Control flows

if
for
while

if statements



In [19]:

    
x = -5

if x < 0:
    x = 0
    print 'Negative changed to zero'
elif x == 0:
    print 'Zero'
elif x == 1:
    print 'Single'
else:
    print 'More'
    
print x









    



Negative changed to zero
0

the body of the if is indented
- indentation is python 's way to grouping statement (code block). C using brackets for grouping.
- each line within a code block must be indented by the same about.
- you should use only tab to indent or spaces to indent
There can be zero or more elif parts, and the else part is optional



In [27]:

    
x = -5
if x < 0:
    print "X is negative"









    



X is negative



In [1]:

    
x = 5
if x == 0: print 'X is zero'

for statements

the for statement in python differs with the other languages.
No loop over an arithmetic progression of numbers. e.g: cannot write
```
for(int i = 0; i < 10; i++)
```
only loop over the items of any sequence



In [5]:

    
for pet in ['cat', 'dog', 'pig']:
    print 'I own a', pet









    



I own a cat
I own a dog
I own a pig

if you need to iterate over a sequence of numbers, using built-in function range() function



In [31]:

    
print range(10)









    



[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]



In [32]:

    
print range(5, 10)









    



[5, 6, 7, 8, 9]



In [33]:

    
print range(0, 10, 3)









    



[0, 3, 6, 9]

Iterate over the indices of a sequence, you can combine range() and len() as follows:



In [7]:

    
a = ['Mary', 'had', 'a', 'little', 'lamb']



In [9]:

    
range(len(a))









    Out[9]:





[0, 1, 2, 3, 4]



In [10]:

    
for i in range(len(a)):
    print i, a[i]









    



0 Mary
1 had
2 a
3 little
4 lamb

break and continue statements

Borrowed from C
In case you don't know... In the loop when encounter:
- break: the loop is immediately terminated and the program control resumes at the next statement following the loop.



In [7]:

    
for n in [1, 2, 3, 4, 5]:
    if n % 2 == 0:
        print 'Found an even number', n
        break









    



Found an even number 2

In the loop when encounter
- continue: instead of forcing termination like break. The continue however, forces the next iteration of the loop to take place, skipping any code in between.



In [9]:

    
for n in [1, 2, 3, 4, 5]:
    if n % 2 == 1:
        continue
    print 'Even number:', n









    



Even number: 2
Even number: 4

while statements

Like while in C



In [24]:

    
i = 0
while i < 5:
    print i,
    i = i + 1

exercise

Calculate dot product of two vector [1, 5, 8, 9, 10] and [9, 2, 5, 7, 3]



In [11]:

    
x = [1, 5, 8, 9, 10]
y = [9, 2, 5, 7, 3]
result = 0

for i in range(len(x)):
    result = result + x[i] * y[i]

print result