Lecture 10: Strings. CS1068+ Introductory Programming in Python. Dr Kieran T. Herley 2018/19. Department of Computer Science University College Cork

Transcription

1 Lecture 10: Strings CS1068+ Introductory Programming in Python Dr Kieran T. Herley 2018/19 Department of Computer Science University College Cork

2 Summary 1

3 Python text: data type str String sequences of letters, digits, other keyboard symbols ; represent text ; no limit on length Examples "" "Donald J. Trump" "1600 Pensylvania Avenue, Washington DC, USA" " " "cs1068 is $#?%!" Note Double quotes delimit string but are not part of it. 2

4 Simple string operations Code Result Explanation len("python") 6 Length of string "Python"[3] "h" Return three-th character from string (counting from zero). "One"+"Two" "OneTwo" Concatenate: glue contents of second onto end of first string. "=" * 8 "========" Concatenate 8 copies of string together NB: + and have different meaning from arithmetic usage 3

5 Simple stuff Assignment and print work as expected. Console Donald Trump Code Editor fname = Donald lname = Trump potus = fname + + lname print(potus) 4

6 Some examples Example "Santa says: " + ("Ho ") * 3 5

7 Some examples Example "Santa says: " + ("Ho ") * 3 Result "Santa says: Ho Ho Ho" 5

8 Some examples Example "Santa says: " + ("Ho ") * 3 Result "Santa says: Ho Ho Ho" Example print( (("#" * 8) + "\n") * 8) 5

9 Some examples Example "Santa says: " + ("Ho ") * 3 Result "Santa says: Ho Ho Ho" Example print( (("#" * 8) + "\n") * 8) Result? ######## ######## ######## ######## ######## ######## ######## ######## 5

10 Some technicalities We can also use single quotes to delimit strings (prefer doubles) Only 97 days to Christmas! We can also use treble quotes (single or double) for multi-line strings """ There was a young student from Cork Who tried Python for a lark But as hard as he tried broken, baffled and brain-fried he remained alas in the dark! """ 6

11 Including tricky characters strings To include a quote character with a string, escape the internal quote symbol thus: print( Barack says, \ Python rocks!\ ) Alternatively can use: print('barack says, Python rocks! ') Other awkward characters: \' single quote \" double quote \n newline \t tab \\ backslash 7

12 Mixing strings and integers Console TypeError Code Editor three = 1 + two Confusion over what + means: for ints it means add, for strings it means concatenate. 8

13 Conversions From/To Integer Real String Integer 17.0 str(17) Automatic 17 conversion Real String Notes: int(3.14) 3 int("123") 123 float("3.14") 3.14 str(3.14) Illegal conversions e.g. int("one") cause ValueError. 2. Function round converts real to nearest integer. 9

14 Iterating through strings Can walk through strings one character at a time. Console D o n a l d Code Editor name = Donald for c in name:...print(c) Loop executes one per character i.e. once with c = D, once with c = o and so on. 10

15 Examples

16 Reversing a string Problem Write a function that takes a string and returns it mirror image i.e. "Thursday!" yields "!yadsruht". Ideas Use separate string to build up answer Step through characters of string one by one For each character prepend it to front of those seen before. 11

17 Voila def backwards(s): # Return a copy of string 's' reversed. t = for c in s: t = c + t return t 12

18 Gettings initials Problem Write a function that takes a string and returns an abbreviation consisting of the first letter of every word therein. Example "Donald J. Trump" yields "DJT" Ideas Assume every word (apart from first) is preceded by a space. Step through string character by character. Identify first letters and concatenenate in a separate variable 13

19 def initials (phrase): first letters = for i in range(len(phrase)): if i == 0 or (phrase[i 1] == ): first letters = first letters + phrase[ i ] return first letters 14

20 Vetting passwords Problem Write a function that takes in a password as a string and returns True if it (i) has length at least 8, (ii) has at least one upper case letter and one lowercase letter and (iii) contains at least one digit. Ideas Can easily check properties of individual character x.isdigit() True if x consists entirely of digits x == x.upper() True if x consists entirely of uppercase Buzz through string char. by char. checking for upper, lower and digits 15

21 def is good(pw): has eight = len(pw) >= 8 has upper, has lower, has num = False, False, False for ch in pw: has upper = has upper or (ch == ch.upper()) has lower = has lower or (ch == ch.lower()) has num = has num or ch.isdigit() return has eight and has upper and has lower and has num 16