How To Write Conditional Statements
Conditional statements are part of every programming language. With conditional statements, we can have code that sometimes runs and at other times does not run, depending on the conditions of the program at that time.
When we fully execute each statement of a program, moving from the top to the bottom with each line executed in order, we are not asking the program to evaluate specific conditions. By using conditional statements, programs can determine whether certain conditions are being met and then be told what to do next.
Let’s look at some examples where we would use conditional statements:
- If the student receives over 65% on her test, report that her grade passes; if not, report that her grade fails
- If he has money in his account, calculate interest; if he doesn’t, charge a penalty fee
- If they buy 10 oranges or more, calculate a discount of 5%; if they buy fewer, then don’t
Through evaluating conditions and assigning code to run based on whether or not those conditions are met, we are writing conditional code.
This tutorial will take you through writing conditional statements in the Python programming language.
We will start with the
if statement, which will evaluate whether a statement is true or false, and run code only in the case that the statement is true.
In a plain text editor, open a file and write the following code:
With this code, we have the variable
grade and are giving it the integer value of
70. We are then using the
if statement to evaluate whether or not the variable grade is greater than or equal (
>= ) to
65. If it does meet this condition, we are telling the program to print out the string
Save the program as
grade.py and run it in a local programming environment from a terminal window with the command
In this case, the grade of 70 does meet the condition of being greater than or equal to 65, so you will receive the following output once you run the program:
Let’s now change the result of this program by changing the value of the
grade variable to
When we save and run this code, we will receive no output because the condition was not met and we did not tell the program to execute another statement.
To give one more example, let us calculate whether a bank account balance is below 0. Let’s create a file called
account.py and write the following program:
When we run the program with
python account.py, we’ll receive the following output:
In the program we initialized the variable
balance with the value of
-5, which is less than 0. Since the balance met the condition of the
if statement (
balance < 0), once we save and run the code, we will receive the string output. Again, if we change the balance to 0 or a positive number, we will receive no output.
It is likely that we will want the program to do something even when an
if statement evaluates to false. In our grade example, we will want output whether the grade is passing or failing.
To do this, we will add an
else statement to the grade condition above that is constructed like this:
Since the grade variable above has the value of
if statement evaluates as false, so the program will not print out
Passing grade. The
else statement that follows tells the program to do something anyway.
When we save and run the program, we’ll receive the following output:
If we then rewrite the program to give the grade a value of
65 or higher, we will instead receive the output
To add an
else statement to the bank account example, we rewrite the code like this:
Here, we changed the
balance variable value to a positive number so that the
else statement will print. To get the first
if statement to print, we can rewrite the value to a negative number.
By combining an
if statement with an
else statement, you are constructing a two-part conditional statement that will tell the computer to execute certain code whether or not the
if condition is met.
Else if statement
So far, we have presented a Boolean option for conditional statements, with each
if statement evaluating to either true or false. In many cases, we will want a program that evaluates more than two possible outcomes. For this, we will use an else if statement, which is written in Python as
elif or else if statement looks like the
if statement and will evaluate another condition.
In the bank account program, we may want to have three discrete outputs for three different situations:
- The balance is below 0
- The balance is equal to 0
- The balance is above 0
elif statement will be placed between the
if statement and the
else statement as follows:
Now, there are three possible outputs that can occur once we run the program: - If the variable
balance is equal to
0 we will receive the output from the
elif statement (
Balance is equal to 0, add funds soon.) - If the variable
balance is set to a positive number, we will receive the output from the
else statement (
Your balance is 0 or above.). - If the variable
balance is set to a negative number, the output will be the string from the
if statement (
Balance is below 0, add funds now or you will be charged a penalty).
What if we want to have more than three possibilities, though? We can do this by writing more than one
elif statement into our code.
grade.py program, let’s rewrite the code so that there are a few letter grades corresponding to ranges of numerical grades:
- 90 or above is equivalent to an A grade
- 80-89 is equivalent to a B grade
- 70-79 is equivalent to a C grade
- 65-69 is equivalent to a D grade
- 64 or below is equivalent to an F grade
To run this code, we will need one
if statement, three
elif statements, and an
else statement that will handle all failing cases.
Let’s rewrite the code from the example above to have strings that print out each of the letter grades. We can keep our
else statement the same.
elif statements will evaluate in order, we can keep our statements pretty basic. This program is completing the following steps:
If the grade is greater than 90, the program will print
A grade, if the grade is less than 90, the program will continue to the next statement…
If the grade is greater than or equal to 80, the program will print
B grade, if the grade is 79 or less, the program will continue to the next statement…
If the grade is greater than or equal to 70, the program will print
C grade, if the grade is 69 or less, the program will continue to the next statement…
If the grade is greater than or equal to 65, the program will print
D grade, if the grade is 64 or less, the program will continue to the next statement…
The program will print
Failing gradebecause all of the above conditions were not met.
Nested If Statements
Once you are feeling comfortable with the
else statements, you can move on to nested conditional statements. We can use nested
if statements for situations where we want to check for a secondary condition if the first condition executes as true. For this, we can have an if-else statement inside of another if-else statement. Let’s look at the syntax of a nested
A few possible outputs can result from this code:
statement1evaluates to true, the program will then evaluate whether the
nested_statementalso evaluates to true. If both cases are true, the output will be:
statement1evaluates to true, but
nested_statementevaluates to false, then the output will be:
statement1evaluates to false, the nested if-else statement will not run, so the
elsestatement will run alone, and the output will be:
We can also have multiple
if statements nested throughout our code:
In the above code, there is a nested
if statement inside each
if statement in addition to the
elif statement. This will allow for more options within each condition.
Let’s look at an example of nested
if statements with our
grade.py program. We can check for whether a grade is passing first (greater than or equal to 65%), then evaluate which letter grade the numerical grade should be equivalent to. If the grade is not passing, though, we do not need to run through the letter grades, and instead can have the program report that the grade is failing. Our modified code with the nested
if statement will look like this:
If we run the code with the variable
grade set to the integer value
92, the first condition is met, and the program will print out
Passing grade of:. Next, it will check to see if the grade is greater than or equal to 90, and since this condition is also met, it will print out
If we run the code with the
grade variable set to
60, then the first condition is not met, so the program will skip the nested
if statements and move down to the
else statement, with the program printing out
We can of course add even more options to this, and use a second layer of nested if statements. Perhaps we will want to evaluate for grades of A+, A and A- separately. We can do so by first checking if the grade is passing, then checkingto see if the grade is 90 or above, then checkingto see if the grade is over 96 for an A+ for instance:
In the code above, for a
grade variable set to
96, the program will run the following:
- Check if the grade is greater than or equal to 65 (true)
- Print out
Passing grade of:
- Check if the grade is greater than or equal to 90 (true)
- Check if the grade is greater than 96 (false)
- Check if the grade is greater than 93 and also less than or equal to 96 (true)
- Leave these nested conditional statements and continue with remaining code
The output of the program for a grade of 96 therefore looks like this:
if statements can provide the opportunity to add several specific levels of conditions to your code.
By using conditional statements like the
if statement, you will have greater control over what your program executes. Conditional statements tell the program to evaluate whether a certain condition is being met. If the condition is met it will execute specific code, but if it is not met the program will continue to move down to other code.
To continue practicing conditional statements, try using different operators, combining operators with
or, and using conditional statements alongside loops. You can also go through our tutorial on How To Make a Simple Calculator Program to gain more familiarity with conditional statements.