Create generator.md

Author: Chaitya62

generator.md

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+# In-Build Generators:
+A generator is a function that generates output whenever you call it and remembers the current state untill the next call is made
+
+A simple example of a generator is ```range()``` function in python
+
+```python 
+# this code will freeze you pc if it generates the entire list
+# and saves it to memory
+#but it doesn't
+range(90000000000000000000000000)
+```
+The reason is this creates a generator which calculates next element every time you call it
+
+## Use of in-build generators
+To create a generator we have to write our expression inside parenthesis '()'.
+```python
+n = 10
+# this creates a generator that gives even i upto n
+gen = (i for i in range(n) if i%2 == 0)
+for i in gen:
+    print(i)
+
+# this creates a list that contains even i upto n
+# as this creates the entire list at once it's not a generators
+# you can call it one line for-loops
+# also known as list comprehensions
+l = [i for i in range(n) if i%2 == 0]
+```
+
+## Custom Generators.
+
+To create more powerfull generators you can use this technique
+
+__Program to calculate fibonacci__
+```python
+# normal method
+def fibo(n):
+    f0 = 0
+    print(0)
+    f1 = 1
+    print(1)
+    for i in range(n-2):
+        temp = f1
+        f1 = f1+f0
+        f0 = temp
+        print(f1)
+        
+# generator method
+def fibo(n):
+    f0 = 0
+    yield f0
+    f1 = 1
+    yield f1
+    for i in range(n-2):
+        temp = f1
+        f1 = f1+f0
+        f0 = temp
+        yield f1
+
+```
+## Important points
+1. Generators consume more time but are memory efficient and do not blow up your ram by calculating everything before hand. 
+2. Normal method is faster but comsumes more ram memory.