## Data I/O, Strings, Dictionary

### Data I/O

# In this lecture we shall concentrate on a few more programming tools which might be useful to you later. Let us try to write a simple interpreter today. In the process we shall learn how to write to a file and recall reading from a file.

#### Reading data from the screen

# This you already do using `input`. Some of you already use `raw_input`, though I did not do it in class. Today I'll formally introduce it. Raw input directly stores the input in a string. You can later process the string in any way you want.

# In[1]:

s = raw_input("Input anything : ")
print type(s)
print s
slst = s.split()
varname = slst[0]
varvalue = float(slst[1])
print "%s has value %f." % (varname, varvalue)


# Out[1]:

#     Input anything : a 7
#     <type 'str'>
#     a 7
#     a has value 7.000000.
# 

#### Command line input

# Sometimes one runs a program where the input to the program is passed via the command line. It is easy to do that in python. For that one has to import a package called `sys`.

# In[2]:

program = """
import sys

x = float(sys.argv[1])
print x*x
"""

progfile = open("files/myprogfile.py", 'w')
progfile.write(program)
progfile.close()


# To see that it works, we can print the file

# In[3]:

progfile = open("files/myprogfile.py", 'r')
print progfile.read()
progfile.close()


# Out[3]:

#     
#     import sys
#     
#     x = float(sys.argv[1])
#     print x*x
#     
# 

# In[4]:

#%run "files/myprogfile.py"


# In[5]:

get_ipython().magic(u'run "files/myprogfile.py" 1.5')


# Out[5]:

#     2.25
# 

#### Raising errors

# Till now we had been trying to deal with errors using conditionals. However to make the program more readable, one is encouraged to use the `try..except` mechanism provided in python.

# In[6]:

xstr = raw_input("Test :")
x = float(xstr)


# Out[6]:

#     Test :34.3
# 

# In[7]:

def read_number() :
    xstr = raw_input("Input a number : ")
    try :
        x = float(xstr)
    except ValueError :
        raise ValueError('Cannot understand the number %s.' % xstr)
    return x


# In[8]:

try :
    x = read_number()
except ValueError, e :
    print e
    sys.exit(1)
        
print x*x


# Out[8]:

#     Input a number : 23.4
#     547.56
# 

### Strings

# Some examples of what you can do with strings.

# In[9]:

eg_str = "This is a sentence. This is another sentence.\nThis is the second line. This is the second\nsentence which started in the second line but\nended in the fourth line."
print eg_str


# Out[9]:

#     This is a sentence. This is another sentence.
#     This is the second line. This is the second
#     sentence which started in the second line but
#     ended in the fourth line.
# 

# In[10]:

# Substrings
eg_str[10:20]


# Out[10]:

#     'sentence. '

# In[11]:

print "The word 'sentence' begins at %d, whereas the word 'sentience' starts at %d." % (eg_str.find('sentence'), eg_str.find('sentience'))


# Out[11]:

#     The word 'sentence' begins at 10, whereas the word 'sentience' starts at -1.
# 

# In[12]:

# Also in works
print ('sentence' in eg_str)
print ('starts' in eg_str)


# Out[12]:

#     True
#     False
# 

# In[13]:

print 'health'.startswith('heal')
print 'Python'.endswith('tail')


# Out[13]:

#     True
#     False
# 

# In[14]:

# Replace
print eg_str.replace('sentence', 'verdict')


# Out[14]:

#     This is a verdict. This is another verdict.
#     This is the second line. This is the second
#     verdict which started in the second line but
#     ended in the fourth line.
# 

# In[15]:

print eg_str.split()
print eg_str.splitlines()
print eg_str.split('a')
print eg_str.split('.')
for str in eg_str.split('.') :
    print str


