A branch is an alternative program-execution path chosen by a condition. In Python, an if-elif-else block checks decisions in order and runs the first block whose condition is True; else runs when none match. section 14.2
Use branches to:
- Choose between outcomes, such as reporting whether a number is positive, negative, or zero. section 14.2
- Create several paths by nesting conditionals inside one another. section 15.3
- Handle expected user choices while clearly stating the allowed input. section 15.0 section 15.1
A number decision
num = int(input("Enter a number: ")) if num > 0: print("num is positive") elif num < 0: print("num is negative") else: print("num is zero") For 2, the first branch runs. For -3, the second runs. For 0, neither condition is true, so else runs. Only one message is printed. section 14.2
Figure 15.1. The boxes represent choices for the user. The text below the boxes represents the situation. The gray arrows show the path with the choice made. The dotted black lines to a smiley face or a sad face represent the end of the program, with survival or not. Out of five possible outcomes, only one leads to survival.
Trace the decision boxes and arrows to see how each choice leads to another situation and eventually to an outcome. section 15.3
