What does l if mean in c. Conditional statement if else
Consistent execution is so common that we rarely remember it as a control structure. The sequence of statements is executed in the order of their natural location in the program, with a possible deviation to call an external fragment (function), but with a mandatory return to the point of call.
In the simplest case, branching is described in the C language using a conditional operator. having the form:
| if (expression) operator_1; else operator_2; |
where is the part else may be absent. First the "expression" is evaluated in brackets; if it is true then it is executed operator_1. If " expression" false (equal to zero - NULL), That operator_1 is skipped and executed operator_2. If in place of conditionally executed operators there should be a group of several language operators, then they are enclosed in curly braces - { }. Often the "expression" in parentheses represents a condition specified using relational and logical operators. Relational operations are denoted in C as follows:
= = equal; ! = not equal;< меньше; >more;
< = меньше или равно; >= greater than or equal to.
Symbol !
in C language denotes logical negation. There are two more logical operations: ||
means or &&
- logical AND. Relational operations take precedence over arithmetic operations, so an expression of the form k > n%i calculated as k > (n%i). A priority &&
higher than ||, but both logical operations are performed after relational and arithmetic operations. In doubtful cases, it is better to use parentheses.
To illustrate the use of the conditional operator, consider a program to determine the largest of three numbers. First if the operator represents a complete conditional construction, in the second case else absent. Note that the semicolon ends the assignment statement max=x, does not violate unity if- operator. If else- the branch is skipped in nested conditions, their interpretation may be ambiguous. To avoid ambiguity, solve it this way: else matches the nearest if, not having his own else.
Example 1.3.
Let's look at an example of a program that uses several nested conditional statements. In this program there is a line float A, B, X declares these three variables as real values. Function format string scanf instructs you to enter two real numbers, which will become the values of the variables A and B respectively.
Example 1.4
| /*SOLUTION OF EQUATION AX=B*/ #include main() { float A,B,X; |
See what branching looks like when the nesting depth of conditional statements is three (Example 1.5). If at least one condition is true, then all the remaining ones are, of course, skipped. When the nesting depth of conditional statements exceeds three, branching loses clarity and clarity.
To implement multitasking branching, they usually resort to a control structure choice ( switch) (see paragraph 9.4). When the branch control structure becomes particularly confusing, curly braces can provide some clarity. They are required when conditional operator contains more than one statement or function, for example
Laboratory work
On the topic of: " Condition operator if - else "
1. Purpose and syntax
The if-else condition statement is used to select the direction of the program's operation depending on the conditions prevailing at a given point in the program at the time of its execution.
General form of writing a conditional operator
if ( <условие>)
<блок операторов 1>;
<блок операторов 2>;
If at the time of execution<условие>true, the program transfers control<блоку операторов 1>and, further, to the first statement outside the if-else construct. Wherein<блок операторов 2>is not executed. Otherwise, if<условие>false, executed<блок операторов 2>, A<блок операторов 1>skipped. Corresponding block diagram
Curly braces in the if-else statement syntax are used to separate blocks 1 and 2 in the text. Try to place the closing brace below the opening brace to improve readability program code. For the same purpose, the text inside curly braces must be shifted to the right several positions.
As a condition in if-else statements any can be used logical expressions, taking the values “true” or “false” (true – false). Below is a table showing the simplest comparison operations between integers and real numbers
Example 1. It is required to write a program that converts temperature on the Celsius scale T C (°C) to temperature on the Kelvin scale T K (K). The T C value is entered by the user from the keyboard.
Solution. We use well-known formula transformation – T K = T C – T 0, where T 0 = –273 °C is the temperature of absolute zero. We will consider the entered T C to be incorrect if it is less than T 0 .
