Conditionals Conditionals: If-Statements Format Example if < - PowerPoint PPT Presentation

Mini-Lecture 11 Conditionals

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) 9/19/18 Conditionals 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 9/19/18 Conditionals 3

Conditionals: “Control Flow” Statements b Branch Point: if b : Evaluate & Choose s1 # statement s1 s3 s3 Statement: Execute if b : b s1 Flow else : Program only s1 s2 takes one path s2 each execution s3 s3 9/19/18 Conditionals 4

Conditionals and Local Variables • temp is needed for swap def max(x,y): """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 4 3 x = y 4 y = temp x y 0 0 temp 3 5 return y 9/19/18 Conditionals 5

Conditionals and Local Variables • temp is needed for swap def max(x,y): """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 5 3 x = y 4 y = temp x y 0 3 temp 3 5 return y 9/19/18 Conditionals 6

Conditionals and Local Variables • temp is needed for swap def max(x,y): """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 3 RETURN 9/19/18 Conditionals 7

Conditionals and Local Variables • Value of max(3,0) ? def max(x,y): """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 9/19/18 Conditionals 8

Conditionals and Local Variables • Value of max(3,0) ? def max(x,y): """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 9/19/18 Conditionals 9

Conditionals and Local Variables • Value of max(0,3) ? def max(x,y): """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 9/19/18 Conditionals 10

Conditionals and Local Variables • Value of max(0,3) ? def max(x,y): """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 9/19/18 Conditionals 11

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 > … 9/19/18 Conditionals 12

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 skips over all the others < statement > … § else means all are false 9/19/18 Conditionals 13

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 9/19/18 Conditionals 14