Structured programming 3 types of control structure Sequential structure Built into Python Selection structure The if statement The if/else statement The if/elif/else statement Repetition structure The while repetition structure The for repetition structure All have one entry point and one exit point

Sequence Control Structure add grade to total add 1 to counter total = total + grade; counter = counter + 1; Fig. 3.1 Sequence structure flowchart with pseudo code.

if Selection Structure print “Passed” Grade >= 60 true false Fig. 3.3 if single-selection structure flowchart.

if/else Structure false true Grade >= 60 print “Passed” print “Failed” false true Fig. 3.4 if/else double-selection structure flowchart.

if/elif/else Selection Structure condition a true false . condition z default action(s) condition b case a action(s) case b action(s) case z action(s) if statement first elif statement last elif statement else statement Fig. 3.5 if/elif/else multiple-selection structure.

while Repetition Structure Repetition Structures Allow a program to repeat an action while a condition is true Using while Repetition Action(s) contained within the body of the loop Condition should evaluate to false at some point Otherwise infinite loop and program hangs

while Repetition Structure true false Product = 2 * product Product <= 1000 Fig. 3.8 while repetition structure flowchart.

Greatest common divisor algorithm Get two integers a, b > 0 from the user If a and b are the same, GCD = a, exit Otherwise, repeat: If a > b: Replace a with a % b. If a is now 0, GCD = b, exit. Case b > a analogous

gcd.py Note % operator for table if/else indentation: dangling else

Running the program Input first integer > 0: 81 Input second integer > 0: 45 a b 81 45 36 45 36 9 The greatest common divisor of 81 and 45 is 9

for Repetition Structure The for loop Function range is used to create a list of values range ( integer ) Values go from 0 up to given integer (i.e., not including) range ( integer, integer ) Values go from first up to second integer range ( integer, integer, integer ) Values go from first up to second integer but increases in intervals of the third integer This loop will execute howmany times: for counter in range ( howmany ): and counter will have values 0, 1,.. howmany-1

Makes the counter go from zero to nine 1 # Fig. 3.18: fig03_18.py 2 # Counter-controlled repetition with the 3 # for structure and range function. 4 5 for counter in range( 10 ): 6 print counter Fig03_18.py Program Output Makes the counter go from zero to nine 0 1 2 3 4 5 6 7 8 9

range function Fig. 3.19 Function range. Python 2.2b2 (#26, Nov 16 2001, 11:44:11) [MSC 32 bit (Intel)] on win32 Type "help", "copyright", "credits" or "license" for more information. >>> range( 10 ) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] Fig. 3.19 Function range. Python 2.2b2 (#26, Nov 16 2001, 11:44:11) [MSC 32 bit (Intel)] on win32 Type "help", "copyright", "credits" or "license" for more information. >>> range( 10, 0, -1 ) [10, 9, 8, 7, 6, 5, 4, 3, 2, 1] Fig. 3.20 Function range with a third value.

Another example Fig03_22.py Program Output 1 # Fig. 3.22: fig03_22.py 2 # Summation with for. 3 4 sum = 0 5 6 for number in range( 2, 101, 2 ): 7 sum += number 8 9 print "Sum is", sum Loops from 2 to 101 in increments of 2 A sum of all the even numbers from 2 to 100 Sum is 2550

3.15 break and continue Statements The break statement Used to make a loop stop looping The loop is exited and no more loop code is executed The continue statement Used to continue the looping process All following actions in the loop body are not executed But the loop will continue to run

When x equals 5 the loop breaks. Only up to 4 will be displayed 1 # Fig. 3.24: fig03_24.py 2 # Using the break statement in a for structure. 3 4 for x in range( 1, 11 ): 5 6 if x == 5: 7 break 8 9 print x, 10 11 print "\nBroke out of loop at x =", x The loop will go from 1 to 10 Fig03_24.py Program Output When x equals 5 the loop breaks. Only up to 4 will be displayed Shows that the counter does not get to 10 like it normally would have 1 2 3 4 Broke out of loop at x = 5

continue skips rest of body but continues loop Fig03_26.py Program Output 1 # Fig. 3.26: fig03_26.py 2 # Using the continue statement in a for/in structure. 3 4 for x in range( 1, 11 ): 5 6 if x == 5: 7 continue 8 9 print x, 10 11 print "\nUsed continue to skip printing the value 5" The loop will continue if the value equals 5 The value 5 will never be output but all the others will 1 2 3 4 6 7 8 9 10 Used continue to skip printing the value 5

Calling/invoking a function Details Functions Purpose Building blocks for the program (separate functionality in independant parts) Avoid code repetition Calling/invoking a function functionName ( argument1, argument2 ) Details Variables created in a function are local to that function Arguments are also local variables A function may return a result using the return statement

Modules A module contains useful function definitions and other elements All of which are related in some way The import keyword is used to include a module A function from a module is invoked like this: moduleName.functionName( arguments )

Functions in math Module 2.3 (#1, Aug 4 2003, 10:37:16) [GCC 3.1.1] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> >>> print sqrt( 900 ) Traceback (most recent call last): File "<stdin>", line 1, in ? NameError: name 'sqrt' is not defined >>> import math >>> print math.sqrt( 900 ) 30.0 Fig. 4.2 Function sqrt of module math.

User-defined functions Functions must be defined before they are used: def functionName ( paramList ): paramList is a comma separated list of parameters The actions of the function then follow They should all be indented appropriately The actions are also called the block or the function body

The function returns the passed value multiplied by itself 1 # Fig. 4.4: fig04_04.py 2 # Creating and using a programmer-defined function. 3 4 # function definition 5 def square( y ): 6 return y * y 7 8 for x in range( 1, 11 ): 9 print square( x ), 10 11 print Fig04_04.py Program Output This is a function definition, the function is called square and is passed the value y The function returns the passed value multiplied by itself This calls the square function and passes it the value x   1 4 9 16 25 36 49 64 81 100

This is a function that receives three values Fig04_05.py 1 # Fig. 4.5: fig04_05.py 2 # Finding the maximum of three integers. 3 4 def maximumValue( x, y, z ): 5 maximum = x 6 7 if y > maximum: 8 maximum = y 9 10 if z > maximum: 11 maximum = z 12 13 return maximum 14 15 a = int( raw_input( "Enter first integer: " ) ) 16 b = int( raw_input( "Enter second integer: " ) ) 17 c = int( raw_input( "Enter third integer: " ) ) 18 19 # function call 20 print "Maximum integer is:", maximumValue( a, b, c ) 21 print # print new line 22 23 d = float( raw_input( "Enter first float: " ) ) 24 e = float( raw_input( "Enter second float: " ) ) 25 f = float( raw_input( "Enter third float: " ) ) 26 print "Maximum float is: ", maximumValue( d, e, f ) 27 print 28 29 g = raw_input( "Enter first string: " ) 30 h = raw_input( "Enter second string: " ) 31 i = raw_input( "Enter third string: " ) 32 print "Maximum string is: ", maximumValue( g, h, i ) This is a function that receives three values Fig04_05.py The function determines the greater of three values and returns it Gets three integers, passes them to the maximumValue function, and displays the results to the user The same process is performed on float and string variables to show the diversity of the function Note: only possible because of dynamic typing! Function parameters not type declared

Fig04_05.py Program Output Enter first integer: 27   Enter first integer: 27 Enter second integer: 12 Enter third integer: 36 Maximum integer is: 36 Enter first float: 12.3 Enter second float: 45.6 Enter third float: 9.03 Maximum float is: 45.6 Enter first string: hello Enter second string: programming Enter third string: goodbye Maximum string is: programming Fig04_05.py Program Output