How to use if statement in Python?

How to use the if statement in Python

The if statement is used to express a possible situation and how to handle the situation. The if statement can be used to express one possibility, two possibilities, or multiple possibilities.

1 A possibility

A single if statement represents a possibility. The if keyword is followed by an expression. When the expression is True, it means that this situation has occurred, then Execute the specified statement to handle the situation, as shown in Figure 1.

Figure 1 The use of a single if statement

Among them, Figure 1① uses the input() function to receive the value input by the user, convert it into int type and Save it in a variable; Figure 1② uses the if statement to judge the variable x. If the value of x is greater than 0, the message "You entered a non-negative number" is output (Figure 1③). It should be noted that the if statement is followed by an expression, the expression is followed by a colon, and the statement to be executed is indented.

The effect of running this code is shown in Figure 2.

Figure 2 Code running effect

As can be seen from Figure 2, when the input integer is 10, it will output "You entered a non- Negative number" message.

2 Two possibilities

If you need to judge whether the value entered by the user is a non-negative number or a negative number, there are two possibilities. In this case, you can use the if...else statement ,As shown in Figure 3.

Figure 3 Use of if...else statement

Among them, when the if statement is used the same as in "1 One possibility", when When the expression of if is False, the processing statement corresponding to the else statement will be called. When a negative number is entered, "You entered a negative number" will be displayed, as shown in Figure 4.

Figure 4 Code running effect

3 Multiple possibilities

If you need to perform positive, zero and negative numbers on the values ​​entered by the user When judging, there are three possibilities. At this time, you can use the if...elif...else statement, as shown in Figure 5.

Figure 5 Usage of if...elif...else statement

Among them, elif is the abbreviation of else if, that is to say, when if When the expression of is False, it will then be judged whether the expression of elif is True. If it is True, the processing code corresponding to elif will be executed; if it is False, the processing code corresponding to else will be executed. The effect of this code execution is shown in Figure 6.

Figure 6 Code running effect

Supplement: Python if statement

The following examples are done by using if...elif...else Statement to determine whether a number is positive, negative or zero:

Example (Python 3.0)

# Filename : test.py
# author by : www.runoob.com
 
# 用户输入数字
 
num = float(input("输入一个数字: "))
if num > 0:
   print("正数")
elif num == 0:
   print("零")
else:
   print("负数")

The output result of executing the above code is:

Enter a number: 3
Positive numbers

We can also use inline if statements to achieve this:

Example (Python 3.0)

# Filename ：test.py
# author by : www.runoob.com
 
# 内嵌 if 语句
 
num = float(input("输入一个数字: "))
if num >= 0:
   if num == 0:
       print("零")
   else:
       print("正数")
else:
   print("负数")

The output result of executing the above code is:

Enter a number: 0
zero

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