C H A P T E R 7 In the first few chapters, we saw some of the basic building blocks of a program. We can now make a program with input, processing, and output. We can even make our input and output a little fancier by using a GUI. We can assign the input to a variable, so we can use it later, and we can use some math to process it. Now we re going to start looking at ways to control what the program does. If a program did the same thing every time, it would be a little boring and not very useful. Programs need to be able to make decisions on what to do. We re going to add some different decision-making techniques to our processing repertoire. Testing, testing Programs need to be able to do different things based on their input. Here are a few examples: If Tim got the right answer, add 1 point to his score. If Jane hit the alien, make an explosion sound. If the file isn t there, display an error message. To make decisions, programs check (do a test) to see if a certain condition is true or not. In the first example above, the condition is got the right answer. Python has only a few ways to test something, and there are only two possible answers for each test: true or false. 65
66 Hello World! Here are some questions Python can ask to test something: Are two things equal? Is one thing less than another? Is one thing greater than another? But wait a minute, got the right answer isn t one of the tests you can do, at least not directly. That means you need to describe the test in a way Python can understand. When we want to know if Tim got the right answer, we d probably know the correct answer, as well as Tim s answer. We could write something like this: If Tim had the correct answer, then the two variables would be equal, and the condition would be true. If he had the wrong answer, the two variables would not be equal, and the condition would be false. Doing tests and making decisions based on the results is called branching. The program decides which way to go, or which branch to follow, based on the result of the test. Python uses the keyword if to test conditions, like this: if timsanswer == correctanswer: print "You got it right!" score = score + 1 print "Thanks for playing." These lines form a block of code because they re indented from the lines above and below
CHAPTER 7 67 A block of code is one or more lines of code that are grouped together. They re all related to a particular part of the program (like an if statement). In Python, blocks of code are formed by indenting the lines of code in the block. The colon (:) at the end of the if line tells Python that a block of instructions is coming next. The block includes every line that is indented from the if line, up until the next line that is not indented. Indenting means that a line of code is pushed over to the right a bit. Instead of starting at the far left, it has some spaces at the beginning, so it starts a few characters away from the left side. If the condition is true, everything in the following block will be done. In the previous short example, the second and third lines make up the block of statements for the if in the first line. Now might be a good time to talk about indenting and blocks of code. Indenting In some languages, indenting is just a matter of style you can indent however you like (or not at all). But in Python, indenting is a necessary part of how you write the code. Indenting tells Python where blocks of code start and where they end.
68 Hello World! Some statements in Python, like the if statement, need a block of code to tell them what to do. In the case of the if statement, the block tells Python what to do if the condition is true. It doesn t matter how far you indent the block, as long as the whole block is indented the same amount. A convention in Python is to use four spaces to indent blocks of code. It would be a good idea to follow this style in your programs. A convention just means lots of people do it that way. Am I seeing double? Are there actually two equal signs in that if statement (if timsanswer == correctanswer)? Yes, there are, and here s why. Am I seeing double? People say, Five plus four is equal to nine, and they ask, Is five plus four equal to nine? One is a statement; the other is a question. In Python we have the same kinds of things statements and questions. A statement might assign a value to a variable. A question might check whether a variable is equal to a certain value. One means you re setting something (assigning it or making it equal). The other means you re checking or testing something (is it equal, yes or no?). So Python uses two different symbols. You already saw the equal sign (=) used for setting or assigning values to variables. Here are a few more examples: correctanswer = 5 + 3 temperature = 35 name = "Bill" For testing whether two things are equal, Python uses a double equal sign (==), like this: if myanswer == correctanswer: if temperature == 40: if name == "Fred":
CHAPTER 7 69 Mixing up = and == is one of the most common mistakes in programming. Several languages use these symbols (not just Python), and lots of programmers use the wrong one in the wrong place every day. Testing or checking is also called comparing. The double equal sign is called a comparison operator. Remember, we talked about operators in chapter 3. An operator is a special symbol that operates on the values around it. In this case, the operation is to test whether the values are equal. Other kinds of tests Lucky for us, the other comparison operators are easier to remember: less than (<), greater than (>), and not equal to (!=). (You can also use <> for not equal to, but most people use!=.) You can also combine > or < with = to make greater than or equal to (>=) and less than or equal to (<=). You might have seen some of these in math class. Not equal In Python 3, the <> syntax for not equal to is no longer supported. In Python 3, you have to use!= for not equal to. You can also chain two greater-than or less-than operators together to make an in-between test, like this: if 8 < age < 12:
70 Hello World! This will check if the variable age has a value between, but not including, 8 and 12. This would be true if age was equal to 9, 10, or 11 (or 8.1 or 11.6, and so on). If we wanted to include the ages 8 and 12, we d do this instead: if 8 <= age <= 12: Comparison operators are also called relational operators (because they test the relation between the two sides: equal or not equal, greater than or less than). A comparison is also called a conditional test or logical test. In programming, logical refers to something where the answer is either true or false. Listing 7.1 shows an example program using comparisons. Start a new file in the IDLE editor, type this program in, and save it call it compare.py. Then Run it. Try running it several times, using different numbers. Try numbers where the first one is bigger, where the first one is smaller, and where the two numbers are equal, and see what you get. Listing 7.1 Using the comparison operators num1 = float(raw_input("enter the first number: ")) num2 = float(raw_input("enter the second number: ")) if num1 < num2: print num1, "is less than", num2 if num1 > num2: print num1, "is greater than", num2 if num1 == num2: print num1, "is equal to", num2 if num1!= num2: print num1, "is not equal to", num2 Remember that this is a double equal sign What happens if the test is false? You ve seen how to make Python do something if the result of a test is true. But what does it do if the test is false? In Python, there are three possibilities: Do another test. If the first test comes out false, you can get Python to test something else with the keyword (which is short for else if ), like this: if answer >= 10: print "You got at least 10!" answer >= 5: print "You got at least 5!" answer >= 3: print "You got at least 3!"
