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2. Week 1 Thursday: If Statements and Conditionals

Review

Let’s first refresh some things you went over in the first two lectures:

• Libraries
  These are collections of variables and commands that other people have written and made accessible. For instance, cin, cout, and endl are not part of the base C++ language, but rather a part of the iostream library. If you want to use any of them, you must type #include <iostream> at the start of the program. Likewise, string variables are only defined in the string library, and math functions are only defined in cmath. You would have to type #include <string> and #include <cmath> respectively.
• Variables
  These hold data from either user input or from the program’s own calculations. To declare a variable for the first time, you must use the format [type] [name] (= [value]);. Everything in the parenthesis is optional, but providing an initial value is generally a good idea. The five primitive types are bool, char, int, float, and double; these are encoded into the C++ language. The sixth type we’ve seen was string, but again that’s part of a library. If you want to change a variable later on, you can use [name] = [value];. Here’s an example:

1int number;             // declares a variable with no value 
2double height = 3.3;    // declares "height" with a value of 3.3
3height = 4.4;           // overwrites the contents of "height" with 4.4

• Input and Output
  cout stands for “console output”. If you want to print things out, you have to “send them” to cout using the << operator. You can chain things together as well, as in:
  cout << [expression] << [expression] << ...;
  This will print out each expression one after another. To insert a line break, use endl. To take input, you need a variable to store the input in. Just type cin >> [variable];!

Hopefully this all looks familiar.

If statements

Our programs have not been particularly responsive, as in they all do the same thing no matter what our user’s input is. Can we reliably trust our user to always provide sensible inputs? If we ask them for their age, what happens if they give us a negative number? If statements provide us with a way to respond to different scenarios. The general syntax is as follows:

1if( [condition] ) {
2    // code 
3}

The [condition] is an expression that is true or false, and the code within the curly braces runs if the condition is true. Typically, it will be a comparison of numbers, such as x <= 15, y > 0, etc. More on conditional expressions below!

You can add an else block at the end:

1if( [condition] ) {
2    // code 
3} else {
4    // more code 
5}

This time, if the condition is true, the first block of code will run; otherwise, the second block will run. Finally, you can chain multiple conditions together:

1if( [condition 1] ) {
2    // code block 1 
3} else if( [condition 2] ) {
4    // code block 2
5} else if( [condition 3] ) {
6    // code block 3 
7} // etc. 

It will go through the conditions in order, one by one, until it finds a true statement. Then, it will run the corresponding code block before exiting. Here is an example:

 1int x = 73; 
 2if(x >= 90) {
 3    cout << "You got an A" << endl; 
 4} else if(x >= 80) {
 5    cout << "You got a B" << endl; 
 6} else if(x >= 70) {
 7    cout << "You got a C" << endl; 
 8} else if(x >= 60) {
 9    cout << "You got a D" << endl; 
10} else {
11    cout << "See me after class." << endl; 
12}

This snippet of code will output You got a C. It will check the condition x >= 90, which is false. Then, it checks x >= 80, which is also false. It finally checks x >= 70, which is true, and so it runs the following code block. It then skips the rest of the else if/else blocks!

There are several things to remark:

• Note where the semicolons are. None of the curly braces are followed by semicolons, and the conditions inside the if statements don’t get semicolons!
• Else blocks are not mandatory. You can have a single if statement or an if statement followed by a few else-if’s without an else statement at the end.
• Curly braces are not mandatory. This will cause the if statement to run the first statement (i.e., up to the first semicolon) only. It is strongly recommended that you use curly braces all the time, but unfortunately programmers can be lazy and omit them. For instance, the output of the following code is pie; can you see why?

1int x = 50; 
2if(x < 0) 
3    cout << "I love "; 
4    cout << "pie" << endl; 

Conditionals

We have to put conditionals inside the if statements, and these will generally take the form of comparisons between numbers. You can compare integers, floats, and doubles with each other. If a and b are numbers, you can perform the following comparisons:

• Greater than (or equal to): a > b, a >= b.
• Less than (or eqaul to): a < b, a <= b.
• Equal to: a == b. There are two equal signs, not one. This is because a = b looks like you’re saying “overwrite the value of a with the value of b”. This is a very common source of error.
• Not equal to: a != b.

These should hopefully be self-explanatory.

Sometimes, however, we may have more than one condition that we want to be true, or several conditions of which we want at least one to be true. If p and q are two boolean expressions (i.e., true or false, such as the comparisons above), you can combine them in the following two ways:

• And: p && q, both p and q must be true.
• Or: p || q, either p or q must be true.

Of course, one can string together any number of conditions using a mix of && and ||; however, you should be careful with the order in which they are combined together. This StackOverflow answer provides a great explanation; in short, && is evaluated first in absence of parentheses.