#!/usr/bin/env python
# coding: utf-8

# In[1]:


'A'


# In[2]:


'ACGT'


# In[3]:


st = 'ACGT'


# In[4]:


len(st) # getting the length of a string


# In[5]:


'' # empty string (epsilon)


# In[6]:


len('')


# In[7]:


import random
random.choice('ACGT') # generating a random nucleotide


# In[8]:


random.choice('ACGT') # repeated invocations might yield different nucleotides


# In[9]:


random.choice('ACGT') # repeated invocations might yield different nucleotides


# In[10]:


random.choice('ACGT') # repeated invocations might yield different nucleotides


# In[11]:


random.choice('ACGT') # repeated invocations might yield different nucleotides


# In[12]:


# now I'll make a random nucleotide string by concatenating random nucleotides
st = ''.join([random.choice('ACGT') for _ in range(40)])
st


# In[13]:


st[1:3] # substring, starting at position 1 and extending up to but not including position 3
# note that the first position is numbered 0


# In[14]:


st[0:3] # prefix of length 3


# In[15]:


st[:3] # another way of getting the prefix of length 3


# In[16]:


st[len(st)-3:len(st)] # suffix of length 3


# In[17]:


st[-3:] # another way of getting the suffix of length 3


# In[18]:


st1, st2 = 'CAT', 'ATAC'


# In[19]:


st1


# In[20]:


st2


# In[21]:


st1 + st2 # concatenation of 2 strings