Conditions and Operators

Run-time user input
Conditions and if-else
Conditions and switch-case
Conditional expressions
If and switch initializer expressions
Constexpr if
Operator precedence

Run-time user input

The programs we have seen in the previous two chapters have been a little predictable in how they run as they have a linear execution path through the main() function. Such simple programs have very little practical use. More complex programs, which alter their control flow based on user input fall into two types:

Batch programs take all of their input at the beginning of their execution, usually from any or all of: program parameters, an environment variable, or an input file.
Interactive programs enact a dialog with the user (the computer operator) while the program is executing. This dialog is often two-way as the user is not necessarily expected to know what input is required without being prompted.

Interactive programs often use either a console or a GUI (Graphical User Interface, historically found on desktop computers, but more often found these days on tablets and smartphones). Interactive console programs often produce output to the console interleaved with user input, while batch programs ususally know all of their input at the beginning of their execution and produce all of their output following this with no further user involvement or action. As an example of a modern alternative, a purely voice-activated device (possibly without a screen) has an interface which interestingly has more in common with an interactive console program than with a GUI application.

Previously we have enountered print() and println() for putting formatted output to the console. Interestingly, there is currently no direct equivalent in Modern C++ for reading input. The getline() functions are not covered until Chapter 7, and as might be guessed from the name read a textual string which must then be processed further in order to obtain a valid value for numerical (or similar) input. For reasons of simplicity, this Chapter only covers the use of stream objects for reading from and writing to the console, and use if these requires the <iostream> header.

As a quick introduction to the stream output object cout (an abbreviation of “Character Output”), string literals, character literals, numeric (and other) values and variables are “put to” the console using (possibly multiple) occurrencies of << (the stream insertion operator). There is no format string as such, the output is created from the object to the right of each <<, in order from left to right. For example:

cout << "The answer is: " << 42 << '\n';  // println("The answer is: {}", 42);

As a complement to cout, the stream input object cin (an abbreviation of “Character Input”) overloads >> (the stream extraction operator) to allow variables to be set from user input. When a cin input expression is reached, the program waits (indefinitely) for the user to type some input and press Enter. The following program outputs a message inviting the user to enter a number, and then prints this number out again on the console. Before cin is used, the variable to be used to accept the input into must have already been defined so that the type of the required input can be deduced. Providing an initial value is preferred (empty braces give it a default value) in case the read by cin fails due to either invalid input, such as the user typing letters where digits were required, or end-of-input (Ctrl-D or Ctrl-Z):

// 03-age1.cpp : get and then display an integer

#include <iostream>
using namespace std;

int main() {
    int alice_age{};
    cout << "Please enter your guess for Alice\'s age: ";
    cin >> alice_age;
    cout << "You guessed " << alice_age << "!\n";
}

Use of cin from the user’s perspective has a few quirks. Perhaps usefully, whitespace (any spaces, tabs or preceding new-lines) is ignored, while perhaps not so usefully, non numerical input is (silently) evaluated to the number zero. Also, the program makes no checks on the range of the input, so numbers such as 200 and -50 are accepted without complaint, and printed out. In fact, the variable alice_age can be set to any value that can be held by type int; however the number must (usually) be entered as a decimal; the prefixes for binary, octal and hexadecimal are by default only interpreted at compile-time for literals within program code, or by conversion functions such as from_chars().

Conditions and if-else

The keyword if is followed by a conditional expression in (mandatory) parentheses, which always evaluates to true or false at run-time (these named Boolean values are implicitly convertible both to and from integer 1 and 0 respectively). (To evaluate conditions at compile-time as well the construct if constexpr can be used; this is discussed later in this Chapter.) There are a number of symbols that are combined to represent mathematical conditions of equality, greater than, and so on. Some of these symbols together with their meanings are shown in the table below:

Symbol	Meaning
==	equal *
!=	not equal
>	greater than
<	less than
>=	greater than or equal
<=	less than or equal

* Note: different from the assignment operator, which is single equals = (confusing the two is a common mistake for new C++ programmers).

Variables of any built-in type can be directly tested by an if expression; non-zero evaluates as true while zero evaluates to false (this is the case for both integer and floating-point types). The following program asks for an integer, and outputs zero or nonzero depending upon the value entered:

// 03-zerotest1.cpp : test an integer value against zero

#include <iostream>
using namespace std;

int main() {
    cout << "Please enter an integer value: ";
    int n{};
    cin >> n;
    cout << "The value entered was ";
    if (n) {
        cout << "nonzero\n";
    }
    else {
        cout << "zero\n";
    }
}

Notice that the scopes for both the if and else clauses are delimited with { and }, and that indentation is used for the cout operations within them. Notice also that the if and the else keywords line up vertically, this style is recommended in order to enable in-editor code folding to work, amongst other reasons. In this program the braces for the if and else clauses are in this case optional because they comprise only a single statement each, however using braces even where not strictly needed is again strongly recommended in case extra code needs to be added to the clauses later (and because code folding often only works in editors where an opening brace exists).

