How to access the index before i in Python?

Question

struggling a bit understanding how to access indices in Python loops. How would you write this code in Python?

int x = 5;
for (int i = 0; i < s.length; i++) {
    if (s[i] + s[i - 1] == x) {
        System.out.println("Success");
    }
}

So far, I have tried the enumerate method, but I don't think it's working as intended.

x = 5
for i, c in enumerate(s):
    if (i, c + (i - 1), c == x):
        print("Success")

Sorry if this has been asked before, but I couldn't really find a solution to this exact way of handling indices in Python loops. Would really appreciate the help.

x = 5 for i in range(len(s)): if s[i]+s[i-1] == x: print('Success') — iamklaus
– iamklaus, Commented Sep 17, 2018 at 11:23
Is it a bug or a feature that the pair of first and last elements of s are tested together? — Eric Duminil
– Eric Duminil, Commented Sep 17, 2018 at 11:26

Mehrdad Pedramfar · Accepted Answer · 2018-09-17 11:45:29Z

2

considering s to be a sequence, assuming is to be a sequence with numbers and considering the first Java-style code in the first part

s = [0,1,2,3,4,5,..]
x = 5

for i in range(1, len(s)):
    if s[i] +s[i-1] == x:
        print("Success")

edited Sep 17, 2018 at 11:45

Mehrdad Pedramfar

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answered Sep 17, 2018 at 11:35

katamit

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1 Comment

Eric Duminil Over a year ago

Note that the Java code starts at 0 and not 1, so it checks if the first and last elements sum up to 5.

Abhinav Upadhyay · Accepted Answer · 2018-09-17 11:29:13Z

0

for i, c in enumerate(s):
    if c + s[i - 1] == x:

c here will be an element from the list referring to s[i] and i will be index variable. In order to access the element at i-1, you need to use s[i - 1]. But when i is 0, you will be comparing s[0] with s[-1] (last element of s) which might not be what you want and you should take care of that.

answered Sep 17, 2018 at 11:29

Abhinav Upadhyay

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2 Comments

Mehrdad Pedramfar Over a year ago

Try this values: s=[0,0,5,5,6,5] and x=5, it should print one time, but it will print two times, that is because of s[i -1] in the first iteration.

Abhinav Upadhyay Over a year ago

I mentioned that in the answer that it might not be what you want to do during the first iteration as i will be 0. The Java code in the question was starting the loop from 0 and I didn't want to deviate from that.

Paul Iyobo · Accepted Answer · 2018-09-17 11:37:10Z

0

the enumerate() method splits the lop target in a tuple containing the index, and the element of the target. Unlike c style languages, when you use a for loop in python without the enumerate, i represents the element. This means that you do not have to insert the index in the list when calling it. Your code in python would be like this. s = [1, 2, 3, 4, 5, 6, 7, 8, 9] # the list named s. x = 5 for i in s: if i + i-1 == x: print("success")

Since we're only working with integers, you could do like this as well

x = 5 for i in range(10): if i + i-1 == x: print("success")

answered Sep 17, 2018 at 11:37

Paul Iyobo

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1 Comment

sadmansh Over a year ago

I want to access the number in the i-th index, not the i-th index itself. In your example, aren't we adding 1 + 0, then 2 + 1, and so on?

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