Line Continuation, Output Formatting, and Decision Structures CS303E: Elements of Computers and Programming

Line Continuation What do you do if your line of Python code is too long? What do you do if your line of Python code is too long? Use the line continuation character! Use the line continuation character! –the backslash character \ –Place at the very end of the line –Python interpreter will assume the next line is part of the same line

Line Continuation: Example >>> sum = \ \ \ >>> sum >>> print “\t\nHello, my name is”, \ “Jarvis” “Jarvis” Hello, my name is Jarvis

Output Formatting We’ve seen this: We’ve seen this: print “The temperature is”,temp,”degrees” –Now, we’ll see another way Better able to control how print displays values Better able to control how print displays values – > –4 / 15 / > 4/15/2010

Output Formatting: Format Operators print “The temperature is %d degrees” % temp String formatting operator: % String formatting operator: % –NOT modulus: modulus operates on numbers –Appears in strings Indicates how and where a value should be printed in a string Indicates how and where a value should be printed in a string Also indicates end of print string and beginning of the values to be printed Also indicates end of print string and beginning of the values to be printed

Output Formatting: Format Operators print “The temperature is %d degrees” % temp Indicates the type and format of the value Indicates the end of the string to be printed and the beginning of the values specified in the string

Output Formatting: Format Specifiers For each, you can also specify width and precision: For each, you can also specify width and precision: %. %. print “The average is %3.2f” % avg

Output Formatting: Embedded Operations You can calculate values in your print statement: print “2+3 = %d” % (2+3) print “x/y = %.2f” % (x/y)

Formatting Example >>> import math # package containing pi >>> math.pi #display the value of pi >>> #Now display with 4 digits after decimal point >>> print “Pi: %.4f” % math.pi Pi: Notes: 1. The f in the format string stands for float. 2. The number appears in the output wherever the format string occurs. 3. The number is rounded to the specified number of digits.

Formatting Example Can specify a minimum field width for the display of a value Can specify a minimum field width for the display of a value Minimum width comes before the decimal point in the format string Minimum width comes before the decimal point in the format string >>> print “Pi: %7.3f” % math.pi >>> print “Pi: %7.3f” % math.pi Pi = Pi = The field width is 7: The field width is 7: –digits and decimal point: width 5 –2 blank spaces to left of number

Formatting: Two or More Values Enclose multiple values in parentheses Enclose multiple values in parentheses Separate the values with commas Separate the values with commas print “First name: %10s, Last Name: %12s” % (“Elvis”, “Presley”) First name: Elvis, Last Name: Presley

Output Formatting: Examples Modify average.py to print 2 decimal places of the average Modify average.py to print 2 decimal places of the average Practice printing strings from raw_input() Practice printing strings from raw_input() Print using multiple values Print using multiple values –The values must be enclosed in parentheses

Question for you: Output Formatting What is the expected output? x = 5.7 y = 2.18 print “x+y=%.1f” % (x+y) A. 7C. 7.9 B. 7.8D. 8

Comparisons Allows you to compare two values Allows you to compare two values Result in a True or False value Result in a True or False value –type Boolean ( bool ) You can compare numbers or strings, literals, variables, or expressions You can compare numbers or strings, literals, variables, or expressions

How do you specify a comparison? Specify the condition using a relational operator Specify the condition using a relational operator OperatorMeaning <Less than >Greater than <=Less than or equal >=Greater than or equal ==Equality !=Not equals

Comparisons: Examples 1. test = 13 < test = 101 >= test = “a” < “b” 4. test = 4 == test = 15 != test = 12 == 3*5

Lexicographic Order Strings are rated according to lexicographic order Strings are rated according to lexicographic order Orders words A-Za-z Orders words A-Za-z –Capital letters first in alphabetical order –Lower-case letters second in alphabetical order NOT in dictionary order NOT in dictionary order

Decisions “Once you make a decision, the universe conspires to make it happen.” -- Ralph Waldo Emerson Gives you the ability to specify different instructions based on a condition Gives you the ability to specify different instructions based on a condition The condition is typically a comparison The condition is typically a comparison if some comparison is true: do something

Decisions: if Statement def main(): command command if( ): if( ): command command main() Indentation matters! (Again) Commands not dependent on the condition Commands only executed if condition is true Commands not dependent on the condition

if examples number = 25 if number > 10: print number, “is greater than 10!” print number, “is greater than 10!”Output: 25 is greater than 10!

Decisions: if-else Statement if( ): if( ): command command else: else: command command Commands only executed if condition is True Commands only executed if condition is False

if-else exercise Write a program that asks the user to enter a number. If the number is 3, print a message indicating that they entered your favorite number, and otherwise, indicate that you don’t like the chosen number. Write a program that asks the user to enter a number. If the number is 3, print a message indicating that they entered your favorite number, and otherwise, indicate that you don’t like the chosen number.

Decisions: if-elif-else Statement if( ): if( ): command command elif( ): elif( ): command command else: else: command command Commands only executed if condition is True Commands only executed if EVERY condition is False Commands only executed if earlier conditions are False and this condition is True You can used as many of these as you like

if-elif-else example number = input(“Please enter your number: “) if number < 10: print number, “is small” print number, “is small” elif number < 100: print number, “is pretty big” print number, “is pretty big” elif number < 500: print number, “is big” print number, “is big”else: print “Wow, a really big number!” print “Wow, a really big number!” Sample Run: Please enter your number: is big

Decisions: Nested if s You can put if statements inside the body of the if (or elif or else ) statement: if(<condition>): if( ): if( ):command else: else:commandelif(<condition>):…

Decisions: Gotchas Exactly ONE of the bodies in if- elif-else will be executed Exactly ONE of the bodies in if- elif-else will be executed –Only the first True condition –THINK about the construction of your if statements before coding Comparison of floats Comparison of floats if(.3 ==.1+.2) is False

Question for you: Decisions What is the expected output? if(125<140): print “first one” print “first one”elif(156>=140): print “second one” print “second one”else: print “third one” print “third one” A. first oneC. third one B. second oneD. first one second one second one