Conditionals and Loops

This lesson is a quick introductory tour of loops within C++. It is not intended for those who are entirely new to programming. Rather, the people who may find it useful include:

• those who have completed our introductory course, but want a quick review
• those who are already familiar with programming in another language, but new to C++
• those who have used C++ in the past, but would benefit from a refresher

It summarises several lessons from our introductory course. Anyone looking for more thorough explanations or additional context should consider Chapter 2 of that course.

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Intro to Programming with C++

Starting from the fundamentals, become a C++ software engineer, step by step.

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Conditionals

The implementation of conditionals and loops in C++ is going to be very comfortable for those coming from other languages. The syntax is likely to be very similar

We have the normal if, else if, and else statements:

1int Health{100};
2
3if (Health > 50) {
4
5} else if (Health > 25) {
6
7} else {
8
9}
10

We have access to ternary statements:

1Health > 50 ? DoSomething() : DoSomethingElse();
2

Finally, we have the typical switch statements. C++ switch statements include “fallthrough” behavior, which we can control using break:

1int Number{3};
2switch (Number) {
3  case 1:
4    std::cout << "One";
5    break;
6  case 2:
7    std::cout << "Two";
8    break;
9  case 3:
10    std::cout << "Three";
11    break;
12  default:
13    std::cout << "Something else";
14}
15

1Three
2

Short Circuit Evaluation

C++ short circuits evaluation of compound boolean expressions. This means once the result of a boolean is known, any further conditionals within that expression are skipped.

As an example of this, the following program does not log anything. The std::cout operations are not executed, because they will not change the result of the boolean expression:

1#include <iostream>
2
3int main(){
4  // The result will be true
5  true || std::cout << "Hi";
6
7  // The result will be false
8  false && std::cout << "Hi";
9}
10

This can situationally be used as a terse alternative to an if statement:

1#include <iostream>
2bool ShouldLog{true};
3
4int main(){
5  ShouldLog && std::cout << "Hi";
6};
7

1Hi
2

Or as an alternative to a series of if-else statements:

1#include <iostream>
2
3bool Log1(){
4  return false;
5};
6
7bool Log2(){
8  std::cout << "Hi from Log2";
9  return true;
10};
11
12bool Log3(){
13  std::cout << "Hi from Log3";
14  return true;
15};
16
17int main(){
18  Log1() || Log2() || Log3();
19}
20

1Hi from Log2
2

Loops

We have the usual 3 looping options that may be recognized from other programming languages: for, while, and do while.

For Loop

A for loop contains three expressions, separated by semicolons:

• An expression to execute at the start of the loop, used to initialize some state if required
• An expression that returns a boolean, indicating whether the loop should continue
• An expression that will run at the end of each iteration of the loop

A typical form of a for loop is below, which will log out the numbers from 1 to 10:

1#include <iostream>
2
3int main(){
4  for (int i{1}; i <= 10; ++i) {
5    std::cout << i;
6  }
7}
8

112345678910
2

Each expression is optional. Below, we provide only the middle expression, which controls whether the loop should continue. We still need to include two semicolons in the appropriate place, to clarify which expressions we’re providing:

1#include <iostream>
2
3int main(){
4  int i{1};
5  for (; i <= 10;) { std::cout << i++; }
6}
7

112345678910
2

While Loop

A while loop accepts a single expression, which determines whether the loop should continue:

1#include <iostream>
2
3int main(){
4  int i{1};
5  while (i <= 10) {
6    std::cout << i++;
7  }
8}
9

112345678910
2

Do While Loop

A do-while loop has a similar syntax to a while loop, except the conditional is moved to after the block:

1#include <iostream>
2
3int main(){
4  int i{1};
5  do {
6    std::cout << i++;
7  } while (i <= 10);
8}
9

112345678910
2

The key difference with a do while loop is that it will always run at least once, even if the boolean that controls the loop is initially false.

1#include <iostream>
2
3int main(){
4  do {
5    std::cout << "I'm running anyway!";
6  } while (false);
7}
8

1I'm running anyway!
2

For-Each Loops and Container Iteration

Those coming from other languages are likely to be familiar with a dedicated way to iterate over the objects in containers like arrays and sets.

C++ includes these techniques too, which we’ll cover in dedicated sections later in this course.

Skipping Loops using continue

The continue keyword skips over the current loop iteration. Below, we use it to omit 3 from our logging:

1#include <iostream>
2
3int main(){
4  for (int i{1}; i <= 10; ++i) {
5    if (i == 3) continue;
6    std::cout << i;
7  }
8}
9

11245678910
2

Ending Loops using break

The break keyword ends our loop entirely. Below, we use it to stop looping once an integer becomes larger than 10:

1#include <iostream>
2
3int main(){
4  int i{1};
5  while (true) {
6    std::cout << i++;
7    if (i > 10) break;
8  }
9}
10

11245678910
2