Python strings. Girls Programming Network School of Information Technologies University of Sydney. Mini-lecture 5

Transcription

1 Python strings Girls Programming Network School of Information Technologies University of Sydney Mini-lecture 5

2 Strings raw input Types Summary 2 Outline 1 Strings 2 Reading input with raw input 3 Types 4 Summary

3 Strings raw input Types Summary 3 Strings hold a sequence of characters ˆ 'Hello World' is a string ˆ We need to be able to distinguish the string from the program ˆ Strings are delimited (a.k.a. separated) using quotes ˆ You can use either single (') or double (") quotes ˆ But they ve got to match ˆ Syntax highlighting in IDLE turns strings green

4 Strings raw input Types Summary 4 What happens when they don t match? ˆ Try entering this string "hello world'... ˆ What about 'don't' or "he said "hi""? ˆ There are two ways to solve these problem: 1 use the opposite quotes: "don't" or 'he said "hi"' 2 escape the quote: 'don\'t' or "he said \"hi\""

5 Strings raw input Types Summary 5 A string of digits is not a number! ˆ Python stores different types of values ˆ The string type (str) isn t the same as the number type (int) ˆ Try this case: 1 >>> 2* >>> "2"*"5" 4 Traceback (most recent call last): 5 File "<stdin>", line 1, in <module> 6 TypeError: can't multiply sequence by non-int of type 'str' 7 >>>

6 Strings raw input Types Summary 6 But some arithmetic operators do work on strings ˆ Addition of two strings is called concatenation 1 >>> >>> "1" + '1' 4 '11' ˆ Multiplication between strings and integers works too: 1 >>> "2"*5 2 '22222' 3 >>> 5*"2" 4 '22222' ˆ Why does multiplication make sense?

7 Strings raw input Types Summary 7 Python provides code for reading from the user ˆ All you need to do is supply a message: 1 >>> name = raw_input('enter your name? ') 2 Enter your name? Jemima 3 >>> name 4 'Jemima' 5 >>> ˆ raw_input is a Python builtin function ˆ It takes a string argument to prompt the user with ˆ Then it waits for input (just like the Python prompt does) ˆ When the user presses Enter the input is returned ˆ It is returned as a string and put in the name variable

8 Strings raw input Types Summary 8 Functions are named pieces of code that do something ˆ When we refer to the code (followed by parentheses) it runs ˆ This is called calling or invoking the function 1 >>> name = raw_input('enter your name? ') 2 Enter your name? Jemima 3 >>> ˆ Our example calls the raw_input function ˆ raw_input goes purple because it is builtin ˆ When a function needs some information to do its work: ˆ we put the extra information inside the parentheses ˆ these are called function arguments

9 Strings raw input Types Summary 9 Function calls are substituted by their answers ˆ When a function call is finished: ˆ the answer (or value the function was calculating) is returned ˆ it effectively replaces the function call in an expression ˆ just like a variable is replaced by its value in expressions ˆ raw_input returns a string containing the input: 1 >>> name = raw_input('enter your name? ') 2 Enter your name? Jemima ˆ now this is the same as: 1 >>> name = 'Jemima' ˆ because raw_input is effectively replaced by the string 'Jemima'

10 Strings raw input Types Summary 10 Converting between strings and integers ˆ You need to explicitly convert between types ˆ The Python int function converts values to integers: 1 >>> int('12345') >>> ˆ Notice that there are no longer quotes around ˆ The Python str function converts values to strings: 1 >>> str(12345) 2 '12345' 3 >>> ˆ This time the quotes have been added because it is now a string

11 Strings raw input Types Summary 11 raw input and int ˆ Often you want to read an integer using raw_input ˆ But raw_input only gives back strings ˆ int to the rescue: 1 >>> num = raw_input('enter a number? ') 2 Enter a number? 5 3 >>> num 4 '5' 5 >>> num = int(raw_input('enter a number? ')) 6 Enter a number? 5 7 >>> num >>>

12 Strings raw input Types Summary 12 You should now be able to: ˆ Create Python strings ˆ Call Python functions ˆ Read input from the user with raw_input ˆ Convert between integers and strings using str or int