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7. Week 3 Thursday: If Statements

If Statements and Conditionals

If statements are our first example of control flow, and their syntax is as follows:

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

The statement, called a conditional, must be a boolean expression. The code within the curly braces only runs if the conditional is true. You can also chain if statements with an else if or an else block:

 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;

Let’s look a bit closer at these boolean conditions. Most often, you’ll be performing a comparison, and we’ve already seen these before: <, >, <=, and >= all do exactly what you think they do. They can only be used to compare two numeric quantities — integers, doubles, characters, etc. You can also compare if two quantities are equal using two equal signs:

 1int x = 37; 
 2cout << "Guess my secret number: " << endl; 
 3int input; cin >> input; 
 4
 5if(input == x) { 
 6    cout << "You guessed it!" << endl; 
 7} else {
 8    cout << "Haha I'm smarter than you " 
 9         << "*spits in your face*" << endl; 
10} 

1int i; cin >> i; 
2if(1 <= i <= 100) {
3    cout << "Your number is between 1 and 100!" << endl; 
4} 

To check if two quantities are not equal, use !=. As a demonstration, consider the following code:

1cout << boolalpha; 
2cout << (20.15 * 100 != 2015) << endl; 

This will print true if 20.15 * 100 is not 2015, false otherwise. Against all common knowledge, this prints false. This is because of the limited precision of doubles in C++! You should in general stay away from using == with doubles.

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.

In addition to this “order of operations” business, C++ engages with something called lazy evaluation. Consider the following two somewhat contrived programs:

 1#include <iostream> 
 2#include <string> 
 3
 4using namespace std; 
 5
 6int main() {
 7    cout << "Please enter a word: " << endl; 
 8    string word; cin >> word; 
 9
10    if(word.length() >= 5 && word.at(4) == 'a') {
11        cout << "You entered a very special word." << endl; 
12    } else {
13        cout << "Your word is kind of lame and stinky." << endl; 
14    } 
15
16    return 0; 
17} 

 1#include <iostream> 
 2#include <string> 
 3
 4using namespace std; 
 5
 6int main() {
 7    cout << "Please enter a word: " << endl; 
 8    string word; cin >> word; 
 9
10    if(word.at(4) == 'a' && word.length() >= 5) { 
11        cout << "You entered a very special word." << endl; 
12    } else {
13        cout << "Your word is kind of lame and stinky." << endl; 
14    } 
15
16    return 0; 
17} 

When providing the input hi, the first program runs perfectly well while the second program crashes. This is because C++ evaluates the two conditions in order. In the first program, it checks if the word.length() >= 5 first. Seeing that it’s false, it has no reason to evaluate the second condition — whether it’s true or not has no impact on the value of the conditional — hence it skips checking word.at(4) and doesn’t crash the program. However, in the second program, it does things in the reverse order and tries to access a nonexistent index.

Let’s analyse this particular problem together. A naïve but entirely plausible solution in pseudocode may look as follows:

1. Let the user enter three integers, \(a\), \(b\), and \(c\).
2. If \(b-a=c-b\), the numbers are evenly spaced.
3. Otherwise, the numbers are not evenly spaced.

This works perfectly well when the numbers are in increasing or decreasing order — we’re checking if the differences between consecutive numbers are the same.

However, this algorithm falls apart entirely when the three numbers are out of order: the user inputting 4 2 6 would cause a = 4, b = 2, c = 6 to be read in, and b - a = -2 while c - b = 4.

One immediate way to remedy this is to first put the numbers in order, then perform the above algorithm. In pseudocode,

1. Let the user enter three integers, \(a\), \(b\), and \(c\).
2. By swapping the contents of \(a\), \(b\), and/or \(c\) as necessary, put the three numbers in ascending or descending order.
3. If \(b-a=c-b\), the numbers are evenly spaced.
4. Otherwise, they are not evenly spaced.

Try to implement these steps algorithmically before peeking at the solution!

 1#include <iostream> 
 2
 3using namespace std; 
 4
 5int main() {
 6    // 1. read user input into 3 integers, a, b, and c 
 7    int a, b, c; 
 8    cout << "Please enter 3 integers: "; 
 9    cin >> a >> b >> c; 
10
11    // 2. rearrange the three integers so they're "in order". 
12    // cases 4 and 5, swapping a and b
13    if((b < a && a >= c) || (b > a && a >= c)) { 
14        int temp = b; 
15        b = a; 
16        a = temp; 
17    } 
18
19    // cases 3 and 6, swapping b and c
20    else if((b < c && c <= a) || (b > c && c >= a)) {
21        int temp = b; 
22        b = c; 
23        c = temp; 
24    } 
25
26    // 3. if b - a and c - b are equal, we are evenly spaced 
27    if( b - a == c - b ) {
28        cout << "Evenly spaced" << endl; 
29    } 
30
31    // 4. otherwise, they aren't evenly spaced. 
32    else {
33        cout << "Unevenly spaced" << endl; 
34    } 
35
36    return 0; 
37}