working with negative numbers in python

Question

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 lt  abs numb        total   total   numa     count   count   1   testing statements if  numa  numb  lt   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

User · Accepted Answer

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

User · Answer

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  lt  0          negate   True         numa   -numa     while numa          if numa  amp  1              accum    numb         numa  gt  gt   1         numb  lt  lt   1     if negate          accum   -accum     print accum   output   3 5 15 -3 5 -15 3 -5 -15 -3 -5 15 -127 -127 16129

User · Answer

How about something like that    Uses no abs   nor mulitiplication  Notes    the abs   function is only used for the optimization trick  This snippet can either be removed or recoded  the logic is less efficient since we re testing the sign of a and b with each iteration  price to pay to avoid both abs   and multiplication operator      def multiply by addition a  b       School exercise  multiplies integers a and b  by successive additions         if abs a   gt  abs b         a  b   b  a       optimize by reducing number of iterations    total   0    while a    0        if a  gt  0           a -  1          total    b       else           a    1          total -  b    return total  multiply by addition 2 3  6 multiply by addition 4 3  12 multiply by addition -4 3  -12 multiply by addition 4 -3  -12 multiply by addition -4 -3  12

User · Answer

Thanks everyone  you all helped me learn a lot  This is what I came up with using some of your suggestions   this is apparently a better way of getting multiple inputs at the same time than the   way I was doing it text   raw input  please give 2 numbers to multiply separated with a comma    split text   text split      numa   int split text 0   numb   int split text 1     standing variables total   0  if numb  gt  0      repeat   numb else      repeat   -numb   for loops work better than while loops and are cheaper  output the total for count in range repeat       total    numa    check to make sure the output is accurate if numb  lt  0      total   -total   print total   Thanks for all the help everyone

User · Answer

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

User · Answer

import time  print   Two Digit Multiplication Calculator   print                                         print    print   Give me two numbers     x   int   input         y   int   input         z   0  print      while x  gt  0      print      z      x   x - 1     z   y   z     time sleep   2      if x    0          print   Final answer    z   while x  lt  0      print      - z       x   x   1     z   y   z     time sleep   2      if x    0          print   Final answer    - z    print

User · Answer

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  lt  0      total   -total  print total

[python] working with negative numbers in python

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