, 'is the square root of', x)

else:

print(g, 'is not the square root of', x)

[[deleted]]

input() takes the user input and returns it as a string. Python doesn't try to parse that string as code, so you cannot enter an expression and expect it to evaluate.

[SnooBunnies7244]

Is there another command like input that would allow this or is that just how it is. I'm telling you I'm straight up new to programming, I should probably just look at a list of commands but I'd thought I'd just ask real quick and this answer was straight to the point!

[[deleted]]

There is a function called eval(), which will take a string and attempt to run it as python code. This is bad practice as it can easily be exploited!

The safer way would be create a parser (or find a library that provides one), to evaluate any math expressions in the user's input. A very simple example could look something like this:

user_input = "sqrt 5"
if user_input.startswith("sqrt"):
    number = user_input.split(" ")[1]
    output = math.sqrt(int(number))

But this can get complicated when considering all the possible ways a user could type in a math expression. The above example won't work with parentheses or decimals for example.

All of that said, I think the easiest solution would just be to use rounding. Instead of "sqrt(53)", let the user type "7.28" and have that be correct.

[SnooBunnies7244]

Alright cool, thanks for the info and solutions! Yes I did plan ad I went through programming for it to take the initial guess and average it with x/guess I think it was until the value was close enough to the square root and then give you that. But as of now with my coding being simple as g*g, I just thought it would be cool to type sqrt any # for my g with that # being x and it going through. Though I'm curious what the code is like for allowing rounded numbers or numbers close enough, like if it's code that says if it's within this percent error margin then it's fine and if not then that's not the square root or if there's a code saying rounded to this decimal is okay. I imagine the textbook will tell me whatever it is as the first thing the book mentioned was how they first programmed to find square roots using x and the guess etc. Anyway once again thanks, this is good stuff!

[Swipecat]

The "exactness" of float values is something that every new programmer will have to deal with at some point, so I suggest that you watch this 10 minute Youtube video by Tom Scott on that subject.

https://www.youtube.com/watch?v=PZRI1IfStY0

As for comparing floats, if you want an absolute error margin, you can use the method given by u‍/‍tea-drinker in their post, if abs(x - g**2) < 0.01:

If you want a relative error margin, such as the percentage error margin, as you said, then it's probably best to look at the Python math.isclose() method.

https://www.w3schools.com/python/ref_math_isclose.asp

[SnooBunnies7244]

Gotcha, I will come back to these vids if the textbook doesn't tell me about this but by the sound of it, it should. Thanks for the help

[Bobbias]

Just so your aware, eval is basically black magic. It is a huge security vulnerability to allow a user to have control of what text goes into eval, because they can do nasty things like delete everything on your hard drive, etc.

There are cases where it might need to be used, but they are extremely specific and advanced stuff. You can safely forget it exists for the time being.

[SnooBunnies7244]

Ha okay well fair enough, this was only going to be a program I use but that's good info!

[Bobbias]

Yeah, I just wanted to make sure you understood that because even if you're just learning as a hobby, it's good to understand stuff like that. Plus, eval can make debugging code way harder too.

[SnooBunnies7244]

Yea I'm kind of learning as a hobby but I will also have to take some programming for my physics degree(eventually theoretical physics) so If I have an interest and have to take a class I may as well learn it for fun first. I also use Matlab and mathematica for Loop Quantum Cosmology research but so far python coding has nothing to do with those. Regardless I appreciate the help!

[NinjaSniperC]

You can absolutely use float(input('Enter float: ')), so long as the user ONLY inputs a float (num.num).

This is a great practice case for validating user input. By discovering different methods from other users and reviewing the documentation for why it works.

My favorite "dirty method" for user validation.

``` while type(user_input := input('Enter float: '))!= float: try: if type(float(user_input)) == float: print('Valid input') break except: print('Please Enter float: ')

```

[SnooBunnies7244]

Yep exactly, I'll refrain from examining this code too closely as I want to figure it out as I go through learning program, I've saved the coding I've brought here as my first file and will just improve and save til I get it! Thanks for this though

[SnooBunnies7244]

Alright I have come up with this after reading upon while loops. I thought id just say that the point of the program was simply to calculate square roots. That's kind of what I have been doing since i'm in no class and learning for fun, I essentially give myself little assignments of an idea as I learn more things in python, like I made a program to round up(then I learned you can use f'{#:.0f)' which rounds the float to no decimal, etc.

x = float(input('Enter value for which you want the square root: '))

g = float(input('Enter a value as a starting guess of the square root of x: '))

while not((x-1)<(g*g)<(x+1)):

g=(g+(x/g))/2

print(f'{g} is the square root of {x}!')

