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Problem in F17: 14.3.3) String Extraction

Problem in S18: 25.3) String Extraction

Learning Objectives

  • Define/Call function with a return value
  • Define/Call function with an argument
  • Determine if situation could benefit from a function

Problem: The expression used below splits a string into two parts and flips them. Extract this expression into a function named flip_halves so we dont have to rewrite the expression for every string. Then replaces the existing expressions with function calls.

print("ABCDEFG"[int(len("ABCDEFG")/2):] + "ABCDEFG"[:int(len("ABCDEFG")/2)])
print("testing"[int(len("testing")/2):] + "testing"[:int(len("testing")/2)])
print("python"[int(len("python")/2):] + "python"[:int(len("python")/2)])

Step 0: Understanding the problem:

The goal here is to put the expression that flips the halves of a string into a function. Doing this will reduce the amount of redundant code, which is the whole point of functions. If we wanted to flip the halves of a string a bunch of times (don't worry about why we'd need to do that), we would have to copy the expression and type in the string every time. If we put this into a function, we could simply call the function, like flip_halves("ABCDEFG"), and it would return the result. This eliminates the need to rewrite code and to memorize or copy/paste the expression.

Step 1: Understanding the expression:

The expression we need to use in flip_halves is the "ABCDEFG"[int(len("ABCDEFG")/2):] + "ABCDEFG"[:int(len("ABCDEFG")/2)] part of the first line, where we need to put our string wherever we see "ABCDEFG". The first part of the expression, "ABCDEFG"[int(len("ABCDEFG")/2):] gets the last half of the string using indexing. The indexing [int(len("ABCDEFG")/2):] begins at int(len("ABCDEFG")/2) which finds the length of the string, divides it by 2 to find the middle, and converts that value to an integer using int() because string indexes must be integers. The end of the indexing (after the colon) is left blank to tell python to go to the end of the string. The second part of the expression "ABCDEFG"[:int(len("ABCDEFG")/2)] gets the first half in a similar way, starting at the beginning by putting nothing before the colon and going until the middle. The + in between these two parts combines the strings generated by the indexing back into one string.

Step 2: Creating the function:

Like in a prior example, Make Address, we start by creating the function header. Our function needs to be called flip_halves and it needs to take a single string, which we can just call string.

def flip_halves(string):

Step 3: Adding the expression:

Since our function needs to perform the expression above on our parameter, string, we need to replace "ABCDEFG" with string:

string[int(len(string)/2):] + string[:int(len(string)/2)]

Notice the quotation marks were removed. This is because instead of working with a string literal, "ABCDEFG", we are working with the variable string.

Now that we have changed the expression to work with the variable string, we need to return the result. This is simply done with the return statement.

return string[int(len(string)/2):] + string[:int(len(string)/2)]

Once our function hits this return statement, it will end and return the value from the expression to whatever part of the code called our function. Now, our completed flip_halves function is:

def flip_halves(string):
	return string[int(len(string)/2):] + string[:int(len(string)/2)]

Step 4: Using our function:

Now that we have a function that will flip the halves of any string we give it, we need to modify the lines given to us to use our function instead of using the expression directly. This is simply done by calling our function, passing it the proper string:

flip_halves("ABCDEFG")

We also need to print the return value:

print(flip_halves("ABCDEFG"))

Doing this for every line that we had originally we get:

print(flip_halves("ABCDEFG"))
print(flip_halves("testing"))
print(flip_halves("python"))

Solution:

Now we can put it all together, making sure our function is before the lines we call it so python knows it exists:

def flip_halves(string):
	return string[int(len(string)/2):] + string[:int(len(string)/2)]
	
print(flip_halves("ABCDEFG"))
print(flip_halves("testing"))
print(flip_halves("python"))

As you can see, writing functions for often repeated parts of code makes things a lot less tedious by removing the need to copy and paste, less error prone for the same reason, and makes it easier to read and understand.