Converting a float to a string without rounding it

Question

I m making a program that  for reasons not needed to be explained  requires a float to be converted into a string to be counted with len    However  str float x   results in x being rounded when converted to a string  which throws the entire thing off  Does anyone know of a fix for it  Here s the code being used if you want to know   len str float x  3

User · Answer

I know this is too late but for those who are coming here for the first time, I'd like to post a solution. I have a float value index and a string imgfile and I had the same problem as you. This is how I fixed the issue

index = 1.0
imgfile = 'data/2.jpg'
out = '%.1f,%s' % (index,imgfile)
print out

The output is

1.0,data/2.jpg

You may modify this formatting example as per your convenience.

User · Answer

Some form of rounding is often unavoidable when dealing with floating point numbers   This is because numbers that you can express exactly in base 10 cannot always be expressed exactly in base 2  which your computer uses    For example    gt  gt  gt   1 0 10000000000000001   In this case  you re seeing  1 converted to a string using repr    gt  gt  gt  repr  1   0 10000000000000001    I believe python chops off the last few digits when you use str   in order to work around this problem  but it s a partial workaround that doesn t substitute for understanding what s going on    gt  gt  gt  str  1   0 1    I m not sure exactly what problems  rounding  is causing you   Perhaps you would do better with string formatting as a way to more precisely control your output   e g    gt  gt  gt     5f     1  0 10000   gt  gt  gt     5f     12345678  0 12346    Documentation here

User · Answer

len repr float x  3     However I must say that this isn t as reliable as you think   Floats are entered displayed as decimal numbers  but your computer  in fact  your standard C library  stores them as binary  You get some side effects from this transition    gt  gt  gt  print len repr 0 1   19  gt  gt  gt  print repr 0 1  0 10000000000000001   The explanation on why this happens is in this chapter of the python tutorial   A solution would be to use a type that specifically tracks decimal numbers  like python s decimal Decimal    gt  gt  gt  print len str decimal Decimal  0 1     3

User · Answer

Other answers already pointed out that the representation of floating numbers is a thorny issue  to say the least   Since you don t give enough context in your question  I cannot know if the decimal module can be useful for your needs   http   docs python org library decimal html  Among other things you can explicitly specify the precision that you wish to obtain  from the docs     gt  gt  gt  getcontext   prec   6  gt  gt  gt  Decimal  3 0   Decimal  3 0    gt  gt  gt  Decimal  3 1415926535   Decimal  3 1415926535    gt  gt  gt  Decimal  3 1415926535     Decimal  2 7182818285   Decimal  5 85987    gt  gt  gt  getcontext   rounding   ROUND UP  gt  gt  gt  Decimal  3 1415926535     Decimal  2 7182818285   Decimal  5 85988     A simple example from my prompt  python 2 6     gt  gt  gt  import decimal  gt  gt  gt  a   decimal Decimal  10 000000001    gt  gt  gt  a Decimal  10 000000001    gt  gt  gt  print a 10 000000001  gt  gt  gt  b   decimal Decimal  10 00000000000000000000000000900000002    gt  gt  gt  print b 10 00000000000000000000000000900000002  gt  gt  gt  print str b  10 00000000000000000000000000900000002  gt  gt  gt  len str b decimal Decimal  3 0     29   Maybe this can help  decimal is in python stdlib since 2 4  with additions in python 2 6   Hope this helps  Francesco

[python] Converting a float to a string without rounding it

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