Question

I am having a little trouble with this question: "Write a program using a for loop that calculates exponentials. Your program should ask the user for a base base and an exponent exp, and calculate base**exp."

This is what I came up with:
x = (raw_input ("Enter a base:"))
y = (raw_input ("Enter an exponent:"))
exponential = pow(x, y)
for solution in exponential:
print "answer =", solution

When I run it, an error comes up for line 3 with this message:
TypeError: unsupported operand type(s) for ** or pow(): 'str' and 'str'

Answer 1

you need to convert x and y into integer or float, because by default raw_input returns a string. You convert one variable to another type by using appropriate type conversion functions such as float() or int()

See more here:
http://docs.python.org/2/library/functions.html#int
http://docs.python.org/2/library/functions.html#float

Answer 2

Thanks! I changed it from raw_input to input since I want it to be an integer..but now a new error is coming up, here is the code:
x = input ("Enter a base:")
y = input ("Enter an exponent:")
a = pow(x, y)
for solution in a:
print solution

and here is the error message:
for solution in a:
TypeError: 'int' object is not iterable

Answer 3

well, pow(x,y) returns a single number -- x to the power o y. Since it's single number you can't iterate over it. You can only iterate over lists. In your case you just need to print a itself.

Now mostlikely what the task question was alluding towards is that you should NOT use internal pow() function, but rather implement it yourself.

Answer 4

So it works when I just print a itself, so a = x**y, but I was confused because the problem stated that it should be written as a for loop, would you be able to show me what that looks like?

Answer 5

Mostlikely they want you to not use ** or pow() but calculate exponent in a loop, looking through multiplication. So let's look at example -- lets say you need to compute 2 to the power of 4.
You can write it out as: 2 ^ 4 = 2 * 2 * 2 * 2
See the looping here? So that's what you need to implement. I'll give you a hint and starting point:

x = input ("Enter a base:")
y = input ("Enter an exponent:")
exponent = 1
for i in range(y):
exponent = ????

so you will only need to fill in ??? :)

Answer 6

Im also still trying to solve this one without sucess...

Answer 7

@filiphdan What issues are you running into? (probably worth discussing in a separate question

Answer 8

c0decracker, thanks for being so thorough and helpful with your answers! I still can't figure this problem out though. First in your code why do you write exponent = 1 in the third line? I'm also not understanding how using the range function helps to translate the code into a loop.

For example..if the goal is to calculate 2**4, and you use "for in in range(y)", the range of y would be (0,1,2,3) ...how do you get from there to a loop of 2 four times?

Answer 9

@kah712 Ok, so let's look at this code line by line, I am going to provide my explanation in the comments after each line:

x = input ("Enter a base:")
# as a result of this line x will contain a value for the base, in our example 2

y = input ("Enter an exponent:")
# as a result of this line y will contain a value of the power, in our example 4
# which means that we need to multiply x by itself 4 times
# so we can write this as : result = x^y = x*x*x*x

exponent = 1
# to utilize the loop to calculate this we need a starting value for the result
# because we are multiplying we need to have a starting value that will not
# change the outcome of multiplication. What value to pick? 1 of course
# because 1*anything = anything :)

for i in range(y):
# ok no we need to loop over multiplication 4 times.
# there are 2 ways of doing it -- one is a general way(see below) and
# second one is "python" way, or in parlance of python developers - pythonic way :)
# range() function is pythonic way of doing it.
# range basically generates a list of N values(where N is the parameter you pass to it)
# so if we say range(4) it will give us (0,1,2,3) so 4 values. Then we use that list to just
# loop over it's values. the values are not important, what's important is that
# there are 4 of them which matches exactly how many times
# we need to iterate over multiplication

exponent = ????

Answer 10

So the other, more general and perhaps simpler way of writing that loop is this:

x = input ("Enter a base:")
y = input ("Enter an exponent:")
exponent = 1
i = 0
while i < y:
exponent = exponent * x
i = i +1

Here we are using variable i as a counter.

Answer 11

ah okay! So I think I'm finally understanding how the range function is working.

So in our example 2^4, range(y) creates a loop of : exponent = exponent * x
Which is basically exponent = 1*2 (four times).
Which gives us 2*2*2*2

Thank you so so much!!!

Answer 12

Almost exactly right :) One important detail. It is actually exponent = exponent * 2 four times.

So let's look at the detail of how it works:
In the first iteration of the look you will have this:
exponent = 1 * 2
so after that exponent will be 2

On the second iteration of the loop you will have:

exponent = 2 * 2 --> 4

On the third iteration:

exponent = 4 * 2 --> 8

And on the forth iteration:

exponent = 8 * 2 --> 16

Makes sense?

Answer 13

Yes it makes complete sense, so during the four loops, the value of "exponent" is actually changing each time it goes through a loop, taking the previous value?

so if we were to do 4^3 the range would be (0,1,2) with exponent = 1,
exponent = exponent * x
exponent = 1*4 ---> exponent = 4
exponent = 4*4 ---> exponent = 16
exponent = 16*4 ---> exponent = 64
print exponent #this would print the last value of exponent, which would be 64!

Answer 14

Yes, exactly right

Answer 15

thanks for the help c0decracker, i finaly got it, it was easier than i thought, i just needed to make the exponent = exponent*x after the for cycle and it was done :)