Before Reading Week. Lists. List methods. Nested Lists. Looping through lists using for loops. While loops

Transcription

1 Before Reading Week Lists List methods Nested Lists Looping through lists using for loops While loops

2 This Week and Maybe Next Special Characters Review Files What are they? Opening and closing files Reading and writing to files Comma Separated Value Files

3 Strings Using Conversion Specifiers We some'mes would like to insert values of variables into strings: A1 = 60 A2 = 75 A3 = 88 We would like: The average of 60, 75 and 88 is How do we print this with our variables? >>>print( The average of, A1,,, A2, and, A3, is, (A1+A2+A3)/3) Does this work?

4 Strings Using Conversion Specifiers We displayed: The average of 60, 75 and 88 is Q. What s wrong? A. Spacing is wrong around commas and periods. We have many more decimal places than wanted. Q. How can we fix it? A. Use conversion specifiers. >>>print( The average of %d, %d and %d is %.2f %(A1, A2, A3, (A1+A2+A3)/3.0)) The average of 60, 75 and 88 is

5 Common Conversion Specifiers %d display the object as a decimal integer %f display the object as a floating point with 6 decimal places %.2f display the object as a floating point with 2 decimal places %s display the object as a string Q. What else do we use % for? A. Modulus. We say that % is overloaded.

6 Invisible Characters How do we indicate a tab, newline or or inside a string? \t tab \ double quote \n new line \ single quote \\ backslash These are called escape sequences. print( This string has a \n newline and a \t tab. This string has a newline and a tab.

7 Files Files what are they? Can think of as a collection of stored information Computer thinks as a sequence of bits possibly arranged in characters Files have names: my_python.py, homework.txt, etc. What can we do with a file? Read from a file and write to a file

8 Opening Files How do we open a file in Python? myfile = open( story.txt, r ) Open is the Python function Story.txt is the name of the file to be opened myfile is a variable that is assigned the file object (also called stream or reader). r is a string indicating what we will do with the file (read, write, append)

9 Using Files After we are finished with a file we must close it: myfile.close() When we write to a file, we have two choices: Write: myfile = open( filename, w ) Append: myfile = open( filename, a ) write replaces the file filename append appends to the file filename

10 Reading Files There are many ways to read from a file. 1. Using a for loop: myfile = open( filename, r ) for line in myfile: <do something with the line> 2. Read the whole file at once into a list of strings: myfile = open( filename, r ) list_of_lines = myfile.readlines()

11 Reading Files 3. Read the entire file into a string: myfile = open( filename, r ) s = myfile.read() 4. Read a specific number of characters: myfile = open( filename, r ) s = myfile.read(10) reads 10 characters s = myfile.read(10) reads then next 10 characters

12 Reading Files 5. Read one line at a time: myfile = open( filename, r ) line = myfile.readline() reads a line line = myfile.readline() reads the next line

13 End Of File (EOF) How do we know when we reach the end of the file? A for loop automatically recognizes EOF. for line in myfile: <do something with the line> Here we have to check for the end of file: line = myfile.readline() reads the next line and s = myfile.read(10) reads 10 characters The EOF character is the empty string.

14 Writing to a file First open the file for writing or appending: myfile = open( story.txt, w ) start the story myfile = open( story.txt, a ) continue the story Then write to the file: myfile.write( Once upon a time ) myfile.write( The end. ) Then close the file: myfile.close()

15 Reading from a CSV file Often we have comma-separated values data files: First Name, Last Name, Utorid Anna, Bretscher, bretsche Joe, Johnson, johnsonj Sally, Jordan, jordansa Why do we like CSV files? Spreadsheet applica@ons like excel understand it Many other programming languages also understand them Easy to generate ourselves

16 Reading from a CSV file How do we read comma-separated values data files: import io import csv csv_file = open( csv_filename.csv, r ) reader = csv.reader(csv_file) for line in reader: # read like an ordinary file # but line is a list

17 While Loops Sometimes we need to loop until a condition is met. For example: Ask user for a password twice If the passwords don t match, ask again until they match or the user quits

18 While Loop Example English example: Ask for password. Ask for password again. While the passwords don t match: Ask again.

19 While Loop Example Python example: password1 = input( Enter password: ) Ask for password again. While the passwords don t match: Ask again.

20 While Loop Example Python example: password1 = input( Enter password: ) password2 = input( Re-enter password: ) While the passwords don t match: Ask again.

21 While Loop Example Python example: password1 = input( Enter password: ) password2 = input( Re-enter password: ) while password1!= password2: Ask again.

22 While Loop Example Python example: password1 = input( Enter password: ) password2 = input( Re-enter password: ) while password1!= password2: password2 = input( Re-enter password: )