if else in a list comprehension

Question

I have a list l   l    22  13  45  50  98  69  43  44  1    For numbers above 45 inclusive  I would like to add 1  and for numbers less than it  5   I tried   x 1 for x in l if x  gt   45 else x 5    But it gives me a syntax error  How can I achieve an if  ndash  else like this in a list comprehension

User · Answer

You can also put the conditional expression in brackets inside the list comprehension:

    l = [22, 13, 45, 50, 98, 69, 43, 44, 1]
    print [[x+5,x+1][x >= 45] for x in l]

[false,true][condition] is the syntax

User · Answer

You must put the expression at the beginning of the list comprehension  an if statement at the end filters elements    x 1 if x  gt   45 else x 5 for x in l

User · Answer

It has to do with how the list comprehension is performed   Keep in mind the following     expression for item in list if conditional     Is equivalent to   for item in list      if conditional          expression   Where the expression is in a slightly different format  think switching the subject and verb order in a sentence    Therefore  your code  x 1 for x in l if x  gt   45  does this   for x in l      if x  gt   45          x 1   However  this code  x 1 if x  gt   45 else x 5 for x in l  does this  after rearranging the expression    for x in l      if x gt  45  x 1     else  x 5

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x 1 if x  gt   45 else x 5 for x in l    And for a reward  here is the comment  I wrote to remember this the first time I did this error      Python s conditional expression is a if C else b and can t be used as    a for i in items if C else b        The right form is    a if C else b for i in items        Even though there is a valid form    a for i in items if C        But that isn t the same as that is how you filter by C  but they can be combined    a if tC else b for i in items if fC

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You can combine conditional logic in a comprehension    ps   PorterStemmer    stop words english   stopwords words  english    best   sorted word scores items    key lambda x  x 1   reverse True   10000   bestwords   set  w for w  s in best      def best word feats words      return dict   word  True  for word in words if word in bestwords       with stemmer  def best word feats stem words      return dict   ps stem word   True  for word in words if word in bestwords       with stemmer and not stopwords  def best word feats stem stop words      return dict   ps stem word   True  for word in words if word in bestwords and word not in stop words english

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I just had a similar problem  and found this question and the answers really useful  Here s the part I was confused about  I m writing it explicitly because no one actually stated it simply in English   The iteration goes at the end    Normally  a loop goes   for this many times      if conditional           do this thing     else          do something else     Everyone states the list comprehension part simply as the first answer did      expression for item in list if conditional      but that s actually not what you do in this case   I was trying to do it that way   In this case  it s more like this     expression if conditional else other thing for this many times

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Make a list from items in an iterable   It seems best to first generalize all the possible forms rather than giving specific answers to questions  Otherwise  the reader won t know how the answer was determined  Here are a few generalized forms I thought up before I got a headache trying to decide if a final else  clause could be used in the last form    expression1 item                                         for item in iterable    expression1 item  if conditional1                        for item in iterable    expression1 item  if conditional1 else expression2 item  for item in iterable    expression1 item  if conditional1 else expression2 item  for item in iterable if conditional2    The value of item doesn t need to be used in any of the conditional clauses  A conditional3 can be used as a switch to either add or not add a value to the output list   For example  to create a new list that eliminates empty strings or whitespace strings from the original list of strings   newlist    s for s in firstlist if s strip

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Here is another illustrative example    gt  gt  gt  print      join   ha  if i else  Ha  for i in range 3           Ha  ha  ha    It exploits the fact that if i evaluates to False for 0 and to True for all other values generated by the function range    Therefore the list comprehension evaluates as follows    gt  gt  gt    ha  if i else  Ha  for i in range 3     Ha    ha    ha

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One way   def change f       if f is None          return unicode f strip        else          return     row    change x  for x in row    Although then you have   row   map change  row    Or you can use a lambda inline

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There isn t any need for ternary if then else  In my opinion your question calls for this answer   row    unicode  x or     strip    for x in row

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gt  gt  gt  l    22  13  45  50  98  69  43  44  1   gt  gt  gt   x 1 if x  gt   45 else x 5 for x in l   27  18  46  51  99  70  48  49  6    Do-something if  lt condition gt   else do-something else

