Lecture 4: Simple Input-Calculate-Output Programs

Transcription

1 Lecture 4: Simple Input-Calculate-Output Programs CS1068+ Introductory Programming in Python Dr Kieran T. Herley 2017/18 Department of Computer Science University College Cork

2 input Statement

3 Python s input statement Role Permits program to invite input (text) from user; program may use or manipulate same subsequently. Syntax Explanation var = input( prompt message here ) 1. Prompt string displayed to nudge user to type line of input; execution pauses until he/she does. 2. Line of text typed in by user in response 3. Assignment of variable name to response (string) 1

4 Simple example Program print( Hello! ) name = input( Who are you? ) print( Welcome, name) Effect Asks user to type in name and prints a welcome message incorporating name 2

5 Our execution environment Console Hello! Code Editor print( Hello! ) name = input( Who are you? ) print( Welcome, name) 3

6 Our execution environment Console Hello! Who are you? Code Editor print( Hello! ) name = input( Who are you? ) print( Welcome, name) 3

7 Our execution environment Console Hello! Who are you? Tom Code Editor print( Hello! ) name = input( Who are you? ) print( Welcome, name) User types Tom then return/enter (four keystrokes) 3

8 Our execution environment Console Hello! Who are you? Welcome Tom Tom Code Editor print( Hello! ) name = input( Who are you? ) print( Welcome, name) 3

9 A technicality Name as typed by user T o m RET Four keystrokes including RET As returned by input statement T o m String of length 3; input statement strips off the RET (newline)! 4

10 Temperature Conversion

11 Temperature conversion program Celsius-to-Fahrenheit formula Program outline t f = 9 5 t c Ask user to enter temperature in Celsius 2. Calculate Fahrenheit equivalent using formula 3. Print Fahrenheit temperature 5

12 First attempt BUGGY! temp c = input( Enter temp in Celsius: ) temp f = temp c 9 / print( Fahrenheit equivalent is, temp f) 6

13 First attempt cont d Console Code Editor Enter...: temp c = input( Enter...: ) temp f = temp c * 9 / print( Fahrenheit..., temp f) 7

14 First attempt cont d Console Code Editor Enter...: 15 temp c = input( Enter...: ) temp f = temp c * 9 / print( Fahrenheit..., temp f) 7

15 First attempt cont d Console Enter...: 15 TypeError Code Editor temp c = input( Enter...: ) temp f = temp c * 9 / print( Fahrenheit..., temp f) Program tries to use string value 15 in float calculation: Program crashes!

16 Type conversions Code Result Explanation int("123") 123 Convert string 123 into int 123; error if string unconvertible str(123) "123" Convert int 123 into str 123 float("4.56") 4.56 Convert string 4.56 into float 4.56; error if string unconvertible str(4.56) "4.56" Convert float 4.56 into str

17 Second attempt OK! temp c = float(input( Enter temp in Celsius: : )) temp f = temp c 9 / print( Fahrenheit equivalent is, temp f) 9

18 Second attempt cont d Console Code Editor Enter...: temp c = float(input( Enter...: )) temp f = temp c * 9 / print( Fahrenheit..., temp f) 10

19 Second attempt cont d Console Code Editor Enter...: 15 temp c = float(input( Enter...: )) temp f = temp c * 9 / print( Fahrenheit..., temp f) 10

20 Second attempt cont d Console Code Editor Enter...: 15 temp c = float(input( Enter...: )) temp f = temp c * 9 / print( Fahrenheit..., temp f) 10

21 Second attempt cont d Console Enter...: 15 Fahrenheit Code Editor temp c = float(input( Enter...: )) temp f = temp c * 9 / print( Fahrenheit..., temp f) 10

22 Simple imperial-to-metric converter

23 Problem definition Problem metres Write program to convert from feet and inches into Examples 5 feet 6 inches is metres 12 feet 4 inches is metres (one inch is metres) 11

24 Strategy Adopt similar input-calculate-output approach as temparature convereter 1. Read amount in feet and inches (seperately) 2. Compute total inches and equivalent in metres 3. Output result 12

25 The program first version f = int(input( Enter feet : )) i = int(input( Enter inches: )) l = 12 f + i m = l print(m) 13

26 The program first version f = int(input( Enter feet : )) i = int(input( Enter inches: )) l = 12 f + i m = l print(m) Observation A bit cryptic Enhancing readability More meaningful variable names More explanatory output (prompt and results) 13

27 The program tidier version # # Simple imperial to metric length converter # Kieran Herley, Sept 2014 # INCHES PER FOOT = 12 METRES PER INCH = print ( cs1068 imperial to metric converter ) feet = int(input( Please enter number of feet: )) inches = int(input( Please enter number of inches: )) length imp = INCHES PER FOOT feet + inches length metric = METRES PER INCH length imp print ( Equals, length metric, metres ) 14

28 Comments # # Simple imperial to metric length converter # Kieran Herley, Sept 2014 # Material after # is a comment Comments have no effect on program meaning or execution Every program should have a comment describing what it does for readability 15

29 Well chosen variable names Choose meaningful variable name that suggest what variable represents. So f feet i inches l length imp m length metric Generally frown on single-letter names 16

30 Named constants METRES PER INCH = length metric = METRES PER INCH length imp Constants are quantities that never 1 change. Use upper-case names for these. 1 or very rarely 17

31 Informative output print( Equals, length metric, metres ) Produces output that looks like Equals metres rather than just

32 The program again # # Simple imperial to metric length converter # Kieran Herley, Sept 2014 # INCHES PER FOOT = 12 METRES PER INCH = print ( cs1068 imperial to metric converter ) feet = int(input( Please enter number of feet: )) inches = int(input( Please enter number of inches: )) length imp = INCHES PER FOOT feet + inches length metric = METRES PER INCH length imp print ( Equals, length metric, metres ) 19