1 Python Chapter 3 Reading strings and printing. © Samuel Marateck

2 To write and run a program: 1.On the shell’s file menu click New Window 2. Type your program. 3. Use a # to begin a comment. 4. Use the Edit window to edit your program. 5. Save your program to the proper directory To save, choose save on the file menu. 6. To run your program, click run module on the run menu,

3 When you save your program, type the program’s name a period and then py. For Instance prog1.py. The program you write is called the source program. When you run your program, the compiler produces what is called the object program. If you look at the directory, in our case you will see prog1.pyc; you will not be able to read this.

4 The entity that runs the object program is called the Python virtual machine.

5 The input() statement Input to a program is read using the input() statement and is read as a string, for instance, name = input(‘Type your input’) The variable name is a string variable. The string ‘Type your input’ is printed in the shell when the program is run.

6 Our program is: #our first program name = input(‘Type your input ’) Print(‘Our input is ‘, name)

7 When your run the program, the following appears in the shell: >>> Type your input You then type anything after the word input. Type your input NYU rocks Here we typed NYU rocks.

8 The computer responds: Our input is NYU rocks The entire session is shown on the next slide.

9 >>> Type your input NYU rocks our input is NYU rocks >>>

10 To be able to type the input on another line, place \n at the end of the string: name = input(‘Type your input \n’)

11 After you save the program and run it, and type asd as input, the following appears on the shell: >>> Type your input asd our input is asd >>>

12 The characters \n does not appear in the shell.; \n is called an escape sequence and n is called a control character and forces a a skip to the next line (carriage return).

13 To see the functions that can be used with a string, type dir(‘’) in the shell. The result is:

14 >>> dir('') ['__add__', '__class__', '__contains__', '__delattr__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getnewargs__', '__gt__', '__hash__', '__init__', '__iter__', '__le__', '__len__', '__lt__', '__mod__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__rmod__', '__rmul__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '_formatter_field_name_split', '_formatter_parser', 'capitalize', 'center', 'count', 'encode', 'endswith', 'expandtabs', 'find', 'format', 'index', 'isalnum', 'isalpha', 'isdecimal', 'isdigit', 'isidentifier', 'islower', 'isnumeric', 'isprintable', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'maketrans', 'partition', 'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill'] >>>

15 To see the effect of one of these functions, type a string, then a period, then the function ending with (). So >>>’asd’.upper() ‘ASD’ upper() converts every letter in the string to uppercase. It leaves all other characters alone. The following are examples of some of these functions.

16 lower()Changes uppercase to Lowercase and leaves all other variables alone. Example: >>>’THE’ ‘the’

17 isalpha()Returns True if the string consists of only letters else False. Example: >>> ‘the’.isalpha() TRUE >>>’the12’. isalpha() False

18 capitalize(). Capitalizes a string. Example: >>>’asd’. capitalize() ‘Asd’

19 strip() Removes the leading and trailing blanks. Example: >>>‘ asd ‘.strip() ‘asd’

20 To convert numeric input to an integer, use the int() function. value = input(‘type your integer’) number = int(value) print(number + 3)

21 Normally, two print statements print their output on two separate lines, Thus print(‘abc’) print(‘def’) Produces >>> abc def >>>

22 To have the output on one line, end the first print with end= ‘’ print(’abc’, end = ‘’) print(‘def’) produces >>>abcdef

23 We’ll investigate the following program: length = 87 width = 5 area = length*width print(‘area =‘, area)

24 If we wrote prin(‘def’) the computer would respond with Traceback (most recent call last): File " ", line 1, in prin('def') NameError: name 'prin' is not defined

25 This is another example of a syntax error. It is also called a compilation error since it occurs during compilation time. Such an error must be corrected before the program will run.

26 The following table describes how the variables are defined. undef means undefined.

27 statementlengthwidtharea length = 8787undef width = 5875undef area=length*width print875435

28 Now let’s analyze: length = 87 width = 5 area = length*width print(‘area =‘, area) width = 3 print(‘area =‘, area)

29 We see that the width has been redefined. What happens in the computer’s memory is that the bits in the location width are reconfigured.

30 statementlengthwidtharea length = 8787undef width = 5875undef area=length*width print width = print873435

31 We see that since the area does not appear on the left side of an assignment statement, it is not redefined and the original value of the area will be printed. This is called a logical error because it is an error in logic and will produce the wrong results. You must detect a logical error yourself.

32 In the corrected program we recalculate the area as is shown in the next slide.

33 length = 87 width = 5 area = length*width print(‘area =‘, area) width = 3 area = length*width print(‘area =‘, area)