Note: Braces for function definitions, including main(), are always mandatory, even in the case of single-statement or empty functions. Function declarations, by contrast, do not have braces; they are analogous to a C++ statement ending with a semi-colon.

Experiment

What happens if you press Ctrl-D (Ctrl-Z then Enter under Windows) when prompted? Can you explain why this is?
Change the program to test for non-zero using the “not equal” operator and a 0. Does this work in the same way?
Change the program again to test for zero (as opposed to non-zero), and change the output statements appropriately so the output remains correct.
Now alter the original program to test a floating-point (double) variable as being zero or non-zero. Do you consider use of 0.0 as better style?
Delete the braces surrounding the if and else clauses. Does the code still compile? What happens if you added a second statement line to the else clause? Or the if clause?

The if statement is a binary choice, however some decisions require more than two options. To enable this, if statements can be chained together. The following program chains a further if onto the tail of the first else clause. Note that in this special case, using braces for the first else clause is not recommended as this would indent the code. The combination else if (with mandatory space) is unambiguous to readers of your code; a second statement to the first else clause (which would necessitate braces) is unlikely to be needed as the (possibly itself chained) if which follows counts as a single statement.

// 03-signtest.cpp : test an integer value for zero, positive or negative

#include <iostream>
using namespace std;

int main() {
    cout << "Please enter an integer value: ";
    int n{};
    cin >> n;
    cout << "The value entered was ";
    if (!n) {
        cout << "zero\n";
    }
    else if (n < 0) {
        cout << "negative\n";
    }
    else {
        cout << "positive\n";
    }
}

Notice that the conditional test if (!n) reverses the logic of the previous program, that is it tests n agains zero and then inverts the previous result, producing true for zero and false for non-zero. We could have used if (n == 0) to get the same result, however the idiom of testing !n is preferred as it also works with objects such as std::ofstream, leading to consistent syntax.

Also, notice that the final else clause catches everything that reached that point, without performing a further test. It is up to the programmer to ensure that by the time control flow reaches here all other possibilities have been tested for (failure to do so is a semantic, or logic error, which can’t usually be caught by the compiler).

Experiment

Modify the above program by removing the else clauses and make it instead perform three different if tests. Consider why this is usually seen to be poor style.

From Mathematics you will be familiar with equality conditions such as 0 ≤ x < 10 specifying that the variable x is between zero (inclusive) and 10 (exclusive). It is not possible to write such conditions directly in C++ as conditional tests are non-associative, however a close approximation usually can be found by combining condition tests with the keywords and and or (which operate the same way as && and ||, which are historically called logical and and logical or). The following is a variation on a previous program which asks the user to guess an age, and says whether or not it is a good guess:

// 03-age2.cpp : get and then test an integer is within range

#include <iostream>
using namespace std;

int main() {
    int alice_age{};
    cout << "Please enter your guess for Alice\'s age: ";
    cin >> alice_age;
    if (6 <= alice_age and alice_age <= 11) {
        cout << "A good guess!\n";
    }
    else {
        cout << "Not a good guess.\n";
    }
}

Experiment

Change the above program so that the test logic is inverted with the output remaining the same, in other words alice_age falling outside the range 6-11 results in a positive condition test. Hint: you will need to use the or keyword and change the order of the output statements.
Now change and to && in the original program. Does it still compile and run? Which style do you prefer?
Now change or to || in the modified program. (Known as the pipe symbol, this is often Shift+Backslash on the keyboard.) Does it compile and run as expected?

Conditions and switch-case

When a test for more than one constant integer value is required, a switch-case block can be employed. The switch expression follows the switch statement and is enclosed in (again mandatory) parentheses and must evaluate to a built-in integral type (that is, between char and long long in size, possibly with the unsigned qualifier). The possible values to test for are listed in a set of case statements that fall within the switch scope, delimited as usual by braces. This example program shows a simple desktop calculator with four arithmetic functions:

// 03-calc.cpp : simple calculator with four functions

#include <iostream>
using namespace std;

int main() {
    int r{}, x{}, y{};
    char op{};
    cout << "Please enter a calculation (number op number, op is one of +-*/):\n";
    cin >> x >> op >> y;
    switch (op) {
    case '+':
        r = x + y;
        break;
    case '-':
        r = x - y;
        break;
    case '*':
        r = x * y;
        break;
    case '/':
        if (y) {
            r = x / y;
        }
        else {
            cerr << "Error: divide by zero.\n";
        }
        break;
    default:
        cerr << "Error: invalid op.\n";
        break;
    }
    cout << "Result: " << r << '\n';
}

