Control Structures Conditionals: If-Statements Format Example if - PowerPoint PPT Presentation

Lecture 6 Control Structures

Conditionals: If-Statements Format Example if < boolean-expression >: # Put x in z if it is positive < statement > if x > 0: … z = x < statement > Execution : if <b oolean-expression > is true, then execute all of the statements indented directly underneath (until first non-indented statement) 9/25/15 Control Structures 2

Conditionals: If-Else-Statements Format Example if < boolean-expression >: # Put max of x, y in z < statement > if x > y: … z = x else : else : < statement > z = y … Execution : if <b oolean-expression > is true, then execute statements indented under if; otherwise execute the statements indented under elsec 9/25/15 Control Structures 3

Conditionals: If-Elif-Else-Statements Format Example if < boolean-expression >: # Put max of x, y, z in w < statement > if x > y and x > z: … w = x elif < boolean-expression >: elif y > z: < statement > … w = y … else : else : w = z < statement > … 9/25/15 Control Structures 4

Conditionals: If-Elif-Else-Statements Format Notes on Use if < boolean-expression >: • No limit on number of elif < statement > § Can have as many as want … § Must be between if , else elif < boolean-expression >: • The else is always optional < statement > § if - elif by itself is fine … … • Booleans checked in order else : § Once it finds a true one, it < statement > skips over all the others … § else means all are false 9/25/15 Control Structures 5

Conditional Expressions Format Example # Put max of x, y in z e1 if bexp else e2 z = x if x > y else y • e1 and e2 are any expression • bexp is a boolean expression • This is an expression! expression, not statement 9/25/15 Control Structures 6

Program Flow vs. Local Variables def max(x,y): • temp is needed for swap """Returns: max of x, y""" § x = y loses value of x # swap x, y § “Scratch computation” # put the larger in y § Primary role of local vars 1 if x > y: • max(3,0) : 2 temp = x max 3 x = y 4 y = temp x y 0 3 temp 3 5 return y 9/25/15 Control Structures 7

Program Flow vs. Local Variables def max(x,y): • Value of max(3,0) ? """Returns: max of x, y""" A: 3 # swap x, y B: 0 # put the larger in y C: Error! if x > y: D: I do not know temp = x x = y y = temp return temp 9/25/15 Control Structures 8

Program Flow vs. Local Variables def max(x,y): • Value of max(3,0) ? """Returns: max of x, y""" A: 3 CORRECT # swap x, y B: 0 # put the larger in y C: Error! if x > y: D: I do not know temp = x x = y • Local variables last until y = temp § They are deleted or § End of the function return temp • Even if defined inside if 9/25/15 Control Structures 9

Program Flow vs. Local Variables def max(x,y): • Value of max(0,3) ? """Returns: max of x, y""" A: 3 # swap x, y B: 0 # put the larger in y C: Error! if x > y: D: I do not know temp = x x = y y = temp return temp 9/25/15 Control Structures 10

Program Flow vs. Local Variables def max(x,y): • Value of max(0,3) ? """Returns: max of x, y""" A: 3 # swap x, y B: 0 # put the larger in y C: Error! CORRECT if x > y: D: I do not know temp = x x = y • Variable existence y = temp depends on flow • Understanding flow return temp is important in testing 9/25/15 Control Structures 11

Local Variables Revisited def max(x,y): • Never refer to a variable """Returns: max of x, y""" that might not exist # swap x, y • Variable “scope” # put larger in temp § Block (indented group) if x > y: where it was first assigned temp = x First assigned § Way to think of variables; x = y not actually part of Python y = temp • Rule of Thumb : Limit variable usage to its scope return temp Outside scope 9/25/15 Control Structures 12

Local Variables Revisited def max(x,y): • Never refer to a variable """Returns: max of x, y""" that might not exist # swap x, y • Variable “scope” # put larger in temp § Block (indented group) temp = y First assigned where it was first assigned if x > y: § Way to think of variables; temp = x not actually part of Python • Rule of Thumb : Limit return temp Inside scope variable usage to its scope 9/25/15 Control Structures 13

