loops and iteration

py4E022-025

While

while loops are called indefinite loops because they keep going until a logical condition becomes False.

# break out of a loop
while True:
    line = input('>')
    if line == 'done':
        break
    print(line)
print('done!')

# finishing an iteration with contine
while True:
    line = input('>')
    if line[0] == '#':
        continue # go up back to the tope of loop 
    if line == 'done':
        break # get out of the loop
    print(line)
print('done!')

for

definite loops iterate through the members of a set.

for i in [5,4,3,2,1]: # i is iteration variable
    print(i)
print('blast off!')

friends = ['ke','xinlei','xjc']
for friend in friends:
    print('hi',friend)
print('ok')

# count
count = 0
sum = 0
print('before',count,sum)
for things in [9,41,12,3,74,15]:
    count += 1
    sum += things
    print(things,count,sum)
print('after',count,sum)

# a none type
smallest = None
print('before',smallest)
for value in [9,41,12,3,74,15]:
    if smallest is None:
        smallest = value
    elif value < smallest:
        smallest = value
    print(smallest,value)
print('after',smallest)

# 'is' is stronger than '==' e.g. 0 == 0.00
# use is for None/ Ture False, not recommended use for integer 

condition

py4E016

astr = 'hello'
try:
  print('hello')
  astr = int(astr) #注意此处错误
except: # 只在try错误的时候触发 每一条
  astr = -1
print('First', istr)