Introduction to Problem Solving and Programming in Python.

Transcription

1 Introduction to Problem Solving and Programming in Python

2 Overview Repeating Types of loops While loops For loop Nested loops 2

3 Loops A loops allows us to execute one or more statements multiple times Contains at least one loop control expression (LCE) that is evaluated each time the loop executes to determine if we should remain in the loop Each iteration in the loop must allow updating the program state (values checked by the LCE) to ensure the loop can eventually exit 3

4 Anatomy of loop control Loop control expression comprises 3 elements Initialization Check Update 4

5 While loops Executes a set of statements while a condition is true 5

6 While loops Executes a set of statements while a condition is true Syntax while expression: statement(s) Example countdown = 10 while countdown >= 0: print (countdown) countdown -= 1 print("blastoff!") 6

7 While loop Loop control expression countdown >= 0 Loop body print (countdown) countdown -= 1 Update statement countdown -= 1 7

8 Loop errors What happens if we fail to perform an update (in the loop body)? countdown = 10 while countdown >= 0: print (countdown) countdown -= 1 What happens if our loop control expression is incorrect? countdown = 10 while countdown <= 0: print (countdown) countdown -= 1 8

9 Common loop errors Infinite loops Initialization error Off by 1 error 9

10 Sentinel controlled loops While loops often use computable values in the LCE Sometimes we don t know how many times the loop will iterate Usually when loop is processing data from external source In these cases, we can use a sentinel value to indicate that the loop should not continue executing 10

11 Properties of a sentinel value Same data type as normal input Outside the valid range of normal input Examples Grade values for a course Data type: float Valid range: Good sentinel: -1 Entering names Data type: string Valid range: Non-empty string Good sentinel: Empty string 11

12 Loop errors Sentinel is a flag, it is not valid input Must be excluded from data set Case study: Student grade statistics Algorithm: Initialize loop control variable and accumulator(s) Ask the user to enter the grade for each student and keep running total When sentinel is entered, calculate and print total and average 12

13 Student grade statistics grade = 0 total = 0 count= 0 while grade >= 0: grade = float(input("enter next grade. -1 to exit: ")) if grade >= 0: count+= 1 total += grade if students > 0: average = total / count else: average = 0 print("total:", total) print("average: ", average) 13

14 For loops Iterates a specified amount of times. Appropriate for sequence iteration. Syntax for single_value_variable in sequence: statement(s) Example for i in range(10, -1, -1): print(i) print("blastoff!") 14

15 range() Python s range() function generates a list of numbers Usage: range(stop) Generates a list of numbers from 0 stop range(start, stop) Generates a list of numbers from start to stop range(start, stop, step) Generates a list of numbers from start to stop in step increments 15

16 Using Else clause with a loop (in Python) When used with While Statements execute when the condition evaluates to false For Statements executed when the loop has iterated over all items in the list 16

17 Nested loops The suite of a loop clause can be almost any valid Python statement, including other loops Syntax while expression: for single_value_variable in sequence: statement(s) Semantics: For every single iteration of the outer loop, the inner loop will iterate to completion 17

18 Case study: drawing rectangles Algorithm Ask user to enter width and height Draw a width x height rectangle using * for each dot 18

19 Case study: drawing rectangles width = int(input("enter rectangle width: ")) height = int(input("enter rectangle height: ")) for i in range(height): for j in range(width): print("* ", end="") print() 19