Tutorials | CS 312: Introduction to Programming | The University of Texas at Austin, Department of Computer Science

if statements

In programming, we might want the computer to do something only in certain cases. Up until now, code runs no matter what. What if we established conditions that have to be met in order for code to run? This is what an if statement does.

In its simplest form, if statements are organized in the following manner:

if (<condition>) {
    <statement>;
    <statement>;
}

If the condition holds true, then the statements within the body will run. Otherwise, we skip over the body entirely and move on to the next line of code.

What's a condition? It's simply an expression that evaluates to either true or false. For instance, 3 * 4 would not be a valid condition, but x == 4 would be one. When in doubt, think whether this expression could be used to solve a yes or no question.

Recall that there is often times ambiguity between using = and ==. The = operator is used for assignment of a new value. For instance, when you do:

int diff = x - y;

you are assigning a new value to the diff variable. While it's true the variables are equal during this line, you could later in your program change the value of diff to something else. So = doesn't necessarily mean the left and right side are the same.

The way you check for equality of the two sides is by using the == operator. If you say diff == 5, then this will check whether diff is equal to 5 without assigning the value 5 to it.

For if statements' conditions, we will almost always be using == inside the condition. Only in advanced code will we be using = inside the condition, but that is mostly beyond the scope of CS 312, especially at this point.

We can add some complexity to our if statement to do things if the condition actually didn't hold true. We call this companion statement an else statement. It must always be used in conjunction with an if statement. In other words, an else statement can't exist alone; it must always follow an if statement.

if (<condition>) {
    <statement>;
    <statement>;
} else {
    <statement>;
    <statement>;
}

An if statement only allows us to check against one condition. An else statement will run if that if statement doesn't hold true. However, if we want to have several different conditions, each with their own bodies, then we should use an else if statement. They always come after if and always before else.

if (<condition 1>) {
    <statement>;
    <statement>;
} else if (<condition 2>) {
    <statement>;
    <statement>;
} else if (<condition 3>) {
    <statement>;
    <statement>;
} else {
    <statement>;
    <statement>;
}

In this compound if block, which consists of if, else if, and else statements, <condition 1> is checked first. If that is met, then we run the contents of the body of the if statement and we don't check any more conditions. Instead, we move past everything. If it's not met, we move on and check against condition 2, our next condition. If that's met, we run the body of the first else if statement, and if not, we move on. We repeat this until we run out of else if conditions to check against. If none of the conditions are met, then we will run the body of the else statement.

It's possible to have a compound if statement with an if and else if(s), but not have an else. In that case, the conditions will still be checked, but once we reach the end of the else ifs, we have nothing else to do, so we end up doing nothing for this entire compound if block.

Example

Let's say we want to output what letter grade you have, given a certain numerical grade. We can do this:

int numGrade = 99;
if (numGrade >= 90) {
    System.out.println("A");
} else if (numGrade >= 80 && numGrade < 90) {
    System.out.println("B");
} else if (numGrade >= 70 && numGrade < 80) {
    System.out.println("C");
} else if (numGrade >= 60 && numGrade < 70) {
    System.out.println("D");
} else {
    System.out.println("F");
}

In this case, we'll print out "A", since 99 >= 90, clearly.

You might notice we're using && in our conditions. This means "and". It's used to denote we want both of these expressions to evaluate as true if we want the entire statement to evaluate to true as well. Otherwise, if both are false or even if one of them is false, the whole condition becomes false.

Notice that we check the conditions sequentially. Therefore, if we know that numGrade >= 90 is false, it will always be the case that by the time we reach numGrade >= 80 && numGrade < 90, numGrade will always be less than 90. Therefore, we can logically say that we don't need to include && numGrade < 90 because it's implied. We can therefore revise our program to be:

int numGrade = 99;
if (numGrade >= 90) {
    System.out.println("A");
} else if (numGrade >= 80) {
    System.out.println("B");
} else if (numGrade >= 70) {
    System.out.println("C");
} else if (numGrade >= 60) {
    System.out.println("D");
} else {
    System.out.println("F");
}

and it will output to be the same thing.

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In loving memory of Haskell, 19??-2001.

CS 312 Tutorials

if statements

Example