String Operations - Computer Science - Lecture Notes, Study notes of Computer Science

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Today ...

• String operations
• Input function

Homework

• HW 3 due
• HW 4 out

String Operations

Python reuses some math operations for strings

• Concatenate two strings with +

>>> first = "Hello" >>> second = "World!" >>> first + second ’HelloWorld!’ >>> first + " " + second ’Hello World!’

• Repeat a string with *

>>> "Ha" * 3 ’HaHaHa’ >>> 3 * "Ha" ’HaHaHa’

• This doesn’t work:

>>> "Ha" * "Ha" TypeError: can’t multiple sequence by non-int of type ’str’

• This also doesn’t work:

>>> "Ha" + 3 TypeError: Can’t convert ’int’ object to str implicitly

The input built-in function

The input command can be used to get user input

• For example >>> ans = input() Hi >>> print(ans) Hi
• The input function waits for user input (ended by a newline)
• Then returns the result (the string entered by the user)
• You can also give an optional prompt >>> name = input("Please enter your name: ") Please enter your name: Shawn >>> print("Your name is:", name) Your name is: Shawn

The input command always returns a string

>>> x = input("Please enter an x value: ") Please enter an x value: 5

>>> y = input("Please enter a y value: ") Please enter a y value: 6

>>> print(x + y) # what happens if >>> x + y 56 # displays ‘56’ (with quotes)

>>> type(x + y)

Q: How can we properly add x and y here?

>>> int(x) + int(y) 11

>>> type(int(x) + int(y))

what about this ...

>>> type(int(x + y))

even though ...

>>> int(x + y) 56

Note that input is only for getting user information

• Only use when user input is needed

Q: For example ... when is this useful?

def add(): x = input("Enter x: ") y = input("Enter y: ") return int(x) + int(y)

• Only when we want user interaction ...
  • NOT for defining a general add function!
• Would be better to get the input, then call an add(x, y) function
• Relying on user input makes our functions less reusable!