Lesson 13 Introduction to Programming Student Resources
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- If-Statements and If/Else-Statements
Input from the User Now let’s try adding an input from you, the user, to a variable. The format may look strange, but the word input is actually a call to a function. By using the word input, we are invoking an already written piece of code whose job is to obtain input and return it. What is placed in the parentheses is a prompting message that will appear in the console window before you type in the input. This prompting message is useful for instructing the user on what to do. Type the following sequence: name2=input("Enter your name: ") We take the value returned and store it in the variable name2. Make sure you type the instruction exactly as shown, including two blank spaces after the colon (:) following the word name. After entering the above command, you will see 'Enter your name: ' in the console window. At this point, type a friend’s name and press the Enter key. You will not see any output. Another function is called print. Although we can output the value of a variable just by typing in the variable name, the print function is more useful in that we can use it to output a number of things. Type the following. Notice that there is a blank space after the word is. There are no blank spaces following the word print, the second quotation mark symbol, or the plus sign. print("My friend is "+name2) What do you see? 
If-Statements and If/Else-Statements Another feature of a program is to decide what should be done based on values stored in variables. For instance, we might want our program to decide and tell us if, based on our age, we should go to recess or to the library. To make a decision, programs use if-statements and if/else-statements. In Python, the syntax is a little bit odd and requires using proper indentation. Type the following after the prompt (>>>), and make sure you use the Tab key to indent when needed. The prompt and the ellipses (…) are automatically displayed; they are not something for you to type. >>> if age >= 15: ... print("Go to the library and study") ... else: ... print("Go play at recess") ... The result should be either the message about going to the library or the message about recess, depending on the value stored in age. Since age should now be 16, the message should tell you to go to the library and study. Notice that the notation >= means “greater than or equal to.” Now try this. Type: age=14 Now retype the if-then-else statement above. What output did you get this time? Notice how the computer makes a choice of what to do. The if-then-else statement is a very powerful tool that we use throughout a program. These statements allow the program to make decisions on its own. For-Loops Another type of feature of a program is known as a loop. A loop causes a set of code to operate a certain number of times. Several types of loops exist, but we will look only at one type of loop in Python called an iterator loop. This loop, known as a for-loop, will iterate once for each item in a list. Let’s try it out with the following code. Make sure to press the Tab key for the second line, and remember that the ellipses (…) are automatically placed there, so do not type them yourself. >>> for number in [0, 1, 2, 3, 4]: ... print(number) ... This will print 0, 1, 2, 3, and 4 on separate lines. 
The for-loop can operate on lists that contain numbers, strings, and individual characters. Type the following two for-loops, adding a tab for each second line: >>> for item in [1, 10, 'a', 'abc']: ... print(item) ... >>> for item in name: ... print(item) ... What do you see? (Remember that for our example, the variable name had Emily Smith stored in it. Your variable name should have your own name.) 
Another function is called range( ). This function returns a list of all of the numbers in the range from 0 to one less than the number you place in parentheses ( ). Type the following: for number in range(5): ... print(number) And you will see, 0, 1, 2, 3, 4 print out. You can also specify the starting point for a range. Retype the previous code but use range(1,6) and you will get the numbers 1 through 5 as output. Let’s use this idea to perform a computation called a factorial. A factorial is the value we get by multiplying a number by all of the numbers less than it down to 1. For instance, the factorial for 5 is 5*4*3*2*1. We write this value as 5!. The following code computes and outputs 5!. Remember to include the tab in the third line. factorial=1 for value in [1, 2, 3, 4, 5]: ... factorial=factorial * value print(factorial) The result should be 120. Directory: ourpages -> auto -> 2017 2017 -> Senior Division Solutions 2017 -> Avant garde adademy of broward 2017 -> Have you always wanted to go to space camp? Dreamt of becoming an astronaut here’s your chance! 2017 -> Atlanta, georgia-april 2017-a total solar eclipse occurs when the moon lines up between the earth and the sun-an extremely rare and beautiful event 2017 -> United states history and geography unit 1: The Rise of Industrial America 1877-1914 2017 -> 1. 1: Analyzing Categorical Data 2017 -> Out-of-Field Notification to Parents/Guardians English 2017 -> Modern History Notes 9-21-2017 Entertainment 2017 -> The secret of change is to focus all of your energy, not on fighting the old, but on building the new 2017 -> The Berlin Conference 1884 Simulation Historical Context Download 0.83 Mb. Share with your friends:
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