Basic Input/Output
The example programs of the previous sections provided little interaction with the user, if any at all. They simply printed simple values on screen, but the standard library provides many additional ways to interact with the user via its input/output features. This section will present a short introduction to some of the most useful.
C++ uses a convenient abstraction called streams to perform input and output operations in sequential media such as the screen, the keyboard or a file. A stream is an entity where a program can either insert or extract characters to/from. There is no need to know details about the media associated to the stream or any of its internal specifications. All we need to know is that streams are a source/destination of characters, and that these characters are provided/accepted sequentially (i.e., one after another).
The standard library defines a handful of stream objects that can be used to access what are considered the standard sources and destinations of characters by the environment where the program runs:
| stream | description |
|---|---|
| cin | standard input stream |
| cout | standard output stream |
| cerr | standard error (output) stream |
| clog | standard logging (output) stream |
We are going to see in more detail only cout and cin (the standard output and input streams); cerr and clog are also output streams, so they essentially work like cout, with the only difference being that they identify streams for specific purposes: error messages and logging; which, in many cases, in most environment setups, they actually do the exact same thing: they print on screen, although they can also be individually redirected.
Standard output (cout)
On most program environments, the standard output by default is the screen, and the C++ stream object defined to access it is cout.
For formatted output operations, cout is used together with the insertion operator, which is written as << (i.e., two "less than" signs).
cout << "Output sentence"; // prints Output sentence on screen
cout << 120; // prints number 120 on screen
cout << x; // prints the value of x on screen
The << operator inserts the data that follows it into the stream that precedes it. In the examples above, it inserted the literal string Output sentence, the number 120, and the value of variable x into the standard output stream cout. Notice that the sentence in the first statement is enclosed in double quotes (") because it is a string literal, while in the last one, x is not. The double quoting is what makes the difference; when the text is enclosed between them, the text is printed literally; when they are not, the text is interpreted as the identifier of a variable, and its value is printed instead. For example, these two sentences have very different results:
cout << "Hello"; // prints Hello
cout << Hello; // prints the content of variable Hello
Multiple insertion operations (<<) may be chained in a single statement:
cout << "This " << " is a " << "single C++ statement";
This last statement would print the text This is a single C++ statement. Chaining insertions is especially useful to mix literals and variables in a single statement:
cout << "I am " << age << " years old and my zipcode is " << zipcode;
Assuming the age variable contains the value 24 and the zipcode variable contains 90064, the output of the previous statement would be:
I am 24 years old and my zipcode is 90064
What cout does not do automatically is add line breaks at the end, unless instructed to do so. For example, take the following two statements inserting into cout:
cout << "This is a sentence.";
cout << "This is another sentence.";
The output would be in a single line, without any line breaks in between. Something like:
This is a sentence.This is another sentence.
To insert a line break, a new-line character shall be inserted at the exact position the line should be broken. In C++, a new-line character can be specified as
(i.e., a backslash character followed by a lowercase n). For example:
cout << "First sentence.
";
cout << "Second sentence.
Third sentence.";
This produces the following output:
First sentence.
Second sentence.
Third sentence.
Alternatively, the endl manipulator can also be used to break lines. For example:
cout << "First sentence." << endl;
cout << "Second sentence." << endl;
This would print:
First sentence.
Second sentence.
The endl manipulator produces a newline character, exactly as the insertion of ' ' does; but it also has an additional behavior: the stream's buffer (if any) is flushed, which means that the output is requested to be physically written to the device, if it wasn't already. This affects mainly fully buffered streams, and cout is (generally) not a fully buffered stream. Still, it is generally a good idea to use endl only when flushing the stream would be a feature and ' ' when it would not. Bear in mind that a flushing operation incurs a certain overhead, and on some devices it may produce a delay.
Standard input (cin)
In most program environments, the standard input by default is the keyboard, and the C++ stream object defined to access it is cin.
For formatted input operations, cin is used together with the extraction operator, which is written as >> (i.e., two "greater than" signs). This operator is then followed by the variable where the extracted data is stored. For example:
int age;
cin >> age;
The first statement declares a variable of type int called age, and the second extracts from cin a value to be stored in it. This operation makes the program wait for input from cin; generally, this means that the program will wait for the user to enter some sequence with the keyboard. In this case, note that the characters introduced using the keyboard are only transmitted to the program when the ENTER (or RETURN) key is pressed. Once the statement with the extraction operation on cin is reached, the program will wait for as long as needed until some input is introduced.
