Loops
Programs are a sequence of instructions, but in some cases those sequences need to be repeated. For example to read all lines from a file, or do a calculation a certain number of times. Python supports both ways to execute a sequence of instructions until a criteria is met or for all elements in sequence.
While and For loops
A while loop is a way to repeat a sequence of instructions until a certain condition is met. The example below will print i as long as it is smaller than 6. The condition is checked at the beginning of the loop and we need to manually increment i.
1#!/usr/bin/env python3
2
3def main():
4 i = 0
5 while i < 6:
6 print(i)
7 i += 1
8
9
10if __name__ == "__main__":
11 main()
A second way is to use a for loop, but here we iterate over a sequence of values instead of evaluating a condition before the next loop. The example below will print the values of the sequence [0, 1, 2, 3, 4, 5] and will generate the same output as the previous example.
1#!/usr/bin/env python3
2
3def main():
4 for i in range(6):
5 print(i)
6
7
8if __name__ == "__main__":
9 main()
With
Old
Example:
1#!/usr/bin/env python3
2
3def main():
4 i = 1
5 while i < 6:
6 print(i)
7 if i == 3:
8 break
9 i += 1
10
11
12if __name__ == "__main__":
13 main()
Example:
1#!/usr/bin/env python3
2
3def main():
4 i = 0
5 while i < 6:
6 i += 1
7 if i == 3:
8 continue
9 print(i)
10
11
12if __name__ == "__main__":
13 main()
Example:
1#!/usr/bin/env python3
2
3def main():
4 i = 1
5 while i < 6:
6 print(i)
7 i += 1
8 else:
9 print("i is no longer less than 6")
10
11
12if __name__ == "__main__":
13 main()
For loop
Example:
1#!/usr/bin/env python3
2
3def main():
4 fruits = ["apple", "banana", "cherry"]
5 for x in fruits:
6 print(x)
7
8
9if __name__ == "__main__":
10 main()
Example:
1#!/usr/bin/env python3
2
3def main():
4 for x in "banana":
5 print(x)
6
7
8if __name__ == "__main__":
9 main()
Example:
1#!/usr/bin/env python3
2
3def main():
4 fruits = ["apple", "banana", "cherry"]
5 for x in fruits:
6 if x == "banana":
7 break
8 print(x)
9
10
11if __name__ == "__main__":
12 main()
Example:
1#!/usr/bin/env python3
2
3def main():
4 fruits = ["apple", "banana", "cherry"]
5 for x in fruits:
6 if x == "banana":
7 continue
8 print(x)
9
10
11if __name__ == "__main__":
12 main()
Example:
1#!/usr/bin/env python3
2
3def main():
4 for x in range(6):
5 print(x)
6
7
8if __name__ == "__main__":
9 main()
Example:
1#!/usr/bin/env python3
2
3def main():
4 for x in range(2, 6):
5 print(x)
6
7
8if __name__ == "__main__":
9 main()
Example:
1#!/usr/bin/env python3
2
3def main():
4 for x in range(2, 30, 3):
5 print(x)
6
7
8if __name__ == "__main__":
9 main()
Example:
1#!/usr/bin/env python3
2
3def main():
4 for x in range(6):
5 print(x)
6 else:
7 print("Finally finished!")
8
9
10if __name__ == "__main__":
11 main()
Example:
1#!/usr/bin/env python3
2
3def main():
4 for x in range(6):
5 if x == 3:
6 break
7 print(x)
8 else:
9 print("Finally finished!")
10
11
12if __name__ == "__main__":
13 main()
Example:
1#!/usr/bin/env python3
2
3def main():
4 adj = ["red", "big", "tasty"]
5 fruits = ["apple", "banana", "cherry"]
6
7 for x in adj:
8 for y in fruits:
9 print(x, y)
10
11
12if __name__ == "__main__":
13 main()