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Then the code in the function creates a new list and attempts to use the second incoming parameter to replace the original list pointed to by that parameter withthe new list. The last line of text in Figure 1 confirms that this was not successful . The copy of the incoming parameter points to the new list but this does not cause the original reference to point to the new list.
This same situation occurs in the Java courses that I teach. I often attempt to explain it this way. Assume that I write my home address on a piece of paperand make a copy of that piece of paper. Then I lose the copy. If a burglar finds the copy, he has a pointer to my house. He could use that pointer in an attemptto break into my house. However, even if he were to scratch out the address on the copy and write a new address on the copy, that would not change the addresswritten on the original. The original would still point to the same old address.
This is essentially what happens in Listing 1 when the code creates a new list and assigns its value to the copy of the original reference. The copy points tothe new list inside the function but the original reference outside the function continues to point to the original list.
In effect, the copies that are received as parameters by a function become local variables within the function. Those variables, and in this case the newlist that is pointed to by one of those variables, all cease to exist when the function terminates unless returned by the function.
Figure 2 shows a visualization of the code in Listing 1 after all of the code has been executed.
Figure 2. Visualization of the code in Listing 1.
I recommend that you create a visualization for the code in Listing 1 and step through the program one instruction at a time. As you do that, pay attention tothe movements of the red and green arrows on the left, the diagram on the right, and the printed material at the bottom. That should help you to betterunderstand the concept of "passing by value."
Listing 2 shows a modification to the code from Listing 1 that is designed to accomplish what Listing 1 attempted to do but was unable to do.
| Listing 2 . Another list-modifier function. |
|---|
# Illustrates pass by value or reference
#---------------------------------------------------def listModifier(listA,listB):
"""Illustrates pass by value or reference"""print("In listModifier")
print("Use incoming parameter to append to listA")listA.append(3.14159)
print("New listA = " + str(listA))print("Assign a new list to listB")
listB = ["A","new","list"]print("New listB = " + str(listB))
return listB#End function definition
#Call the functionprint("Create two lists")
aList = ["ab","cd","ef"]bList = ["The","old","list"]
print("aList = " + str(aList))print("bList = " + str(bList))
print("Call listModifier")bList = listModifier(aList,bList)
print("Back from listModifier")print("aList = " + str(aList))
print("bList = " + str(bList)) |
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