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# Course A201: Introduction to Programming - PowerPoint PPT Presentation

Course A201: Introduction to Programming. 09/30/2010. Outlines for this week. How to write for loops Function range() Python membership operator: in Write nested for loops to print out certain shapes More on Strings String Indexing Function len () String functions

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### Course A201:Introduction to Programming

09/30/2010

• How to write for loops

• Function range()

• Python membership operator: in

• Write nested for loops to print out certain shapes

• More on Strings

• String Indexing

• Function len()

• String functions

• Finish Part-1, understand what to do in Part-2

for loops vs while loops

number = 1

While number< 11:

print(number)

count += 1

number_list = range(1, 11, 1)

for number innumber_list:

print(number)

• The output will be exactly the same

[functions]

• input1, input2, input3, …

• You give me some inputs

I am a function.

I will perform a certain job.

• I give you some outputs back, explicitly or implicitly

• output1, output2, output3, …

• Example of how to use range():

>>> range()

ERROR! range expected at least 1 arguments

>>> range(10)

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

>>> ans = range(10)

>>> ans

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

• 10 is not included!

• So, we know that range() will require at least one argument, let’s see how it works when there’re two:

>>> range(3, 11)

[3, 4, 5, 6, 7, 8, 9,10]

>>> range(-10,1)

[-10, -9, -8, -7, -6, -5, -4, -3, -2, -1, 0]

>>> range(10,1)

[]

• Empty! when second argument is smaller than the first one, and there is no third argument.

• Three arguments?

>>> range(3, 11, 2)

[3, 5, 7, 9]

>>> range(10, 1, -1)

[10, 9, 8, 7, 6, 5, 4, 3, 2]

>>> range(-10, -1, -1)

[]

>>> range(2, 8, 0)

ERROR! range() step argument must not be zero

• The 3rd argument is the ‘step argument’. The default value is 1.

• Empty!

name = “Linger Xu”

foraain name:

print(aa)

number_list = range(-10, 1)

for number innumber_list:

print(number)

• The variable name is a string, “Linger Xu”

[foraain name] fetch every letter in this string sequentially, and put it into aa

• The variable number_list contains a list of numbers: [for number innumber_list ] fetch every number in this list sequentially, and put it into number

• Go throught every member of a specified sequence:

• -10, -9, -8, …

• user = “5198”

if “1” in user:

print(“Number 1 is in”, user)

if “0” not in user:

print(“Number 0 is not in”, user)

• membership operator: in / not in

• The variable after in must be holding a sequence of values, such as string and list.

number_list = 9

for number innumber_list:

print(number)

ERROR! 'int' object is not iterable

• Finish first problem in Part1 , Assignment 4

Write a program that counts for the user using a for loop as shown in class. Let the user enter the starting number, the ending number, and the amount by which to count.

• How to print out:

1 2 3 4 5 6 7

1 2 3 4 5 6 7

1 2 3 4 5 6 7

1 2 3 4 5 6 7

1 2 3 4 5 6 7

row_size = 5

column_size = 7

for row in range(row_size):

for column in range(column_size ):

print(column+1, end=“ ”)

print()

• Loop within a loop. Look out for indentation!!

• What will happen if you indent the second print also?

row_size = 5

column_size = 7

for row in range(row_size):

for column in range(column_size ):

print(column+1, end=“ ”)

print()

• How to print out:

1 1 1 1 1 1 1

2 2 2 2 2 2 2

3 3 3 3 3 3 3

4 4 4 4 4 4 4

5 5 5 5 5 5 5

row_size = 5

column_size = 7

for row in range(row_size):

for column in range(column_size ):

print(row+1, end=“ ”)

print()

• Just change column in 5th line to row

• Finish second problem in Part1 , Assignment 4

Scalable Patterns: What do the following codes print?

• Understand the 1st problem in Part2

String: Indexing

• str=“killer rabbit”

• Ex: str[0] returns “k”

str[1] returns “i”

str[3] returns “l”

str[-3] returns “b”

str[-14] returns Error! Index out of range!

String: Indexing

• Try this program (in textbook Chapter 4):

import random

word = "index"

print("The word is: ", word, "\n“)

high = len(word)

low = -len(word)

for i in range(10):

position = random.randrange(low, high)

print("word[", position, "]\t", word[position])

String: Indexing

• Count the occurrence of one letter in a string

str1=“killer rabbit”

target =“i”

count = 0

for letter in str1:

if letter == target:

count += 1

print(“There’re ” + str(count) + “ ‘i‘s in string: ” + str(str1) )

Function len()

• Return an integer that represents how many elements are there in this specified sequence

>>> user = input(“Type a word: “)

>>> user = “I like Python.”

>>> len(user)

14

>>> user = “5198”

>>> len(user)

4

String: methods

quote = "I like Python.“

• quote.upper() -> capitalize everything

–“I LIKE PYTHON.”

• quote.lower() -> small letter everything

–“I like python.”

• quote.title() -> capitalize the first letter of every word

–“I Like Python.”

String: methods

quote = "I like Python.“

• quote.strip() -> removes spaces, tabs, newlines before and after

• quote.replace(“like”, “dislike programming in”)

–“I dislike programming in Python.”

• Try quote.center(50)

String methods vs Built-in functions

• When you want to use a string method, you’ve got to use the dot“.”:

>>> quote = "I like Python."

>>> quote.upper()

'I LIKE PYTHON.'

>>> upper(quote) <- Error!

• While built-in function

>>> len(quote)

>>> range(10)

String: Indexing and slicing

• Understand the 2nd and 3rd problem in Part2