How to fix the error message for list index out of range when splitting a string using Python?

Question

I am currently working on taking a string containing a linear expression of the form mx + b and returning a list containing the slope (m) and vertical intercept (b) as float values.

The program works good if the string contains both m and b (i.e.; 5x+3). However, it doesn't work if the string just contains m (i.e.; 5x).

I get a

IndexError: list index out of range.

I know what the issue is, but I guess I am still lacking the skills to correct it. I wanted to reach out to see what I could do to fix it.

def getLinearParameters(mx_plus_b):
    equation_values = mx_plus_b.split("+")
    m = equation_values[0][:-1]
    b = equation_values[1]
    return [float(m), float(b)]

print(getLinearParameters("5x"))

Add a default value to your `b = …` line with https://stackoverflow.com/a/41387299/1707015 ? What should `b` be if it is missing? Zero? — qräbnö, Dec 12 '20 at 13:49
how diverse is your set of inputs going to be? E.g.: `'-2.3e6 + 10.8z'`? In other words: spaces? Floating point numbers? Other variable names? — Pierre D, Dec 12 '20 at 13:56
You might want to read [parsing-math-expression-in-python-and-solving-to-find-an-answer](https://stackoverflow.com/questions/13055884/parsing-math-expression-in-python-and-solving-to-find-an-answer) for inspiration — Patrick Artner, Dec 12 '20 at 14:01

octopus · Answer 1 · 2020-12-12T16:51:34.947

2

You need to put the code inside Try and add catch in case the exception occurred and handle it

try:
    #Statments
except IndexError:
    #Error occurred
    #Do something

In the except you can add a default Value for example.

edited Dec 12 '20 at 16:51

answered Dec 12 '20 at 13:50

octopus

151
1
8

score 2 · Answer 2 · answered Dec 12 '20 at 13:56

2

You can use try except. except catches the exception and handle the exception
Try like this:

def getLinearParameters(mx_plus_b):
    equation_values = mx_plus_b.split("+")
    m = equation_values[0][:-1]
    try:
        b = equation_values[1]
    except:
        b = 0
    return [float(m), float(b)]
print(getLinearParameters("5x"))

answered Dec 12 '20 at 13:56

ppwater

2,315
4
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2

`getLinearParameters("42")` ;o) but he'll find that one out too – Patrick Artner Dec 12 '20 at 14:00
@PatrickArtner What do you mean? – ppwater Dec 12 '20 at 14:02
execute it, print the result, see what I mean ... programming is often about edgecases that when not thought through will cause you some problems in the future ;) – Patrick Artner Dec 12 '20 at 14:09
Apart from that ("sting is a list"): How about `… except (KeyboardInterrupt, MemoryError, SyntaxError, SystemExit): raise; except Exception as e: …` to make it more robust? – qräbnö Dec 12 '20 at 14:39
Ah, I understand what you mean – ppwater Dec 13 '20 at 02:41

score 2 · Answer 3 · answered Dec 12 '20 at 14:05

2

Avoid using try - except blocks to fix an error, as that is a band-aid, not a proper solution. Instead, use an if - else statement:

def getLinearParameters(mx_plus_b):
    equation_values = mx_plus_b.split("+")
    m = equation_values[0][:-1]
    
    if len(equation_values) > 1:
        b = equation_values[1]
    else:
        b = 0
        
    return [float(m), float(b)]

print(getLinearParameters("5x"))

answered Dec 12 '20 at 14:05

Red

26,798
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+1 for band-aid. Where can I read more about that? On the other hand, I think try catch is okay, it's Python after all and not a Java philosophy? – qräbnö Dec 12 '20 at 14:10
1

@qräbnö Here is an article: https://towardsdatascience.com/do-not-abuse-try-except-in-python-d9b8ee59e23b – Red Dec 12 '20 at 14:17
Credits: https://stackoverflow.com/a/65208608/13552470 – Red Dec 12 '20 at 14:28

How to fix the error message for list index out of range when splitting a string using Python?

3 Answers3