Line Continuation, Output Formatting, and Decision Structures

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

2. Line Continuation • What do you do if your line of Python code is too long? • 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

3. Line Continuation: Example >>> sum = 2.35 + 8 \ + 13.6 + \ 25 >>> sum 48.950000000000003 >>> print “\t\nHello, my name is”, \ “Jarvis” Hello, my name is Jarvis

4. Output Formatting • We’ve seen this: print “The temperature is”,temp,”degrees” • Now, we’ll see another way • Better able to control how print displays values • 88.33333333 -> 88.33 • 4 / 15 / 2010 -> 4/15/2010

5. Output Formatting:Format Operators print “The temperature is %d degrees” % temp • String formatting operator: % • NOT modulus: modulus operates on numbers • Appears in strings • 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

6. 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

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

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

9. Formatting Example >>> import math # package containing pi >>> math.pi #display the value of pi 3.1415926535897931 >>> #Now display with 4 digits after decimal point >>> print “Pi: %.4f” % math.pi Pi: 3.1416 Notes: • The f in the format string stands for float. • The number appears in the output wherever the format string occurs. • The number is rounded to the specified number of digits.

10. Formatting Example • Can specify a minimum field width for the display of a value • Minimum width comes before the decimal point in the format string • >>> print “Pi: %7.3f” % math.pi • Pi = 3.142 • The field width is 7: • digits and decimal point: width 5 • 2 blank spaces to left of number

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

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

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

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

15. How do you specify a comparison? • Specify the condition using a relational operator

16. Comparisons:Examples • test = 13 < 15 • test = 101 >= 99 • test = “a” < “b” • test = 4 == 2+2 • test = 15 != 16 • test = 12 == 3*5

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

18. 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 • The condition is typically a comparison if some comparison is true: do something

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

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

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

22. 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.

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

24. if-elif-else example number = input(“Please enter your number: “) if number < 10: print number, “is small” elif number < 100: print number, “is pretty big” elif number < 500: print number, “is big” else: print “Wow, a really big number!” Sample Run: Please enter your number: 355 355 is big

25. Decisions:Nested ifs You can put if statements inside the body of the if (or elif or else) statement: if(<condition>): if(<some other condition>): command else: command elif(<condition>): …

26. Decisions:Gotchas • 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 if(.3 == .1+.2) is False

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