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Comments on What are statements and expressions?
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What are statements and expressions?
When I have tried to read technical explanations of the syntax rules for programming languages, and when I am trying to decipher error messages, I often encounter the terms expression and statement. It comes across that these two are related to each other somehow.
I understand that these terms have something to do with the actual code written in a programming language - not, for example, special sorts of values calculated by the program when it runs - right? But what do they mean exactly? How can I use these concepts to improve my understanding of a programming language?
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In computer programming, an expression is something that yields a value.
A statement performs an action.
For example, let us look at some pseudocode. Let's assume that we want to calculate the sum of 3 variables:
sum = a + b + c;
print(sum);
print(sum); is a statement: it performs an action.
a + b + c is an expression: it yields a value.
Now you may be wondering: is sum = a + b + c a statement, or an expression?
The answer is that it's a statement, but it contains an expression. a + b + c yields a value, and then an action is taken: the value is assigned to a variable.
In this example, we have an arithmetic expression. But most operations on strings and booleans are also expressions!
For example, we could have a conditional:
if (a > 3 && p == 5) { ... }
In this condition statement, the part a > 3 && p == 5 is an expression. A boolean expression.
Or, we might be concatenating strings:
fullName = firstName + " " + lastName;
In this line of code, firstName + " " + lastName is a string expression. It yields a value. The line as a whole is a statement: it evaluates an expression and stores the result in a new variable.
In general, expressions occur inside statements. An expression yields a value, but after you have your value, you'll want to do something with it - store it somewhere, or output it, or send it as an argument to another function.
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