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Print assigned conditional

in this video we're gonna look at statements that allow our programs to decide what to do based on a condition so for example here's a function called ABS value it takes in a number X and it returns the absolute value of that number so for example if we pass in negative 5 for X then what's going to happen is X is assigned negative 5 we jump to the function and the first thing we hit is this new type of statement called an if statement and we have a condition here X is less than 0 and in this case that condition is true because negative 5 is less than 0 and therefore we're going to do the statement or statements that are indented underneath that if so in this case we're taking the current value of x we are multiplying it by negative 1 so negative 1 times negative 5 is positive 5 that positive 5 is then stored back into the variable and now when we come down here and return X we're gonna be returning positive 5 which is what we'll see at the shell if on the other hand we passed in a positive integer for X then when we hit this if statement the condition is not true because 10 is not less than 0 and therefore we're gonna skip this line that's indented and go to whatever comes afterwards so this time we're not going to change the value of x we're just going to return the value that was passed in and you can see that in either case what we're getting is the absolute value of what was passed in and so the if statement allows us to decide whether we should change the value or not so this is known as an if statement and this is the simplest form of conditional execution we have the keyword if followed by some condition followed by a colon and then we have a block of one or more statements that are indented the same amount and that is what I'm calling the true block in this case our true block only has a single statement but it could have more than one and in general we're going to evaluate the condition if it's true we're going to execute the true block but if it's false we're going to skip over it sometimes rather than just choosing whether to do something or not we want to choose between two options and for this we can expand our if statement to include an else component also with a colon after it and then another block of statements which I'm calling a false block so if the condition is true we're going to execute the true block otherwise we're going to execute the false block and so you can see that the flowchart is a choice between two options either the true block which again is one or more statements or the false block which can be one or more statements it's all based on whether the condition is true or not here's an example of a two-way decision I have a function called pass/fail it's taking a numeric average if the average is greater than or equal to 60 we're going to assign the string pass to the variable grade otherwise we're going to assign the string fail to the variable grade and then we'll return the appropriate string now what are the options for the conditions so we have less than or greater than we can do less than or equal to notice the equal comes after we can do greater than or equal to if you want to compare things to see if they're equal you need to use two equal signs not just one because a single equal sign is used for assignment and then not equals is an exclamation point followed by an equal sign when we evaluate a condition in Python we get one of two values capital T true or capital F false for example if I enter 10 less than 20 at the shell I'll get true because 10 is less than 20 if I enter the string Jones equals equals the string Baker I'm getting false because those two strings are not equal to each other and it's important to realize these values true and false are not strings they are actually literal values a special type known as the bull type if I use the type function on the literal value true you can see that it gives me the type bool I can also use the type function on an expression like xxx greater than 6 and because it evaluates to a bool I'm going to also see that type any expression that evaluates to true or false is known as a boolean expression and true and false are boolean values we can form more complex conditions by using what are called the logical operators and or and not and they either combine boolean expressions or they modify the value of a boolean expression and I've summarized how they work here on this slide you can find more information in the textbook now when we talk about blocks our true block and our false blocks but that notion of a block is a more general one and as we said it can contain more than one statement so for example here our true block has two statements so if the string that we pass in for the variable year is equal to the string frosh we're gonna print these two lines but otherwise we're gonna print these three lines and so that's another example of a two-way decision how do you know where a block begins and ends well a block begins whenever we increase the amount of indenting so because we're adding additional indenting here this starts a new block and as long as we maintain at least that much indenting the block is going to continue and a block ends when one of two things happens either we reach a line with less indenting than the start of the block so for example here you can see the else has less indenting than the first line of the true block and so the true block ends when we hit this else line the other option is we reach the end of the program so here we're beginning our false block because we increase the indenting and that false block continues as long as we have it Lee lunch indenting but then we end up hitting the end of the program down here so that false block ends this function is supposed to take a numeric average and return the appropriate letter grade but as it stands it doesn't work let's trace through a case to see why so if we pass in ninety five for the average we begin with the first if statement and the condition is true so we assign the string a to grade but the next condition is also true ninety five is greater than or equal to 80 and so now the grade becomes B and all of these conditions are true so we keep updating the grade and then we finally skip this last assignment because it's in the false block and we return the string D so we get the incorrect return value here what we'd like to do is not have separate if statements here but one big multi-way decision and we can do that in Python by changing these later conditions from ifs to elif's and by doing that we create one big multi way decision and we're going to be choosing from among these possible options and so now when we pass in ninety five the first condition is true we do it's true block and we skip all of the others and we return the string a more generally you can have after the first if as many LS as you want each with their own condition and their own true block the conditions are evaluated in order the first condition that is true gets its true block executed and then we skip all the rest and we're only going to do the false block if all of the conditions are false and here's what the flowchart looks like for that so again as soon as you get a condition that's true you do its block you skip all the rest