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coding utf-8  def my function get list        my list    0  1  2  3  4  5         You may use map   to convert each item in the list to a string         and then join them to print my list      print  Affichage de my list   0    format      join map str  my list          return my list   my result list                 number in my list   4     Condition is False   append number in my list   4 in my result list        number in my list   2    Condition is True   append number in my list   2 in my result list           number in my list   2    0      Condition  If the number in my list is even     for number in my list in my function get list      For each number in my list    print  Affichage de my result list   0    format      join map str  my result list        venv    python list comp py  Affichage de my list  0  1  2  3  4  5   Affichage de my result list  0  5  4  7  8  9   So  for you  row         unicode x strip     x is not None  for x in row

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Like in  a if condition1 else b for i in list1 if condition2   the two ifs with condition1 and condition2 doing two different things  The part  a if condition1 else b  is from a lambda expression   lambda x  a if condition1 else b   while the other condition2 is another lambda   lambda x  condition2   Whole list comprehension can be regard as combination of map and filter   map lambda x  a if condition1 else b  filter lambda x  condition2  list1

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you can do this row    unicode x strip    if x    None else    for x in row   Some Syntax for List comprehension    item if condition else item for item in items   f item  if condition else value for item in items   item if condition for item in items   value if condition else value1 if condition1 else value2

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You could move the conditional to   v    22  13  45  50  98  69  43  44  1     x 1 if x  gt  45 else x 5   for x in v     But it s starting to look a little ugly  so you might be better off using a normal loop  Note that I used v instead of l for the list variable to reduce confusion with the number 1  I think l and O should be avoided as variable names under any circumstances  even in quick-and-dirty example code

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The specific problem has already been solved in previous answers  so I will address the general idea of using conditionals inside list comprehensions   Here is an example that shows how conditionals can be written inside a list comprehension   X    1 5  2 3  4 4  5 4   n   1 5  5 1   a         Original list    Extract non-strings from X to new list X non str    el for el in X if not isinstance el  str      When using only  if   put  for  in the beginning    Change all strings in X to  b   preserve everything else as is X str changed     b  if isinstance el  str  else el for el in X     When using  if  and  else   put  for  in the end   Note that in the first list comprehension for X non str  the order is       expression for item in iterable if condition   and in the last list comprehension for X str changed  the order is      expression1  if  condition  else expression2 for item in iterable   I always find it hard to remember that expression1 has to be before if and expression2 has to be after else  My head wants both to be either before or after    I guess it is designed like that because it resembles normal language  e g   I want to stay inside if it rains  else I want to go outside   In plain English the two types of list comprehensions mentioned above could be stated as   With only if      extract apple for apple in apple box if apple is ripe   and with if else     mark apple if apple is ripe else leave it unmarked for apple in apple box

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The other solutions are great for a single if   else construct  However  ternary statements within list comprehensions are arguably difficult to read   Using a function aids readability  but such a solution is difficult to extend or adapt in a workflow where the mapping is an input  A dictionary can alleviate these concerns   row    None   This    is    a    filler    test    string   None   d    None       filler    manipulated    res    d get x  x  for x in row   print res         This    is    a    manipulated    test    string

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You can totally do that  It s just an ordering issue    unicode x strip    if x is not None else    for x in row    In general    f x  if condition else g x  for x in sequence    And  for list comprehensions with if conditions only    f x  for x in sequence if condition    Note that this actually uses a different language construct  a conditional expression  which itself is not part of the comprehension syntax  while the if after the for   in is part of list comprehensions and used to filter elements from the source iterable     Conditional expressions can be used in all kinds of situations where you want to choose between two expression values based on some condition  This does the same as the ternary operator    that exists in other languages  For example   value   123 print value   is    even  if value   2    0 else  odd

User · Answer

The reason you re getting this error has to do with how the list comprehension is performed   Keep in mind the following     expression for item in list if conditional     Is equivalent to   for item in list      if conditional          expression   Where the expression is in a slightly different format  think switching the subject and verb order in a sentence    Therefore  your code  x 1 for x in l if x  gt   45  does this   for x in l      if x  gt   45          x 1   However  this code  x 1 if x  gt   45 else x 5 for x in l  does this  after rearranging the expression    for x in l      if x gt  45  x 1     else  x 5

[python] if else in a list comprehension

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