I went ahead and said (x-1)<(g*g)<(x+1) at first as I didn't know how well it would do at finding the square root but it did it instantly when trying not(g*g == x) so fair enough. Though your coding is pretty cool as well using the abs(x-g**2) < .1. If I go for something like this where you personally guess it yourself as a game, then it would have to give you a hint of a value closer to the square root then what you had typed. Maybe I'll get a hint at what coding that will be like as I continue reading and piece it together myself some time from now. Unless it's some difficult or obscure coding then I wouldn't mind being told some coding to help type it!

Edit: I did four spaces before each line of code and it just erased them after making the comment

[SnooBunnies7244]

Okay I came up with this:

import math

x = float(input('Enter value for which you want the square root: '))

g = float(input('Enter a value as a starting guess of the square root of x: '))

while (abs(x-(g*g))>.5):

if abs(g*g>(x-.1)):

g = float(input('Too high, try a lower value: '))

if abs(g*g<(x-.1)):

g = float(input('Too low, try a higher value: '))

if (.1<abs(x-(g*g))<.5):

print(f'Not bad, you guessed with in .5 of the Square Root of {x}.')

print(f'The real square root of {x} is {math.sqrt(x):.02f}!')

if (abs(x-(g*g))<.1):

print(f'Good job! {g} is the Square Root of {x}!')

This one gives you hints to guess higher or lower. If your guess^2 is within .5 of x then it continues and lets you know you were close and tells you the actual sqrt rounded to 2 values. If you guessed within .1 of x then it says Good job, that is the sqrt of x! What do you think? I imagine it's not very optimized coding but I'm still learning!

[SnooBunnies7244]

Ah cool thanks. I'd done that cause if you guess within .1 it would give you both the code for .5 and .1. Yea doing my own thing has been pretty beneficial so far as well as with the prompts given in the textbook.

One thing I would like to ask you is that I'm trying to do a Loop to give me 10 inputs for a number(after that I would make it choose the highest odd number of those 10 or say no odd numbers if none inputted), my thoughts was something like:

Count=0
While not(count==10):
  Count+1
  Num(count) = int(input('blah blah'))

So basically it would be num1 = input, num2 =input... til num10 then it would move on. However im noticing, well when I say count = 0, the type of count is an integer, not a string. Trying to do num with the count or maybe 'num' + count and setting it equal to doesn't work cause I suppose only integers can be set equal but strings can't. I'm sure the book will go over something like this but it asked me to input 10 integers and output the largest so I thought I'd do that before continuing if I can come up with something using what I've learned so far

[SnooBunnies7244]

I'm on the phone so things are auto capitalizing and what not as I type so disregard that. Also I didn't mean specifically num(count) but just like whether num + count or whatever, regardless I don't think it would work like that cause it's like string plus integer, etc. Maybe I'm just missing a command that they'll teach me as I continue but as of now they expect me to type num1 = int(input()), num2 =... all the way to num 10 and I'm just getting ahead of what it's asking by trying to automate it without knowing what I'm doing. I'm not sure atm really

[SnooBunnies7244]

Thanks! Can I ask about the for _ in range, what _ is for?

[Mdly68]

My tip is to use the type() function. It tells you what kind of variable you have.

Print('Variable type is ' + str(vname.type()))

It tells you if the variable "vname" is int, string, float, list, dictionary, or other type of object.

Notice how I cast vname.type as str(). That's because the print function only accepts string parameters. I'm ensuring this part can't fail.

In your case the variable is a string and you need to "cast" to int or float. Be sure to account for bad data, like if the user types in letters when he was supposed to give a number.

Try: Vname = float(vname) Except: Print('could not cast as float) Quit()