Notice that “getting” multiple variables from cin allows for the input of three values together, optionally separated by whitespace or newlines. This permissiveness can be useful in some cases but doesn’t handle erroneous input very well so is often unsuitable to be used in production code (as error recovery involves clearing the error state, possibly losing input in the process). The four case statements each check for a valid integer (actually a character literal) stored in op and program flow jumps to the one that matches, if any. The break statements are necessary and cause control flow to jump to the closing brace of the switch block; if they were not present flow would fall through to the next case statement, which is rarely desirable. The default case statement is optional, and program flow always continues here if none of the case statements match, if it is not present the compiler will often produce a warning.

Notice also the use of cerr to output error messages to the standard error stream; by default cerr echos to the terminal (the same as for cout) but this output can be redirected at run-time to a text file (or a null device). The if test for zero divisor should be familiar syntax by now and prevents a possible floating-point exception. In this case, and in the case of an error, the result variable contains the default value zero.

Experiment

Change the type of the input and result variables to double and make sure the program still compiles and runs correctly.
Add a case clause for the exponentiation operator '^' which calls the function pow(x,y) (C++ has no built-in exponentiation operator, ^ in code actually means bitwise exclusive-or). Hint: you will need #include <cmath> and possibly also -lm on the link path.
Go back to using int variables and add the modulo operator % to the list of valid operators. You will need to add a suitable case clause. Note that this operation gives the remainder from a division, so divide-by-zero needs to be caught here as well.
Rewrite the case values as plain decimal integers, obtained from a table showing ASCII characteres against their numbers. Then try using hexadecimal values, and then octal values.
Rewrite the whole switch-case block as multiple if-else-if statements. Test all control-flow paths.

The need for break statements at the end of each case clause has already been mentioned. Occasionally the behavior of program flow falling through to the next case can be useful. More often, multiple case matches using the same code is the desired behavior. The following program demonstrates the former of these:

// 03-fallthrough.cpp : demonstrate case clauses without break

#include <iostream>
using namespace std;

int main() {
    cout << "Please enter an integer between zero and three:\n";
    int n{};
    cin >> n;
    switch (n) {
    case 0:
        cout << "Number is less than 1\n";
        [[fallthrough]];
    case 1:
        cout << "Number is less than 2\n";
        [[fallthrough]];
    case 2:
        cout << "Number is less than 3\n";
        break;
    case 3:
        cout << "Number is exactly 3\n";
        break;
    default:
        cout << "Number out of range!\n";
        break;
    }
}

Notice that case 1: “falls through” into case 2:, and case 0: falls through into both of these. Some compilers will warn where break is missing from a case clause as it is a common programming mistake; this warning can be suppressed by writing [[fallthrough]] (this is a C++ attribute) where the compiler is expecting to find break (immediately before the next case). Using this attribute in the way shown here provides clarity to both human reader and compiler; it is not necessary where case statements follow on immediately with no code between.

Experiment:

See if you can correctly predict the output of this program with user input of 0 through 3.
Remove the attributes and try to compile this program. See if your compiler gives a warning; if not, try to enable a warning flag in your compiler options.

Conditional expressions

The need to choose between two values based on a condition test is so common that C++ has a built-in operator to do exactly that. Consider the following condition test:

if (condition) {
    value = first;
}
else {
    value = second;
}

The pseudocode shown here is indentical in meaning to the following conditional expression:

value = (condition) ? first : second;

The parentheses around the condition in the conditional expression are in fact optional, because the ternary operator ?: has lower precedence than the (in-)equality tests, however they are often included to aid code readability. Using if generates code which is in most cases equally efficient but sometimes a conditional expression is preferred style. Note that first and second need to be of the same type, or convertible to the same common type, as the type of the entity assigned to value needs to be determined at compile-time.

The program below is identical to 03-zerotest1.cpp in operation except that it has been written using the ternary operator:

// 03-zerotest2.cpp : test an integer value against zero and use conditional expression

#include <iostream>
using namespace std;

int main() {
    cout << "Please enter an integer value: ";
    int n{};
    cin >> n;
    cout << "The value entered was " << ( (n) ? "nonzero\n" : "zero\n" );
}

Note: the parentheses around the whole conditional expression are needed as << has a higher precedence than ?:.