The extraction operation on cin uses the type of the variable after the >> operator to determine how it interprets the characters read from the input; if it is an integer, the format expected is a series of digits, if a string a sequence of characters, etc.
// i/o example
#include <iostream>
using namespace std;
int main ()
{
int i;
cout << "Please enter an integer value: ";
cin >> i;
cout << "The value you entered is " << i;
cout << " and its double is " << i*2 << ".
";
return 0;
}
//Please enter an integer value: 702
//The value you entered is 702 and its double is 1404.
As you can see, extracting from cin seems to make the task of getting input from the standard input pretty simple and straightforward. But this method also has a big drawback. What happens in the example above if the user enters something else that cannot be interpreted as an integer? Well, in this case, the extraction operation fails. And this, by default, lets the program continue without setting a value for variable i, producing undetermined results if the value of i is used later.
This is very poor program behavior. Most programs are expected to behave in an expected manner no matter what the user types, handling invalid values appropriately. Only very simple programs should rely on values extracted directly from cin without further checking. A little later we will see how stringstreams can be used to have better control over user input.
Extractions on cin can also be chained to request more than one datum in a single statement:
cin >> a >> b;
This is equivalent to:
cin >> a;
cin >> b;
In both cases, the user is expected to introduce two values, one for variable a, and another for variable b. Any kind of space is used to separate two consecutive input operations; this may either be a space, a tab, or a new-line character.
cin and strings
The extraction operator can be used on cin to get strings of characters in the same way as with fundamental data types:
string mystring;
cin >> mystring;
However, cin extraction always considers spaces (whitespaces, tabs, new-line...) as terminating the value being extracted, and thus extracting a string means to always extract a single word, not a phrase or an entire sentence.
To get an entire line from cin, there exists a function, called getline, that takes the stream (cin) as first argument, and the string variable as second. For example:
// cin with strings
#include <iostream>
#include <string>
using namespace std;
int main ()
{
string mystr;
cout << "What's your name? ";
getline (cin, mystr);
cout << "Hello " << mystr << ".
";
cout << "What is your favorite team? ";
getline (cin, mystr);
cout << "I like " << mystr << " too!
";
return 0;
}
//What's your name? Homer Simpson
//Hello Homer Simpson.
//What is your favorite team? The Isotopes
//I like The Isotopes too!
Notice how in both calls to getline, we used the same string identifier (mystr). What the program does in the second call is simply replace the previous content with the new one that is introduced.
The standard behavior that most users expect from a console program is that each time the program queries the user for input, the user introduces the field, and then presses ENTER (or RETURN). That is to say, input is generally expected to happen in terms of lines on console programs, and this can be achieved by using getline to obtain input from the user. Therefore, unless you have a strong reason not to, you should always use getline to get input in your console programs instead of extracting from cin.
stringstream
The standard header
string mystr ("1204");
int myint;
stringstream(mystr) >> myint;
This declares a string with initialized to a value of "1204", and a variable of type int. Then, the third line uses this variable to extract from a stringstream constructed from the string. This piece of code stores the numerical value 1204 in the variable called myint.
// stringstreams
#include <iostream>
#include <string>
#include <sstream>
using namespace std;
int main ()
{
string mystr;
float price=0;
int quantity=0;
cout << "Enter price: ";
getline (cin,mystr);
stringstream(mystr) >> price;
cout << "Enter quantity: ";
getline (cin,mystr);
stringstream(mystr) >> quantity;
cout << "Total price: " << price*quantity << endl;
return 0;
}
//Enter price: 22.25
//Enter quantity: 7
//Total price: 155.75
In this example, we acquire numeric values from the standard input indirectly: Instead of extracting numeric values directly from cin, we get lines from it into a string object (mystr), and then we extract the values from this string into the variables price and quantity. Once these are numerical values, arithmetic operations can be performed on them, such as multiplying them to obtain a total price.
With this approach of getting entire lines and extracting their contents, we separate the process of getting user input from its interpretation as data, allowing the input process to be what the user expects, and at the same time gaining more control over the transformation of its content into useful data by the program.
Statements and flow control
A simple C++ statement is each of the individual instructions of a program, like the variable declarations and expressions seen in previous sections. They always end with a semicolon (;), and are executed in the same order in which they appear in a program.
But programs are not limited to a linear sequence of statements. During its process, a program may repeat segments of code, or take decisions and bifurcate. For that purpose, C++ provides flow control statements that serve to specify what has to be done by our program, when, and under which circumstances.