Variation on max def max(x,y): Which is better? """Returns: Matter of preference max of x, y""" if x > y: return x There are two returns ! else: But only one is executed return y 9/25/15 Control Structures 14

For Loops: Processing Sequences # Print contents of seq The for-loop: x = seq[0] for x in seq: print x print x x = seq[1] print x … • Key Concepts x = seq[len(seq)-1] § loop sequence: seq print x § loop variable : x • Remember : § body : print x § Cannot program … § Also called repetend § Reason for recursion 9/25/15 Control Structures 15

For Loops: Processing Sequences • loop sequence: seq The for-loop: • loop variable : x for x in seq: • body : print x print x To execute the for-loop: 1. Check if there is a “next” element of loop sequence True seq has 2. If not, terminate execution put next more elts elt in x 3. Otherwise, put the element in the loop variable 4. Execute all of the body False print x 5. Repeat as long as 1 is true 9/25/15 Control Structures 16

Example: Summing the Elements of a List def sum(thelist): """Returns: the sum of all elements in thelist Precondition: thelist is a list of all numbers (either floats or ints)""" # Create a variable to hold result (start at 0) # Add each list element to variable # Return the variable 9/25/15 Control Structures 17

Example: Summing the Elements of a List def sum(thelist): """Returns: the sum of all elements in thelist Precondition: thelist is a list of all numbers (either floats or ints)""" result = 0 • loop sequence: thelist for x in thelist: result = result + x • loop variable : x • body : result=result+x return result 9/25/15 Control Structures 18

Example: Summing the Elements of a List def sum(thelist): """Returns: the sum of all elements in thelist Precondition: thelist is a list of all numbers (either floats or ints)""" Accumulator result = 0 variable • loop sequence: thelist for x in thelist: result = result + x • loop variable : x • body : result=result+x return result 9/25/15 Control Structures 19

For Loops and Conditionals def num_ints(thelist): """Returns: the number of ints in thelist Precondition: thelist is a list of any mix of types""" # Create a variable to hold result (start at 0) # for each element in the list… # check if it is an int # add 1 if it is # Return the variable 9/25/15 Control Structures 20

For Loops and Conditionals def num_ints(thelist): """Returns: the number of ints in thelist Precondition: thelist is a list of any mix of types""" result = 0 for x in thelist: if type(x) == int: Body result = result+1 return result 9/25/15 Control Structures 21

Modifying the Contents of a List def add_one(thelist): """(Procedure) Adds 1 to every element in the list Precondition: thelist is a list of all numbers (either floats or ints)""" for x in thelist: DOES NOT WORK! x = x+1 # procedure; no return 9/25/15 Control Structures 22

More Complex For-Loops • Combine with a counter • Nest conditionals inside § Variable that increments § Body is all indented code each time body executed § Can put other control § Tracks position in seq structures inside the body • Example : • Example : cnt = 0 nints = 0 # num of ints for x in seq: for x in seq: print `x`+' at '+`cnt` if type(x) == int: cnt = cnt + 1 # incr nints = nints + 1 9/25/15 Control Structures 23

for-loops: Beyond Sequences def blanklines(fname): • Work on iterable objects """Return: # blank lines in file fname § Object with an ordered Precondition: fname is a string""" collection of data # open makes a file object § This includes sequences file = open('myfile.txt') § But also much more # Accumulator count = 0 • Examples : for line in file: # line is a string § Text Files (built-in) if len(line) == 0: # line is blank § Web pages ( urllib2 ) count = count+1 • 2110 : learn to design f.close() # close file when done return count custom iterable objects 9/25/15 Control Structures 24

Beyond Sequences: The while-loop while < condition >: statement 1 repetend or body … statement n • Relationship to for-loop § Broader notion of true condition “still stuff to do” repetend § Must explicitly ensure false condition becomes false 9/25/15 Control Structures 25