﻿ Python: Find the index of a given string at which a given substring starts - w3resource

# Python: Find the index of a given string at which a given substring starts

## Python String: Exercise-81 with Solution

Write a Python program to find the index of a given string at which a given substring starts. If the substring is not found in the given string return 'Not found'.

Sample Solution:-

Python Code:

``````def find_Index(str1, pos):
if len(pos) > len(str1):

for i in range(len(str1)):

for j in range(len(pos)):

if str1[i + j] == pos[j] and j == len(pos) - 1:
return i

elif str1[i + j] != pos[j]:
break

print(find_Index("Python Exercises", "Ex"))
print(find_Index("Python Exercises", "yt"))
print(find_Index("Python Exercises", "PY"))
```
```

Sample Output:

```7
1
```

Flowchart:

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## Python: Tips of the Day

Check if a given key already exists in a dictionary:

In is the intended way to test for the existence of a key in a dict.

```d = {"key1": 10, "key2": 23}

if "key1" in d:
print("this will execute")

if "nonexistent key" in d:
print("this will not")
```

If you wanted a default, you can always use dict.get():

```d = dict()

for i in range(100):
key = i % 10
d[key] = d.get(key, 0) + 1
```

and if you wanted to always ensure a default value for any key you can either use dict.setdefault() repeatedly or defaultdict from the collections module, like so:

```from collections import defaultdict

d = defaultdict(int)

for i in range(100):
d[i % 10] += 1
```

but in general, the in keyword is the best way to do it.

Ref: https://bit.ly/2XPMRyz