# Out[15]:

#     ['This', 'is', 'a', 'sentence.', 'This', 'is', 'another', 'sentence.', 'This', 'is', 'the', 'second', 'line.', 'This', 'is', 'the', 'second', 'sentence', 'which', 'started', 'in', 'the', 'second', 'line', 'but', 'ended', 'in', 'the', 'fourth', 'line.']
#     ['This is a sentence. This is another sentence.', 'This is the second line. This is the second', 'sentence which started in the second line but', 'ended in the fourth line.']
#     ['This is ', ' sentence. This is ', 'nother sentence.\nThis is the second line. This is the second\nsentence which st', 'rted in the second line but\nended in the fourth line.']
#     ['This is a sentence', ' This is another sentence', '\nThis is the second line', ' This is the second\nsentence which started in the second line but\nended in the fourth line', '']
#     This is a sentence
#      This is another sentence
#     
#     This is the second line
#      This is the second
#     sentence which started in the second line but
#     ended in the fourth line
#     
# 

# In[16]:

# Checking type of characters in a string
print "'2334' contains only digits : %s" % '2334'.isdigit()
print "'a123' contains only digits : %s" % 'a123'.isdigit()
print "Space '    \n  \t  ' : %s" % ' \n  \t   '.isspace()
print "'' is a space : %s" % ''.isspace()


# Out[16]:

#     '2334' contains only digits : True
#     'a123' contains only digits : False
#     Space '    
#       	  ' : True
#     '' is a space : False
# 

# In[17]:

# Removing initial and trailing characters
print "+" + '    This is a   stupid sentence.           \n'.strip() + "+"


# Out[17]:

#     +This is a   stupid sentence.+
# 

# In[18]:

# delimiter.join(list of strings) 

list_of_sentences = ["Sky is blue", "Classes are boring", "Examples are stupid"]
print '. '.join(list_of_sentences) + '.'
print "-"*50
print ".\n".join(list_of_sentences) + '.'


# Out[18]:

#     Sky is blue. Classes are boring. Examples are stupid.
#     --------------------------------------------------
#     Sky is blue.
#     Classes are boring.
#     Examples are stupid.
# 

### Dictionary

# In[19]:

names = ['Eric', 'Ila', 'Emma', 'John', 'Umesh', 'Asha', 'Akash', 'Kate', 'Uma', 'Sam']
scores = [7,8,6,9,10,6,8,7,7,9]

score_dict={'Eric' : 7, 'Ila' : 8}
print "Ila scored %d." % score_dict['Ila']

# One can add.
score_dict={}
print score_dict

for i in range(len(names)) :
    score_dict[names[i]] = scores[i]

print score_dict


# Out[19]:

#     Ila scored 8.
#     {}
#     {'Emma': 6, 'Akash': 8, 'Sam': 9, 'Ila': 8, 'Asha': 6, 'Kate': 7, 'Umesh': 10, 'Uma': 7, 'John': 9, 'Eric': 7}
# 

# In[20]:

for name in score_dict :
    print "%s scored %d" % (name, score_dict[name])


# Out[20]:

#     Emma scored 6
#     Akash scored 8
#     Sam scored 9
#     Ila scored 8
#     Asha scored 6
#     Kate scored 7
#     Umesh scored 10
#     Uma scored 7
#     John scored 9
#     Eric scored 7
# 

# In[21]:

def print_score(n) :
    if n in score_dict :
        print "%s scored %d." %(n, score_dict[n])
    else :
        print "%s is not on list." % n


# In[22]:

print_score('Peter')
print_score('Asha')


# Out[22]:

#     Peter is not on list.
#     Asha scored 6.
# 

# In[23]:

def tabulate_scores(sd) :
    print "Name     :    Score"
    for name in sorted(sd) :
        print "%5s    :%9d" % (name, sd[name])
    return None


# In[24]:

tabulate_scores(score_dict)


# Out[24]:

#     Name     :    Score
#     Akash    :        8
#      Asha    :        6
#      Emma    :        6
#      Eric    :        7
#       Ila    :        8
#      John    :        9
#      Kate    :        7
#       Sam    :        9
#       Uma    :        7
#     Umesh    :       10
#