# 13.3. Complex Conditionals¶

Sometimes, the decisions we need to make in a program are complex - they depend on more than one piece of information. For instance, the program below is designed to find out how many credits a student has and if they have picked a major. If the student is in their first year and does not have a major, we want to recommend a First Year Experience class to them:

The code as written works, but is awkward. We repeat statements (like the print message) and do a lot of nesting of if statements that make it tricky to parse the exact situation the messages will be displayed in.

What we want to express is something more like this version that avoids duplication and makes the structure more simple.

if credits is between 0 and 45:
if madeDecision is "N" or "No":
print("You should sign up for a FYE class")


To express ideas kind of like those while making decisions, we can use the keywords and, or, and not. An and used to join two expressions is only true if both expressions are true. An or joining two logical expressions means that if either or both of the expressions is true, the whole expression is true. A not negates the logical value that follows it. If it was true, then a not changes the result to false. If it was false, the not changes the result to true.

Expression

Meaning

(a < b) or (c < d)

The whole expression is true if a is less than b or c is less than d (or both parts are true).

(a < b) and (c < d)

The whole expression is true if a is less than b and also c is less than d.

not a < b

Only true if a is actually greater than or equal to b. The logical expression not a < b is the same as a >= b.

Here is the same program written using and and or. Hopefully you agree that it is more simple and clear than what we started with. On the following page, we will look more closely at how and, or, and not work.

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