Lecture4
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This document summarizes a CS151 lecture that covered functions in Python. It showed examples of defining functions using def, passing arguments, and returning values. It also explained the range function and how it can be used in for loops. Finally, it provided a full example of writing functions to check if a string is a palindrome by reversing it and comparing the original to the reversed string.
![CS151 Spring 2019
The range function in Python
You can always go right to the source documentation and lookup what a python
function does: https://docs.python.org/3/
Top right hand corner give a search box: range [enter]
class range(start, stop[, step])
start integer, remember we have 0 based indexing.
stop integer, can be an integer, boolean or function (i.e. len(myString))
step what increment or decrement to use, usually 1 or -1 but could be anything.](https://image.slidesharecdn.com/lecture4-190125181310/85/Lecture4-6-320.jpg)
![CS151 Spring 2019
Range example
def printString(myString):
for myIndex in range( 0, len(myString)-1, 1):
print(myString[myIndex])
This function will use a for loop to print from 0 to string length minus 1,
incrementing by +1 on each iteration.
Why -1 on string length?
Let myString = ‘Palindrome’ len(myString) returns 10 not 9
len(myString)-1 returns 9 which is the last character.
Index 0 1 2 3 4 5 6 7 8 9
string P a l i n d r o m e](https://image.slidesharecdn.com/lecture4-190125181310/85/Lecture4-7-320.jpg)
![CS151 Spring 2019
Convert to Python
def reverse(myString):
tempString = ‘’ #create an empty string
for myIndex in range(len(myString)-1, -1, -1): # more on this next
tempString += myString[myindex] #copy to new string
return(tempString)
def isPalindrome(myString):
revString = reverse(myString)
if myString == revString:
return(True)
else:
return(False)
#Call isPalindrome
print(isPalindrome(‘123454321’)](https://image.slidesharecdn.com/lecture4-190125181310/85/Lecture4-11-320.jpg)
Lecture4
- 1. CS 151 Lecture 4 Rudy Martinez
- 2. CS151 Spring 2019 Goals ● Show how to SSH using NoMachine ● Show how to startup Spyder3 ● Explain functions in python3 ● Explain range(start, stop, step) function ● Explain __name__ == “__main__”
- 3. CS151 Spring 2019 Functions in Python3 Functions , like math, takes an argument(s) and returns zero or more values. Structure def funcName(arg1, arg2): def : keyword to specify define a function funcName: you can name your function anything but making it self describing helps. (i.e. isPalindrome) arg1,arg2: Arguments you pass to the function can be zero or more
- 4. CS151 Spring 2019 Functions cont ... Structure cont… def funcName(arg1, arg2): Statements return( value ) Your function can contain any number of statements to do what ever the function is designed to do. return(): This is what you return to the calling program.
- 5. CS151 Spring 2019 Example function def add2numbers(myInt1, myInt2): # Trivial example tempInt = myInt1 + myInt2 return(tempInt) This is trivial and pedantic to show an example, it could easily be written with one line: return(myInt1 + myInt2) However, your programs will most likely not be so simple. Also there are no error checking, this function assumes you will use it correctly. This can be a bad assumption and you should check that myInt is actually an integer.
- 6. CS151 Spring 2019 The range function in Python You can always go right to the source documentation and lookup what a python function does: https://docs.python.org/3/ Top right hand corner give a search box: range [enter] class range(start, stop[, step]) start integer, remember we have 0 based indexing. stop integer, can be an integer, boolean or function (i.e. len(myString)) step what increment or decrement to use, usually 1 or -1 but could be anything.
- 7. CS151 Spring 2019 Range example def printString(myString): for myIndex in range( 0, len(myString)-1, 1): print(myString[myIndex]) This function will use a for loop to print from 0 to string length minus 1, incrementing by +1 on each iteration. Why -1 on string length? Let myString = ‘Palindrome’ len(myString) returns 10 not 9 len(myString)-1 returns 9 which is the last character. Index 0 1 2 3 4 5 6 7 8 9 string P a l i n d r o m e
- 8. CS151 Spring 2019 A real example program A Palindrome is a sentence or word that is the same forward and backward. String = 123454321 Bad algorithm: Algorithm for Palindrome: Reverse the given string and compare to original string
- 9. CS151 Spring 2019 A real example program, continued A Palindrome is a sentence or word that is the same forward and backward. String = 123454321 Good algorithm: Algorithm for Palindrome: Copy original string Reverse string Compare each string If they are equal then return True, else return False.
- 10. CS151 Spring 2019 A real example program, continued String = 123454321 Best algorithm: Algorithm for Palindrome: Reverse string Loop from end of original string and append to a new string Compare temp string to original string If they are equal then return True, else return False.
- 11. CS151 Spring 2019 Convert to Python def reverse(myString): tempString = ‘’ #create an empty string for myIndex in range(len(myString)-1, -1, -1): # more on this next tempString += myString[myindex] #copy to new string return(tempString) def isPalindrome(myString): revString = reverse(myString) if myString == revString: return(True) else: return(False) #Call isPalindrome print(isPalindrome(‘123454321’)
- 12. CS151 Spring 2019 Explanation The for loop in reverse may look weird. for myIndex in range(len(myString)-1, -1, -1): Remember: So len(myString) returns 10, but our index is 9 so we subtract one to accomodate 0 based indexing. We stop at -1, since the for loop stops at 0 not -1, the operator is < not <= Finally our step is decrementing by 1, so -1 Index 0 1 2 3 4 5 6 7 8 9 String P a l i n d r o m e
- 13. CS151 Spring 2019 More Explanation We do not need to write the reverse function, however, we will see how we can call reverse from another python program using an import statement. How can we make this program better? Error checking? Whitespace, punctuation removal?