# [Python-Dev] List comprehension syntax

Huaiyu Zhu HZhu@knowledgetrack.com
Fri, 14 Jul 2000 18:42:11 -0700 (PDT)

```I just read the whole thread on list comprehension.  Here's my proposed
syntax, given by examples.  This notation is very safe as it has been used
in mathematics for years.

a = [1, 2, 3];  b = [4, 5, 6]

#----------------------------------------------------
# Parallel loops:

c = tuples(a,b)             # c = [(1,4), (2,5), (3,6)]
c = lists(a,b)              # c = [[1,4], [2,5], [3,6]]
c = xtuples(a,b)            # c = ((1,4), (2,5), (3,6))
c = xlists(a,b)             # c = ([1,4], [2,5], [3,6])

#----------------------------------------------------
# prefix x has the same effect as in xrange in loops

for pair in xtuples(a,b):
print pair              # print (1,4); print (2,5); print (3,6)
for pair in xtuples(a,b):
print pair              # print [1,4]; print [2,5]; print [3,6]
for i, j in xlists(a,b): pass

#----------------------------------------------------
# list comprehension - all elements that satisfy conditions

c = [x*2, y: x, y in xtuples(a,b)]          # c = [(2,4), (4,5), (6,6)]
c = [[x, y]: x, y in xtuples(a,b)]          # c = [[1,4], [2,5], [3,6]]
c = [x: x in a+b; x%2=0]                    # c = [2, 4, 6]
c = [x in a+b: x%2=0]                       # ditto
c = [x,y: x, y in xtuples(a,b); x>1; x+y<8] # c = [(2,5)]
c = [x,y in xtuples(a,b): x>1; x+y<8]       # ditto

#----------------------------------------------------
# Corresponding to old syntax

c = [x in a: f(x)]          # c = filter(f, x)
c = [f(x): x in a]          # c = map(f, a)

#----------------------------------------------------
# Iterated loops (all elements that satisfy condition):

c = [x, y: x in a; y in b]  # c = [(1,4), (1,5), (1,6), (2,4), (2,5), ...]
c = [[x,y]: x in a; y in b] # c = [[1,4], [1,5], [1,6], [2,4], [2,5], ...]

for i, j in [i, j: i in a; j in b]:
print i, j              # print 1,4; print 1,5; ...

for i in a; j in b:
print i, j              # ditto

--
Huaiyu Zhu                       hzhu@users.sourceforge.net
Matrix for Python Project        http://MatPy.sourceforge.net

```