Experiment

Modify this program to remove the code duplication "zero\n".
Change the program to use two nested conditional expressions and produce the same output as 03-signtest.cpp (this is quite tricky to get right).

If and switch initializer expressions

An extension to the if and switch conditional expression syntax is to precede the conditional expression with an initializer and a semi-colon. In fact this is quite flexible, just about any legal C++ expression can be used. The scope of a variable defined in such an initializer has the scope of both the if and else clauses (if present) for an if statement, and the switch body when used with a switch statement.

The following example defines a variable within an if statement:

// 03-ifinitializer.cpp : use of variable initializer in if statement

#include <iostream>
using namespace std;

int main() {
    cout << "Please enter a positive number:\n";
    unsigned n{};
    cin >> n;
    cout << "The least significant digit was ";
    if (auto m = n % 10; m < 5) {
        cout << "less than five (" << m << ")\n";
    }
    else {
        cout << "five or more (" << m << ")\n";
    }
}

The variable defined in the initializer can optionally be used in the condition test, as shown here.

Experiment

Try to use n and m after the closing brace of the else clause.
Rewrite this program to use a switch statement instead of if. Pay close attention to the conditional expression needed. The new program should correctly handle all inputs and produce identical output to the one shown.

Constexpr if

The conditional expression following an if statement is evaluated at run-time. However, if the values and entities within the conditional expression are constexpr (see the previous article) then the if clause can be made to execute at compile-time, meaning that control flow at run-time is both known and fixed (and hence optimizations can be made).

// 03-ifconstexpr.cpp : demonstrate compile-time if

#include <iostream>
using namespace std;

int main() {
    constexpr auto int_size = sizeof(int);
    if constexpr (int_size == 4) {
        cout << "32 bit ints\n";
    }
    else if constexpr (int_size == 8) {
        cout << "64 bit ints\n";
    }
    else {
        cout << "Man, you have weird ints!\n";
    }
}

Testing this program in the online Compiler Explorer results in only one of the three string literals actually being embedded in the assembly language output, therefore proving that it is a compile-time evaluation. The ability to perform an if-test at compile-time, as well as assign from constexpr-returning function calls, means that the constexpr functionality of C++ is in fact Turing Complete. It also allows floating-point numbers and even some user-defined types (with constexpr constructors) as well as Standard Library types to be used and evaluated at compile-time.

Experiment

Rewrite the program testing π from the previous Chapter to use if constexpr instead of static_assert()

Operator precedence

C++ has quite a lot of operators, many of which are inherited from C and operate on the built-in types in exactly the same ways. Some are unary (operate on one value) while others are binary (operate on two values). Unary operators exist which are prefix (written before the object) or postfix (written after the object) and two (++ and --) are both. Binary operators are exclusively infix (written between the two objects they operate on). There is also exactly one ternary operator (which we have seen in this Chapter, the conditional operator), which operates on three values. Some operators are left-to-right associative and others are right-to-left associative, except for the scope resolution operator and comparison operators which are non-associative.

The table below is intended to be a complete list, and as such introduces operators not previously covered; the highest precedence operators are listed first:

Operator	Associativity	Description	Example
`::`	n/a	Global/namespace/class scope	`::name`, `namespace::entity`
`()`	Left to right	Parentheses or function call	`(expression)`, `function(params)`
`[]`	Left to right	Array subscript	`array[index]`
`.` / `->`	Left to right	Member access	`object.member`, `pointer->member`
`++` / `--`	Right to left	Pre/post increment/decrement	`++x`, `x++`, `--x`, `x--`
`+` / `-`	Left to right	Addition/subtraction	`x + y`, `x - y`
`*` / `/` / `%`	Left to right	Multiplication/division/modulo	`x * y`, `x / y`, `x % y`
`<<` / `>>`	Left to right	Bitwise shift left/right	`x << y`, `x >> y`
`<` / `<=` / `>` / `>=`	None	Comparison operators	`x < y`, `x >= y`
`==` / `!=`	None	Equality/inequality	`x == y`, `x != y`
`&&`	Left to right	Logical AND	`x && y`
`\|\|`	Left to right	Logical OR	`x \|\| y`
`=` / `+=` / `-=`	Right to left	Assignment operators	`x = y`, `x += y`, `x -= y`
`throw`	Right to left	Exception throw	`throw exception`
`,`	Left to right	Comma sequencing	`x, y`

This course is largely based on the material from Richard Spencer’s repository, where he shares his knowledge and passion for modern C++ programming.

Conditions and Operators

Table of contents

Run-time user input

Conditions and if-else

Conditions and switch-case

Conditional expressions

If and switch initializer expressions

Constexpr if

Operator precedence