CODEWITHSUNDEEP
pythonif-statementequality
jesiKat
jesiKat

Reputation: 37

Python If Statement Never Evaluates to True

I apologize if my questions seem trivial. I would rather ask this in a chat room; however, my reputation is too low at the moment, so I am unable to ask anything in the Python chat room. I am currently learning Python for a class and the teacher gave us some practice problems to work to get us spun up. The function I am building now takes a number list and converts it to a string. The issue that I am having is that my if statement never evaluates to true. I have tried a few ways to work with the variables and have added many print statements to see if they should ever be equal, but to no avail. Thanks again in advance. I promise I am only asking after researching and trying many ways, but am now at a loss...Here is my code: 

def nlist2string(nlist):
    characters = ['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z']
    numbers = ['0','1','2','3','4','5','6','7','8','9','10','11','12','13','14','15','16','17','18','19','20','21','22','23','24','25']
    newList = []
    nListLen = len(nlist)         # var msgLen will be an integer of the length

    print 'Number list before conversion: ', nlist

    index = 0
    while index < nListLen:
        print 'Index at: ', nlist[index]
        num = nlist[index]
        print 'Is num equal to nlist indexed? ', num
        newNum = num % 26

        i = 0
        while i < 26:
            num1 = newNum
            num2 = numbers[i]
            print 'num1 = ', num1
            print 'num2 = ', num2
            if (num1 == num2):
                newList.append(characters[i])
                print 'Here is the current newList: ', newList
            else:
                print 'They never equal each other.'
            i = i + 1
        index = index + 1
    return newList

    numMessage = [28, 0, 33]
    convertedNumMsg = nlist2string(numMessage)
    print 'Number list after conversion: ', convertedNumMsg

Upvotes: 1

Views: 1126

Answers (4)

Asad Saeeduddin
Asad Saeeduddin

Reputation: 46628

The answer above properly addresses the question, but as a general tip: when reducing a list of values to a single value, use the reduce function. I realise of course that this is a learning exercise, but it can be useful to know the relevant inbuilt function for the job. That makes your function much shorter:

def nlist2string(nlist):

    def convert_to_alpha(s):
        if isinstance(s,str): //check if s is already a string
            return s          //if it is, return it unchanged
        else:
            return str(unichr(s+97)) //otherwise, get the corresponding alphabet
                                     //and return that
    def reduce_func(x,y):
        //convert the numbers to alphabets
        //and join the two together
        return convert_to_alpha(x) + convert_to_alpha(y) 

    return reduce(reduce_func, nlist)

For example, the output from:

l = [7,4,11,11,14]
print nlist2string(l)

is the string "hello".

The reduce function takes two arguments, a function that will be used to collapse the list into a single value, and a list.

As a more simple example of what reduce does:

function add(x,y):
    return x + y

print reduce(add, [1, 4, 3, 10, 5])
//Output: 23

Upvotes: 1

pradyunsg
pradyunsg

Reputation: 19406

You have a list of strings, of numbers 0-25.
In Python, a string is never equal to a number, So, num1 == num2 is always False.

So, it should be 

numbers = [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]

is more appropriate(and will work).

Or even better 

numbers = range(26)

And if you don't want to edit numbers's value, use this conditional:

if num1 == int(num2):

This will convert num2 into an integer, which is what you want to do.
Also, in that case, you can use map (Built-in Function), for more readability like so:

numbers = map(str, range(26))

Upvotes: 1

coiax
coiax

Reputation: 640

In Python, string literals and numbers are separate types of object, and do not compare equal.

1 == "1"

returns False.

You're iterating through your list of strings of numerals, but it's not a list of actual numbers.

You can use the range() function (it's a python built-in) to generate a number list instead of typing it out by hand.

Upvotes: 0

Andrew Clark
Andrew Clark

Reputation: 208405

You are trying to compare integers to strings, try changing your definition for numbers to the following:

numbers = [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]

Or alternatively, numbers = range(26).

Currently when comparing num1 and num2, you will be doing comparisons like 4 == '4', which will never be true:

>>> 4 == '4'
False

As an alternative to changing how you create your numbers list, you could convert num2 to an integer or num1 to a string before the comparison, so either num1 == int(num2) or str(num1) == num2.

Upvotes: 5

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