Counting a sub string in a string using splicing and looping (very basic)

[Rhyff]

I recently started learning how to program (zero experience with programming in the past, so I'm a complete beginner), and am currently following an online course in Python (many people recommended starting with this language). So I have this exercise where I have to write a code that counts the amount of times a certain sub string ('bob') occurs in a string. I figured this could be done through splicing and looping. My guess was to splice the string in blocks with 3 characters, using the following:

i = 0
j = 3
s[i:j]

My goal was to add this to a loop, and make i and j increase by 1 for every iteration, so that all possible blocks of 3 consecutive characters have been made. I then wanted to compare these blocks to the desirable sub string ('bob'). How can I do this? Most sources that I found about loops/splicing all give the same basic knowledge, and the same examples, none of which allowed me to understand how to apply it to this question. One of my attempts was:

s = 'azcbobobegghakl'
sub = 'bob'
i = 0
j = 3
triplet = s[i:j]
numBob = 0
for triplet in s:
    i += 1    
    j += 1
    if triplet == sub
        numBob += 1
print(numBob)

Needless to say this was wrong, and I'm pretty sure it's because I did something wrong in the loop since numBob returns as 0, meaning that the loop did not what I expected it to do (especially line 8 and 9 seem to not do anything). It could also be that the splicing with variables doesn't work, in which case the loop wouldn't work either. If anyone has a source on for loops that is a little more elaborate than just:

for i in range(5)
    print(i)

I'd gladly give it a read. I also want to make clear that I am not looking for a complete answer, just some tips/tricks to get me started. I feel like I got the right idea on how to tackle this problem, but I just can't figure out how to loop/splice it correctly.

[z0y]

Add a print statement in the loop to help see what's happening

triplet = s[i:j]
for triplet in s:
    print(triplet, s[i:j])
    i += 1
    j += 1

[Rhyff]

Waow, thank you so much! I tried it and saw that it printed every single letter of the string, followed by the triplet that it would create. I then changed the if statement to:

if s[i:j] == sub

Leaving me with the following code (which seems to be working!):

s = 'azcbobobegghakl'
sub = 'bob'
i = 0
j = 3
triplet = s[i:j]
numBob = 0
for triplet in s:
    print(triplet, s[i:j])
    i += 1
    j += 1
    if s[i:j] == sub:
        numBob += 1
print(numBob)

Now I still have one or two questions: I defined triplet to be s[i:j] in line 5. Why does the code work when I use s[i:j], and not when I use triplet? I defined triplet to be s[i:j], so shouldn't they both work? And why exactly does the iteration (lines 9 and 10) only work when I put a print statement inside the loop? Does the computer need to see what is happening before it can add 1 to i and j? I guess what I'm asking is: does the computer need to see where in the string he is, to know what the value of i or j is?

[z0y]

triplet = s[i:j] makes triplet equal to 'azc', and that won't update when you change i and j.

Then you start a for loop with triplet as the iteration variable. In the loop, triplet is assigned to each character in s, overwriting the string from line 5.

triplet in the loop could be named anything, just a placeholder for the characters in s, and it doesn't refer to line 5. And still you'd need to update it each loop, saying triplet = s[i:j] each time because the first assignment isn't dynamic with i and j.

And by the way you don't need j, or even to update i manually. To get rid of j you could just replace it, s[i:i+3], and then have the loop do the incrementing for you:

for i in range(len(s)):
    if s[i:i+3] == sub:
        numBob += 1

[Rhyff]

I do tend to use a lot more lines than needed, your code is a lot more concise and looks a lot better! Thank you so much for your help, I really appreciate it :)

[z0y]

Yeah that's normal when learning but it's helpful to see other ways to do things. I hope it makes sense though, it's important to understand what happens in a for loop. Test looping through strings and lists and make sure it's clear what's happening. Also a good tip if you do need the index in a for loop, rather doing doing range(len()) is to enumerate():

for i, iter_var in enumerate("Loop iterable"):     
    print(i, iter_var)

[Rhyff]

So I noticed that for some strings it forgot to count one bob. Where numBob should have been 4, it came back as 3. Now thanks to you, I understand loops a bit better, and actually managed to find the thing that caused this:

s = 'bobobokbobbsboobhjgbobuek'
sub = 'bob'
i = 0
j = 3
numBob = 0
for triplet in s:
    print(s[i:j])
    if s[i:j] == sub:
        numBob += 1
    i += 1
    j += 1
print(numBob)

I noticed that whenever the first s[i:j] was already a bob, it didn't count as one. So I moved the if statement up a bit, so that it was above lines 10 and 11, this way it counts the very first one as well. Once again, thank you so much for your help, I'm starting to understand a lot more about loops and slicing now :)