working with negative numbers in python
Solution 1
Perhaps you would accomplish this with something to the effect of
text = raw_input("please give 2 numbers to multiply separated with a comma:")
split_text = text.split(',')
a = int(split_text[0])
b = int(split_text[1])
# The last three lines could be written: a, b = map(int, text.split(','))
# but you may find the code I used a bit easier to understand for now.
if b > 0:
num_times = b
else:
num_times = -b
total = 0
# While loops with counters basically should not be used, so I replaced the loop
# with a for loop. Using a while loop at all is rare.
for i in xrange(num_times):
total += a
# We do this a times, giving us total == a * abs(b)
if b < 0:
# If b is negative, adjust the total to reflect this.
total = -total
print total
or maybe
a * b
Solution 2
Too hard? Your TA is... well, the phrase would probably get me banned. Anyways, check to see if numb
is negative. If it is then multiply numa
by -1
and do numb = abs(numb)
. Then do the loop.
Solution 3
The abs() in the while condition is needed, since, well, it controls the number of iterations (how would you define a negative number of iterations?). You can correct it by inverting the sign of the result if numb
is negative.
So this is the modified version of your code. Note I replaced the while loop with a cleaner for loop.
#get user input of numbers as variables
numa, numb = input("please give 2 numbers to multiply seperated with a comma:")
#standing variables
total = 0
#output the total
for count in range(abs(numb)):
total += numa
if numb < 0:
total = -total
print total
Solution 4
Try this on your TA:
# Simulate multiplying two N-bit two's-complement numbers
# into a 2N-bit accumulator
# Use shift-add so that it's O(base_2_log(N)) not O(N)
for numa, numb in ((3, 5), (-3, 5), (3, -5), (-3, -5), (-127, -127)):
print numa, numb,
accum = 0
negate = False
if numa < 0:
negate = True
numa = -numa
while numa:
if numa & 1:
accum += numb
numa >>= 1
numb <<= 1
if negate:
accum = -accum
print accum
output:
3 5 15
-3 5 -15
3 -5 -15
-3 -5 15
-127 -127 16129
dman762000
Updated on December 02, 2021Comments
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dman762000 over 2 years
I am a student in a concepts of programming class. The lab is run by a TA and today in lab he gave us a real simple little program to build. It was one where it would multiply by addition. Anyway, he had us use absolute to avoid breaking the prog with negatives. I whipped it up real quick and then argued with him for 10 minutes that it was bad math. It was, 4 * -5 does not equal 20, it equals -20. He said that he really dosen't care about that and that it would be too hard to make the prog handle the negatives anyway. So my question is how do I go about this.
here is the prog I turned in:
#get user input of numbers as variables numa, numb = input("please give 2 numbers to multiply seperated with a comma:") #standing variables total = 0 count = 0 #output the total while (count< abs(numb)): total = total + numa count = count + 1 #testing statements if (numa, numb <= 0): print abs(total) else: print total
I want to do it without absolutes, but every time I input negative numbers I get a big fat goosegg. I know there is some simple way to do it, I just can't find it.