Not all arguments converted during string formatting (Proving Collatz Conjecture)

Question

When I run the code below I get an error. I have looked here on StackOverflow, but I ended up not solving my problem.

print "insert value"
value = raw_input ()

flag = False
i = 0

while flag:
    if value == 1:
        flag = True
    elif value % 2 == 0:
        value = value / 2
    else:
        value = value * 3
        value = value + 1
    i = i + 1

print "After", i, "attempts the conjecture has been demonstrated"

I get an error in the elif logical test value% 2 == 0 which says

not all arguments converted during string formatting

I think the problem is in the variable type but I tried the input function and forcing the int type, value = int (input (....)), but that also didn't work.

What exactly is value % 2 == 0: supposed to be doing? – iScrE4m Dec 22 '15 at 02:51 — iScrE4m, Dec 22 '15 at 02:51
it's supposed to check if value is even – otherOne Dec 22 '15 at 02:52 — otherOne, Dec 22 '15 at 02:52

score 4 · Accepted Answer · answered Dec 22 '15 at 02:55

4

In IDLE this worked for me

value = int(raw_input ())

converting raw input into integer

while True:
    if value == 1:
        break
    elif value % 2 == 0:
        value = value / 2
    else:
        value = value * 3
        value = value + 1
    i = i + 1

a bit simpler way to do while without having arbitrary flag

answered Dec 22 '15 at 02:55

iScrE4m

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Im a student and it worked thanks I will accepted as soon as I can (3mins) – otherOne Dec 22 '15 at 03:00

score 2 · Answer 2 · answered Dec 22 '15 at 08:06

iScrE4m has shown how to fix your program, but I'll explain why you got that

TypeError: not all arguments converted during string formatting

error message.

When the Python interpreter sees % after a string it interprets the % as the string interpolation operator, which is used to convert data and insert it into a string.

For example

a = 2
b = 3
print "%d + %d = %d" % (a, b, a + b)

output

2 + 3 = 5

In your code, value is a string so the interpreter tries to do string interpolation on value % 2 but then it sees that value isn't a valid format string to format the data 2, so it gives you that TypeError.

In modern code use of %-style string formatting is discouraged and the str.format method is preferred. The equivalent to the above example is:

print "{0} + {1} = {2}".format(a, b, a + b)

In Python 2.7 and later you can omit the field numbers:

print "{} + {} = {}".format(a, b, a + b)

I have a few more comments on your code.

When calling a function in Python it's conventional to not put a space after the function name.

There's no need to use print to print a prompt before calling raw_input: you can (and should) supply the prompt as an argument to raw_input.

When performing integer division you should use the integer division operator //. A single slash works ok in Python 2, but in Python 3 it won't behave as expected here because it will perform a floating-point division. Eg, 10 / 2 will result in 5.0.

When you need to perform a single operation on a variable and save the result of that operation back to the same variable you can use an augmented assignment operator. Eg,
instead of i = i + 1 you can write i += 1;
instead of value = value // 2 you can write value //= 2.

Also, instead of

value = value * 3
value = value + 1

you could do

value *= 3
value += 1

However, when performing multiple operations on a variable it's a little more compact and efficient to do them on one line. So it would be more usual to do:

value = value * 3 + 1

Putting it all together, here's a re-worked version of your code.

value = int(raw_input("Enter value: "))
i = 0
while True:
    if value == 1:
        break
    elif value % 2 == 0:
        value //= 2
    else:
        value = value * 3 + 1
    i += 1    

print "After {} attempts the conjecture has been demonstrated.".format(i)

Not all arguments converted during string formatting (Proving Collatz Conjecture)

2 Answers2