CHAPTER 7 71 if answer>=10 answer>=5 answer>=3 least 3! least 5! least 10! You can have as many statements as you want after the if. Do something else if all the other tests come out false. You do this with the else keyword. This always goes at the end, after you ve done the if and any statements: if answer >= 10: print "You got at least 10!" answer >= 5: print "You got at least 5!" answer >= 3: print "You got at least 3!" print "You got less than 3." if answer>=10 answer>=5 answer>=3 else Got less than 3! least 3! least 5! least 10!
72 Hello World! Move on. If you don t put anything else after the if block, the program will continue on to the next line of code (if there is one) or will end (if there is no more code). if answer>=10 answer>=5 answer>=3 least 3! least 5! least 10! Try making a program with the code above by adding a line at the beginning to input a number: answer = float(raw_input ("Enter a number from 1 to 15")) Remember to save the file (you pick the name this time), and then run it. Try it a few times with different inputs to see what you get. Testing for more than one condition What if you want to test for more than one thing? Let s say we made a game that was for eight-year-olds and up, and we want to make sure the player is in at least third grade. There are two conditions to meet. Here is one way we could test for both conditions: age = float(raw_input("enter your age: ")) grade = int(raw_input("enter your grade: ")) if age >= 8: if grade >= 3: print "You can play this game." print "Sorry, you can't play the game." Notice that the first print line is indented eight spaces, not just four spaces. That s because each if needs its own block, so each one has its own indenting.
CHAPTER 7 73 Reminder Remember, if you re using Python 3, you need to replace raw_input() with input(), and you need to use parentheses with print, like this: print("you can play this game.") Using and That last example will work fine. But there is a shorter way to do the same thing. You can combine conditions like this: age = float(raw_input("enter your age: ")) grade = int(raw_input("enter your grade: ")) if age >= 8 and grade >= 3: print "You can play this game." print "Sorry, you can't play the game." Combine conditions with and We combined the two conditions using the and keyword. The and means both of the conditions have to be true for the following block to execute. if age>=8 and grade>=3 You can play! (Only get here if both conditions are true) You can t play! You can t play!
74 Hello World! You can put more than two conditions together with and: age = float(raw_input("enter your age: ")) grade = int(raw_input("enter your grade: ")) color = raw_input("enter your favorite color: ") if age >= 8 and grade >= 3 and color == "green": print "You are allowed to play this game." print "Sorry, you can't play the game." If there are more than two conditions, all the conditions have to be true for the if statement to be true. There are other ways of combining conditions too. Using or The or keyword is also used to put conditions together. If you use or, the block is executed if any of the conditions are true: color = raw_input("enter your favorite color: ") if color == "red" or color == "blue" or color == "green": print "You are allowed to play this game." print "Sorry, you can't play the game." if color = red or color = blue or color = green else You can t play! You can play! Using not You can also flip around a comparison to mean the opposite, using not: (Get here if any condition is true) color = raw_input("enter your favorite color: ") if color == "red" or color == "blue" or color == "green": print "You are allowed to play this game." print "Sorry, you can't play the game."
CHAPTER 7 75 This line means the same as this one: if not (age < 8): if age >= 8: In both cases, the block executes if the age is 8 or higher, and it doesn t if the age is lower than 8. In chapter 4, you saw math operators like +, -, *, and /. In this chapter, you saw the comparison operators <, >, ==, and so on. The and, or, and not keywords are also operators. They re called logical operators. They re used to modify comparisons by combining two or more of them (and, or) or reversing them (not). Table 7.1 lists all the operators we ve talked about so far. Table 7.1 List of math and comparison operators Operator Name What it does Math operators = Assignment Assigns a value to a name (variable). + Addition Adds two numbers together. This can also be used to concatenate strings. - Subtraction Subtracts two numbers. += Increment Adds one to a number. -= Decrement Subtracts one from a number. * Multiplication Multiplies two numbers together. / Division Divides two numbers. If both numbers are integers, the result will be just the integer quotient, with no remainder. % Modulus Gets the remainder (or modulus) for integer division of two numbers. ** Exponentiation Raises a number to a power. Both the number and the power can be integers or floats. Comparison operators == Equality Checks whether two things are equal. < Less than Checks whether the first number is less than the second number. > Greater than Checks whether the first number is greater than the second number. <= Less than or equal to >= Greater than or equal to Checks whether the first number is less than or equal to the second number. Checks whether the first number is greater than or equal to the second number.!= <> Not equal to Checks whether two things are not equal. (Either operator can be used.)