Many of the flow control statements explained in this section require a generic (sub)statement as part of its syntax. This statement may either be a simple C++ statement, -such as a single instruction, terminated with a semicolon(;)- or a compound statement. A compound statement is a group of statements (each of them terminated by its own semicolon), but all grouped together in a block, enclosed in curly braces: {}:
{ statement1; statement2; statement3; }
The entire block is considered a single statement (composed itself of multiple substatements). Whenever a generic statement is part of the syntax of a flow control statement, this can either be a simple statement or a compound statement.
Selection statements: if and else
The if keyword is used to execute a statement or block, if, and only if, a condition is fulfilled. Its syntax is:
if (condition) statement
Here, condition is the expression that is being evaluated. If this condition is true, statement is executed. If it is false, statement is not executed (it is simply ignored), and the program continues right after the entire selection statement.
For example, the following code fragment prints the message (x is 100), only if the value stored in the x variable is indeed 100:
if (x == 100)
cout << "x is 100";
If x is not exactly 100, this statement is ignored, and nothing is printed.
If you want to include more than a single statement to be executed when the condition is fulfilled, these statements shall be enclosed in braces ({}), forming a block:
if (x == 100)
{
cout << "x is ";
cout << x;
}
As usual, indentation and line breaks in the code have no effect, so the above code is equivalent to:
if (x == 100) { cout << "x is "; cout << x; }
Selection statements with if can also specify what happens when the condition is not fulfilled, by using the else keyword to introduce an alternative statement. Its syntax is:
if (condition) statement1 else statement2
where statement1 is executed in case condition is true, and in case it is not, statement2 is executed.
For example:
if (x == 100)
cout << "x is 100";
else
cout << "x is not 100";
This prints x is 100, if indeed x has a value of 100, but if it does not, and only if it does not, it prints x is not 100 instead.
Several if + else structures can be concatenated with the intention of checking a range of values. For example:
if (x > 0)
cout << "x is positive";
else if (x < 0)
cout << "x is negative";
else
cout << "x is 0";
This prints whether x is positive, negative, or zero by concatenating two if-else structures. Again, it would have also been possible to execute more than a single statement per case by grouping them into blocks enclosed in braces: {}.
Iteration statements (loops)
Loops repeat a statement a certain number of times, or while a condition is fulfilled. They are introduced by the keywords while, do, and for.
The while loop
The simplest kind of loop is the while-loop. Its syntax is:
while (expression) statement
The while-loop simply repeats statement while expression is true. If, after any execution of statement, expression is no longer true, the loop ends, and the program continues right after the loop. For example, let's have a look at a countdown using a while-loop:
// custom countdown using while
#include <iostream>
using namespace std;
int main ()
{
int n = 10;
while (n>0) {
cout << n << ", ";
--n;
}
cout << "liftoff!
";
}
//10, 9, 8, 7, 6, 5, 4, 3, 2, 1, liftoff!
The first statement in main sets n to a value of 10. This is the first number in the countdown. Then the while-loop begins: if this value fulfills the condition n>0 (that n is greater than zero), then the block that follows the condition is executed, and repeated for as long as the condition (n>0) remains being true.
The whole process of the previous program can be interpreted according to the following script (beginning in main):
- n is assigned a value
- The while condition is checked (n>0). At this point there are two possibilities:
- condition is true: the statement is executed (to step 3)
- condition is false: ignore statement and continue after it (to step 5)
- Execute statement:
cout << n << ", ";
--n;
(prints the value of n and decreases n by 1)
- End of block. Return automatically to step 2.
Continue the program right after the block:
print liftoff! and end the program.
A thing to consider with while-loops is that the loop should end at some point, and thus the statement shall alter values checked in the condition in some way, so as to force it to become false at some point. Otherwise, the loop will continue looping forever. In this case, the loop includes --n, that decreases the value of the variable that is being evaluated in the condition (n) by one - this will eventually make the condition (n>0) false after a certain number of loop iterations. To be more specific, after 10 iterations, n becomes 0, making the condition no longer true, and ending the while-loop.
Note that the complexity of this loop is trivial for a computer, and so the whole countdown is performed instantly, without any practical delay between elements of the count (if interested, see sleep_for for a countdown example with delays).
