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I was trying a simple recursive problem in python and was stumped by the for-loop-esq implementation in python. The implementation uses the range(x,y,z) function.

Here is the code in python: 

num = 25
f1 = 0
f2 = 1

for num in range(1,num,1):
    num = f1 + f2
    f1 = f2
    f2 = num

print("num is : ", num)

Same code in java:

int num = 25;
int f1 = 0;
int f2 = 1;
for(int i = 1; i < num; i++){
    num = f1 + f2;
    f1 = f2;
    f2 = num;
}
System.out.println("# at 25 is : " + num);

The java implementation will never spit out the right answer (75025) because num is overwritten in the first line of the for-loop (num = f1 + f2). Where as in python, the code executes and delivers the right answer. How come? Does python implement some sort of i counter under the hood? I'm using the pyCharm debugger to test this.

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4
  • This is not recursion, recursion is when a function calls itself, which is not the case here, you are simply looping Commented Jun 6, 2018 at 17:57
  • despite the different behavior between java and python, why do you need to change a variable that shoud be used to control the loop? if you want to execute 25 times, don't change the variable. If you want a more "flexible" loop, use while instead Commented Jun 6, 2018 at 18:00
  • @ArthurAttout You're right, I was trying the for-loop implementation of the problem. Commented Jun 6, 2018 at 18:08
  • @RicardoPontual The whole question stems from a typo and curiosity. I know what the correct implementation is. Commented Jun 6, 2018 at 18:10

3 Answers 3

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I believe the difference is that the range function in Python has arguments passed by value. So even after you give the num variable a new value, the function still holds on to the 25 you gave it originally, and thus iterates 25 times.

In Java the loop is checking the value of num after every iteration to see when to stop. Since the value of num changed to 1 during the first iteration, it met the criteria to stop the loop, so it only iterates one time.

I hope that makes sense!

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2 Comments

Ah! that does make sense. Now I'm decending down a "Pass By Ref vs Pass by Value" rabbit hole. First legitimate answer. Thanks! stupidpythonideas.blogspot.com/2013/11/…
I'm glad I could help clarify!
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In the first iteration of the for loop, you set num to 1 (num = f1 + f2). Since num is used in your loop condition, the loop will stop after the first iteration. To get it working the same way as the python code you will need another variable and keep num at 25

    int num = 25;
    int f1 = 0;
    int f2 = 1;
    int res = 0;
    for (int i = 1; i < num; i++) {
        res = f1 + f2;
        f1 = f2;
        f2 = res;
    }
    System.out.println("# at 25 is : " + res);
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1 Comment

Lars, that wasn't the question. Thanks.
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My guess is that the function in range(1, num, 1) only sees something like this in range(1,25,1) or in other word it is function with arguments passed by value.

The return value of the function range should be something like [1,2,3 ...25].

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