CMU 15-112: Fundamentals of Programming and Computer Science
Class Notes: String Formatting (Optional)

• This note is optional and will not be covered in quizzes or exams.
• Note that string formatting with f strings is not optional and is included in the main (required) Strings notes rather than here.

1. String formatting with the % operator (optional)
2. String formatting with the format method (optional)

1. String formatting with the % operator (optional)   video
   
   format a string with %s
   

   breed = "beagle"
   print("Did you see a %s?" % breed)

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   format an integer with %d
   

   dogs = 42
   print("There are %d dogs." % dogs)

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   format a float with %f
   

   grade = 87.385
   print("Your current grade is %f!" % grade)

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   format a float with %.[precision]f
   You can control how many fractional digits of a float are included in the string by changing the number to the right of the decimal point.

   grade = 87.385
   print("Your current grade is %0.1f!" % grade)
   print("Your current grade is %0.2f!" % grade)
   print("Your current grade is %0.3f!" % grade)
   print("Your current grade is %0.4f!" % grade)

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   format multiple values
   

   dogs = 42
   cats = 18
   exclamation = "Wow"
   print("There are %d dogs and %d cats. %s!!!" % (dogs, cats, exclamation))

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   format right-aligned with %[minWidth]
   

   dogs = 42
   cats = 3
   print("%10s %10s" % ("dogs", "cats"))
   print("%10d %10d" % (dogs, cats))

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   format left-aligned with %-[minWidth]
   

   dogs = 42
   cats = 3
   print("%-10s %-10s" % ("dogs", "cats"))
   print("%-10d %-10d" % (dogs, cats))

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2. String formatting with the format method (optional)
   
   Using .format()
   

   x, y, z = 3, 8, 42
   s = 'Huzzah!'
   
   # {k} is replaced by the kth argument to format():
   print('My favorite numbers are {0}, {1} and {2}. {3}'.format(x,y,z,s))
   
   # You can access the arguments in any order:
   print('{2} {1} {0}!'.format('mice', 'chase', 'cats'))
   
   # Or even multiple times:
   print('{0}{1}{0}'.format('abra', 'cad'))

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   Text alignment
   You can also use different specifiers to control how the strings are displayed.

   print(repr('{:<30}'.format('left aligned')))  # {:<30} = 30 characters left aligned
   print(repr('{:>30}'.format('right aligned'))) # {:>30} = 30 characters right aligned
   print(repr('{:^30}'.format('centered')))      # {:^30} = 30 characters center aligned
   
   # Text alignment with filler characters
   print('{:#<30}'.format('left aligned'))
   print('{:$>30}'.format('right aligned'))
   print('{:*^30}'.format('centered'))

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   String/Character specifiers
   

   print("{!r} uses repr and shows quotes. {!s} doesn't.".format('repr()', 'str()'))
   print("{:c}{:c}{:c}{:c}{:c}{:c}{:c}".format(75, 105, 109, 99, 104, 101, 101))

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   Numeric specifiers
   We can control what base a number is displayed in:

   print("int: {0:d}, bin: {0:b}, oct: {0:o}, hex: {0:x}, Hex: {0:X}".format(42))

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   Precision specifiers
   We can control the precision with which a number is displayed:

   import math
   print('{:,}'.format(1234567890))      # Print with commas every 3rd place
   print('{:0.2e}'.format(9876543210))   # Scientific notation with 2 decimal places
   print('{:0.4f}'.format(math.pi * 10)) # Display the float with 4 decimal places
   print('{:.3%}'.format(19/22))         # Display as a percentage with 3 decimal places

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   Example: degrees to radians
   This uses several .format() specifiers to display the conversion between degrees and radians in a pretty way.

   import math
   for i in range(0, 360, 45): 
       print("{:c}{:>4}º = {:.3f} radians".format(10209, i, math.radians(i)))

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