The do-while loop
A very similar loop is the do-while loop, whose syntax is:
do statement while (condition);
It behaves like a while-loop, except that condition is evaluated after the execution of statement instead of before, guaranteeing at least one execution of statement, even if condition is never fulfilled. For example, the following example program echoes any text the user introduces until the user enters goodbye:
// echo machine
#include <iostream>
#include <string>
using namespace std;
int main ()
{
string str;
do {
cout << "Enter text: ";
getline (cin,str);
cout << "You entered: " << str << '
';
} while (str != "goodbye");
}
//Enter text: hello
//You entered: hello
//Enter text: who's there?
//You entered: who's there?
//Enter text: goodbye
//You entered: goodbye
The do-while loop is usually preferred over a while-loop when the statement needs to be executed at least once, such as when the condition that is checked to end of the loop is determined within the loop statement itself. In the previous example, the user input within the block is what will determine if the loop ends. And thus, even if the user wants to end the loop as soon as possible by entering goodbye, the block in the loop needs to be executed at least once to prompt for input, and the condition can, in fact, only be determined after it is executed.
The for loop
The for loop is designed to iterate a number of times. Its syntax is:
for (initialization; condition; increase) statement;
Like the while-loop, this loop repeats statement while condition is true. But, in addition, the for loop provides specific locations to contain an initialization and an increase expression, executed before the loop begins the first time, and after each iteration, respectively. Therefore, it is especially useful to use counter variables as condition.
It works in the following way:
- initialization is executed. Generally, this declares a counter variable, and sets it to some initial value. This is executed a single time, at the beginning of the loop.
- condition is checked. If it is true, the loop continues; otherwise, the loop ends, and statement is skipped, going directly to step 5.
- statement is executed. As usual, it can be either a single statement or a block enclosed in curly braces { }.
- increase is executed, and the loop gets back to step 2.
- the loop ends: execution continues by the next statement after it.
Here is the countdown example using a for loop:
// countdown using a for loop
#include <iostream>
using namespace std;
int main ()
{
for (int n=10; n>0; n--) {
cout << n << ", ";
}
cout << "liftoff!
";
}
//10, 9, 8, 7, 6, 5, 4, 3, 2, 1, liftoff!
The three fields in a for-loop are optional. They can be left empty, but in all cases the semicolon signs between them are required. For example, for (;n<10;) is a loop without initialization or increase (equivalent to a while-loop); and for (;n<10;++n) is a loop with increase, but no initialization (maybe because the variable was already initialized before the loop). A loop with no condition is equivalent to a loop with true as condition (i.e., an infinite loop).
Because each of the fields is executed in a particular time in the life cycle of a loop, it may be useful to execute more than a single expression as any of initialization, condition, or statement. Unfortunately, these are not statements, but rather, simple expressions, and thus cannot be replaced by a block. As expressions, they can, however, make use of the comma operator (,): This operator is an expression separator, and can separate multiple expressions where only one is generally expected. For example, using it, it would be possible for a for loop to handle two counter variables, initializing and increasing both:
for ( n=0, i=100 ; n!=i ; ++n, --i )
{
// whatever here...
}
This loop will execute 50 times if neither n or i are modified within the loop:
n starts with a value of 0, and i with 100, the condition is n!=i (i.e., that n is not equal to i). Because n is increased by one, and i decreased by one on each iteration, the loop's condition will become false after the 50th iteration, when both n and i are equal to 50.
Range-based for loop
The for-loop has another syntax, which is used exclusively with ranges:
for ( declaration : range ) statement;
This kind of for loop iterates over all the elements in range, where declaration declares some variable able to take the value of an element in this range. Ranges are sequences of elements, including arrays, containers, and any other type supporting the functions begin and end; Most of these types have not yet been introduced in this tutorial, but we are already acquainted with at least one kind of range: strings, which are sequences of characters.
An example of range-based for loop using strings:
// range-based for loop
#include <iostream>
#include <string>
using namespace std;
int main ()
{
string str {"Hello!"};
for (char c : str)
{
cout << "[" << c << "]";
}
cout << '
';
}
//[H][e][l][l][o][!]
Note how what precedes the colon (:) in the for loop is the declaration of a char variable (the elements in a string are of type char). We then use this variable, c, in the statement block to represent the value of each of the elements in the range.
This loop is automatic and does not require the explicit declaration of any counter variable.
Range based loops usually also make use of type deduction for the type of the elements with auto. Typically, the range-based loop above can also be written as:
for (auto c : str)
cout << "[" << c << "]";
Here, the type of c is automatically deduced as the type of the elements in str.
Jump statements
Jump statements allow altering the flow of a program by performing jumps to specific locations.
