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PH2150 Scientific Computing Skills Control Structures in Python • In general, statements are executed sequentially, top to bottom. • There are many instances when you want to break away from this e.g, • Branch point: choose to go in one direction or another based on meeting some condition. • Repeat: You may want to execute a block of code several times. • Programming languages provide various control structures that allow for more complicated execution paths. • Here we will focus on two such examples conditionals and loops: • Will see the if, while and for statements and associated else, elif, range, break and continue commands PH2150 Scientific Computing Skills The if statement is used to check a condition: if the condition is TRUE, we run a block of statements (the if-block), else (FALSE) we process another block of statements (else-block) or continue if no optional else clause. if guess==number: #if followed by condition we are testing, colon indicates begin the if-block <Block of statements> # indent shows that you are in the if-block, when the block of statements is completed you will drop back into the main program. if guess==number: #if TRUE execute statements <Block of statements> # skips else else:#if FALSE executes the second block <Block of statements> # returns to main program after these statements. PH2150 Scientific Computing Skills The elif statement allows you to check multiple expressions for TRUE and execute a block of code as soon as one of the conditions evaluates to TRUE.. if guess==number: #1st condition <Block of statements> # elif condition2: # checks for a 2nd condition which if TRUE runs second block of statements. <Block of statements> # elif condition3: # checks for a 3rd condition which if TRUE runs 3rd block of statements. <Block of statements> # elif condition4: # checks for a 4th condition which if TRUE runs 4th block of statements. <Block of statements> # else: # optional else if all above statements are FALSE <Block of statements> # PH2150 Scientific Computing Skills You may want to check for another condition after a condition resolves to TRUE. Here you can use the nested if construct. In a nested if construct, you can have an if...elif...else construct inside another if...elif...else if condition1: # <Block of statements> # runs statements then checks next if. if condition2: #checks for a 2nd condition which if TRUE runs second block of statements. <Block of statements> # elif condition3: # checks for a 3rd condition which if TRUE runs 3rd block of statements. <Block of statements> # else: # if condition 1 is TRUE, but 2 and 3 are FALSE. <Block of statements> # else: # optional else if condition 1 is FALSE <Block of statements> # PH2150 Scientific Computing Skills Loops: for loops are traditionally used when you have a piece of code which you want to repeat number of times. As an alternative, there is the while Loop, the while loop is used when a condition is to be met, or if you want a piece of code to repeat forever. for letter in 'Python': # loops for each character in string print 'Current Letter :', letter fruits = ['banana', 'apple', 'mango'] for fruit in fruits: # loops for each element in list print 'Current fruit :', fruit for index in range(len(fruits)): # loops for each element in list print 'Current fruit :', fruits[index] FruitsA=[x for x in fruits if x[0]==‘a’] # returns [‘apple’] PH2150 Scientific Computing Skills Control Structures in Python: range() The range() function creates a list of numbers determined by the input parameters. range([start,] stop[, step]) -> list of integers When step is given, it specifies the increment (or decrement). >>> range(5) [0, 1, 2, 3, 4] >>> range(5, 10) [5, 6, 7, 8, 9] >>> range(0, 10, 2) [0, 2, 4, 6, 8] e.g. How to get every second element in a list? for i in range(0, len(data), 2): print data[i] PH2150 Scientific Computing Skills Loops: the while loop is used when a condition is to be met, or if you want a piece of code to repeat forever. while expression: #while TRUE execute block of statements <Block of statements> var = 1 while var == 1 : # This constructs an infinite loop num = raw_input("Enter a number :") print "You entered: ", num if num=='Q': # This gives a route out of loop print “Goodbye” break # break will terminate the loop