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1. Control Flow : in Order to Control the Flow of Execution of a Program There Are Three Categories of Statements in Python.They Are: 1

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1. Control Flow : in Order to Control the Flow of Execution of a Program There Are Three Categories of Statements in Python.They Are: 1 13A05806 Python Programming UNIT – III 1. Control flow : In order to control the flow of execution of a program there are three categories of statements in python.They are: 1. Selection statements 2. iteration statements 3. Jump statements 1. 1. Selection Statements Decision making is valuable when something we want to do depends on some user input or some other value that is not known when we write our program. This is quite often the case and Python, along with all interesting programming languages, has the ability to compare values and then take one action or another depending on that outcome. Figure: If Statement Figure: if-else Statement For instance, you might write a program that reads data from a file and takes one action or another based on the data it read. Or, a program might get some input from a user and then take one of several actions based on that input. To make a choice in Python you write an if statement. An if statement takes one of two forms. It may be just an if statement. In this case, if the condition evaluates to true then it will evaluate the then statements. If the condition is not true the computer will skip to the statements after the if statement. Department of CSE-GPCET 13A05806 Python Programming <statements before if statement> if <condition>: <then statements> <statements after if statement> Figure depicts this graphically. An if statement evaluates the conditional expression and then goes to one of two places depending on the outcome. Notice the indentation in the if statement above. The indentation indicates the then statements are part of the if statement. Indentation is very important in Python. Indentation determines the control flow of the program. If the condition evaluates to true, a detour is taken to execute the then statements before continuing on after the if statement. Generally, we want to know if some value in our program is equal to, greater, or less than another value. The comparison operators, or relational operators, in Python allow us to compare two values. Any value in your program, usually a variable, can be compared with another value to see how the two values relate to each other. Sometimes, you may want your program to do one thing if a condition is true and something else if a condition is false. Notice that the if statement does something only when the condition evaluates to true and does not do anything otherwise. If you want one thing to happen when a condition is true and another to happen if the condition is false then you need to use an if-else statement. An if- else statement adds a keyword of else to do something when the condition evaluates to false. An if- else statement looks like this. <statements before if statement> if <condition>: <then statements> else: <else statements> <statements after if statement> If the condition evaluates to true, the then statements are executed. Otherwise, the else statements are executed. Figure depicts this graphically. The control of your program branches to one of two locations, the then statements or the else statements depending on the outcome of the condition. Again, indentation is very important. The else keyword must line up with the if statement to be properly paired with the if statement by Python. If you don‘t line up the if and the else in exactly the same columns, Python will not know that the if and the else go together. In addition, the else is only paired with the closest if that is in the same column. Both the then statements and the else statements must be indented and must be indented the same amount. Python is very picky about indentation because indentation in Python determines the flow of control in the program. In either case, after executing the if or the if-else statement control proceeds to the next statement after the if or if-else. The statement after the if-else statement is the next line of the program that is indented the same amount as the if and the else. Example: Consider a program that finds the maximum of two integers. The last line before the if- else statement is the y = assignment statement. The first line after the if-else statement is the print(―Done.‖) statement. Department of CSE-GPCET 13A05806 Python Programming x = int(input("Please enter an integer: ")) y = int(input("Please enter another integer: ")) i f x > y: print(x,"is greater than",y) e l s e : print(y,"is greater than or equal to",x) print("Done.") Output: Please enter an integer: 20 Please enter another integer: 48 48 is greater than or equal to 20 Example: Finding the Max of Three Integers x = int(input("Please enter an integer: ")) y = int(input("Please enter another integer: ")) z = int(input("Please enter a third integer: ")) i f y > x: i f z > y: print(z, "is greatest.") e l s e : print(y, "is greatest.") e l s e : i f z > x: print(z, "is greatest.") e l s e : print(x, "is greatest.") print("Done.") Output: Please enter an integer: 30 Please enter another integer:20 Please enter a third integer: 5 30 is greatest. Done The need to select between several choices that Python has a special form of the if statement to handle this. It is the if-elif statement. In this statement, one, and only one, alternative is chosen. The first alternative whose condition evaluates to True is the code that will be executed. All other alternatives are ignored. The general form of the if-elif statement is: <statements before if statement> if <first condition>: <first alternative> elif <second condition>: Department of CSE-GPCET 13A05806 Python Programming <second alternative> elif <third condition>: <third alternative> else: <catch-all alternative> <statements after the if statement> There can be as many alternatives as are needed. In addition, the else clause is optional so may or may not appear in the statement. If we revise our example using this form of the if statement it looks a lot better. Not only does it look better, it is easier to read and it is still clear which choices are being considered. In either case, if the conditions are not mutually exclusive then priority is given to the first condition that evaluates to true. This means that while a condition may be true, its statements may not be executed if it is not the first true condition in the if statement. 1.2 Iteration Statements Iteration refers to repeating the same thing over and over again. In the case of string sequences, you can write code that will be executed for each character in the string. The same code is executed for each character in a string. However, the result of executing the code might depend on the current character in the string. To iterate over each element of a sequence you may write a for loop. A for loop looks like this: for loop <statements before for loop> for <variable> in <sequence>: <body of for loop> <statements after for loop> In this code the <variable> is any variable name you choose. The variable will be assigned to the first element of the sequence and then the statements in the body of the for loop will be executed. Then, the variable is assigned to the second element of the sequence and the body of the for loop is repeated. This continues until no elements are left in the sequence. If you write a for loop and try to execute it on an empty sequence, the body of the for loop is not executed even once. The for loop means just what is says: for each element of a sequence. If the sequence is zero in length then it won‘t execute the body at all. If there is one element in the sequence, then the body is executed once, and so on. For loops are useful when you need to do something for every element of a sequence. Since computers are useful when dealing with large amounts of similar data, for loops are often at the center of the programs we write. Example : Consider the following program. s = input("Please type some characters and press enter:") for c in s: print(c) print("Done") Output: Department of CSE-GPCET 13A05806 Python Programming If the user enters how are you? the output is: h o w a r e y o u ? Done Each character of the sequence is printed on a separate line. Notice that there are blank lines, or what appear to be blank lines, between the words. This is because there are space characters between each of the words in the original string and the for loop is executed once for every character of the string including the space characters. Each of these blank lines really contains one space character. Example: Use for loop to find sum of 1 to 5 Example: Use for loop to find the sum of numbers numbers of a list. sum=0 sum=0 for i in range(1,6): a =[1,2,4,5] sum=sum+i for i in a: sum=sum+i print("sum of 1 to 5 numbers=",sum) print("sum of numbers of a list=",sum) Output: Ouput: sum of 1 to 5 numbers= 15 sum of numbers of a list= 12 While Loops Python's while statement is its most general iteration construct. In simple terms, it repeatedly executes a block of indented statements, as long as a test at the top keeps evaluating to a true value.
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