The break statement
break leaves a loop, even if the condition for its end is not fulfilled. It can be used to end an infinite loop, or to force it to end before its natural end. For example, let's stop the countdown before its natural end:
// break loop example
#include <iostream>
using namespace std;
int main ()
{
for (int n=10; n>0; n--)
{
cout << n << ", ";
if (n==3)
{
cout << "countdown aborted!";
break;
}
}
}
//10, 9, 8, 7, 6, 5, 4, 3, countdown aborted!
The continue statement
The continue statement causes the program to skip the rest of the loop in the current iteration, as if the end of the statement block had been reached, causing it to jump to the start of the following iteration. For example, let's skip number 5 in our countdown:
// continue loop example
#include <iostream>
using namespace std;
int main ()
{
for (int n=10; n>0; n--) {
if (n==5) continue;
cout << n << ", ";
}
cout << "liftoff!
";
}
//10, 9, 8, 7, 6, 4, 3, 2, 1, liftoff!
The goto statement
goto allows to make an absolute jump to another point in the program. This unconditional jump ignores nesting levels, and does not cause any automatic stack unwinding. Therefore, it is a feature to use with care, and preferably within the same block of statements, especially in the presence of local variables.
The destination point is identified by a label, which is then used as an argument for the goto statement. A label is made of a valid identifier followed by a colon (:).
goto is generally deemed a low-level feature, with no particular use cases in modern higher-level programming paradigms generally used with C++. But, just as an example, here is a version of our countdown loop using goto:
// goto loop example
#include <iostream>
using namespace std;
int main ()
{
int n=10;
mylabel:
cout << n << ", ";
n--;
if (n>0) goto mylabel;
cout << "liftoff!
";
}
//10, 9, 8, 7, 6, 5, 4, 3, 2, 1, liftoff!
Another selection statement: switch.
The syntax of the switch statement is a bit peculiar. Its purpose is to check for a value among a number of possible constant expressions. It is something similar to concatenating if-else statements, but limited to constant expressions. Its most typical syntax is:
switch (expression)
{
case constant1:
group-of-statements-1;
break;
case constant2:
group-of-statements-2;
break;
.
.
.
default:
default-group-of-statements
}
It works in the following way: switch evaluates expression and checks if it is equivalent to constant1; if it is, it executes group-of-statements-1 until it finds the break statement. When it finds this break statement, the program jumps to the end of the entire switch statement (the closing brace).
If expression was not equal to constant1, it is then checked against constant2. If it is equal to this, it executes group-of-statements-2 until a break is found, when it jumps to the end of the switch.
Finally, if the value of expression did not match any of the previously specified constants (there may be any number of these), the program executes the statements included after the default: label, if it exists (since it is optional).
Both of the following code fragments have the same behavior, demonstrating the if-else equivalent of a switch staement:
| switch example |
|---|
switch (x) {
case 1:
cout << "x is 1";
break;
case 2:
cout << "x is 2";
break;
default:
cout << "value of x unknown";
}
|
| if-else equivalent |
if (x == 1) {
cout << "x is 1";
}
else if (x == 2) {
cout << "x is 2";
}
else {
cout << "value of x unknown";
}
|
The switch statement has a somewhat peculiar syntax inherited from the early times of the first C compilers, because it uses labels instead of blocks. In the most typical use (shown above), this means that break statements are needed after each group of statements for a particular label. If break is not included, all statements following the case (including those under any other labels) are also executed, until the end of the switch block or a jump statement (such as break) is reached.
If the example above lacked the break statement after the first group for case one, the program would not jump automatically to the end of the switch block after printing x is 1, and would instead continue executing the statements in case two (thus printing also x is 2). It would then continue doing so until a break statement is encountered, or the end of the switch block. This makes unnecessary to enclose the statements for each case in braces {}, and can also be useful to execute the same group of statements for different possible values. For example:
switch (x) {
case 1:
case 2:
case 3:
cout << "x is 1, 2 or 3";
break;
default:
cout << "x is not 1, 2 nor 3";
}
Notice that switch is limited to compare its evaluated expression against labels that are constant expressions. It is not possible to use variables as labels or ranges, because they are not valid C++ constant expressions.
To check for ranges or values that are not constant, it is better to use concatenations of if and else if statements.
References
[1] Basic Input/Output
[2] Statements and flow control
- 变更记录
| 时间 | 地点 | 修改人 | 备注 |
|---|---|---|---|
| 2020-09-10 | 佛山 | PatrickLee | 首发 |