// – KelvinvsCelsius–
#include
#include
#pragma argsused
floatT0 = -273; // declare and initialize T0
floatTc, Tk; // declare real Tc and Tk
cout<<» VvediteTc=»; // выводим приглашение
cin>>Tc; // request Tc
if
(
Tc
<
T
0)
// check condition Tc cout<<» Tc < T0!»; // условие истинно, выводим на }
// error message screen Tk = Tc-T0; // condition is false, calculate cout<< «Tk =» << Tk; // Tk и выводим на экран getch(); // delay before pressing a key return 0; // end the program Type the above code, compile it and run the program. Examine the results of operation at various values of T C . 2. Shortened recording options In programming, it is common to find that some action is required in response to existing conditions (for example, if incorrect input data is received from the user, then issue an error message and exit the program). In such cases, C++ programs may use a shorthand notation of the conditional statement with the missing else block. The general form of such a record if
(
<условие>)
<блок операторов>; Here, if the condition is true, control is transferred to the block of statements in curly braces. If the condition is false, this block is skipped. The corresponding block diagram differs from the previous one in the absence of one “arm” Another shorthand option is used when any of the if or else blocks consist of only one statement. In this case, the absence of curly braces delimiting this block is allowed. if
(
<условие>)
operator 1; operator 2; Here, operators 1 and 2 can be not only simple one-line arithmetic operations or I/O operators, but also complex multi-line constructs, such as other (nested) conditional statements or loop operators, which will be discussed below. 3. Nested statements Conditional statements can be nested within each other, in accordance with the software algorithm that they implement. An arbitrary degree of their “nesting” is allowed. If one if-else statement is nested within another, then the first statement is included in the second fully, and not just any one of its if or else parts. Partial overlap of their individual blocks is unacceptable. In the example above, one of the statements (shown in bold) is nested within another. Entry B) is incorrect due to the fact that the else block of the inner conditional statement partially overlaps with both the if and else blocks of the outer statement. Example 2. The user enters three integers a, b, c from the keyboard. It is necessary to display the largest of these numbers. Solution. One of the possible algorithms for solving this problem is shown in the following block diagram. The scheme can be implemented programmatically using nested if-else statements // – Selecting the largest of 3 numbers – #include #include #pragma argsused int main (int argc, char* argv) float a, b, c; // declare three variables cout<< «Vvedite a –»; // вводимзначения a, b, c cout<< «Vvedite b –»; cout<< «Vvedite c –»; if (a>b) // if a > b if (a>c) // if a > c cout<<» max = «< else // otherwise, i.e. if a<= с cout<<» max = «< else // otherwise, i.e. if a<= b if (b>c) // and if b > c cout<<» max = «< else // otherwise, i.e. if b<= а cout<<» max = «< getch(); // delay before pressing any key Analyze the block diagram of this algorithm and its software implementation. Modify the algorithm and program code to find the smallest of three numbers. 4. Compound logical expressions The condition in an if-else statement can be expressed as more than just a comparison of two numeric values. For example, double conditions are very common, which in mathematics are written as “a< b < c». Запись означает, что значение b лежит в диапазоне между значениями a и c. В программе такие условия должны быть переформулированы с использованием простых операций сравнения и логических операций «И», «ИЛИ», «НЕ» In particular, the expression “a< b < c» сформулируем как «a меньше b, и b меньше c». На С++ это будет записано как (a if((a Example 3. On an empty chessboard there is a white pawn in position (n, m), and a black bishop in position (k, l). Here the first coordinate is the column number of the chessboard, the second is the row number (both vary in the range from 1 to 8). Assess the current situation according to three options The pawn is under attack The elephant is under attack The bishop and pawn are “safe”. Solution. Let's take into account that the pawn can attack the two positions closest to it diagonally forward, and the bishop attacks both of its diagonals completely. From here the conditions can be formulated · “((k = n+1) OR (k = n‑1)) AND (l = m+1)” – pawn attack on bishop, · “(k+l = n+m) OR (k-l = n-m)” – bishop attack on pawn, · otherwise the pieces are safe. // – Chess composition – #include #include int main (int argc, char* argv) cout<<«Koordinaty beloi peshki:«< cout<<» vvedite n –»; cout<<» vvedite m –»; cout<<«Koordinaty chernogo slona:«< cout<<» vvedite k –»; cout<<» vvedite l –»; if(((k==n+1)||(k==n‑1))&&(l==m+1)) // bishop attacked cout< if((k+l==n+m)||(k-l==n-m)) // pawn attacked cout< else // no attack cout< The ability to control program flow allows selective execution of individual sections of code, and this is a very valuable feature of programming. The if statement allows us to execute or not execute certain sections of code, depending on whether the statement's condition is true or false. One of the most important purposes of the if statement is that it allows the program to take action based on what the user has entered. A trivial example of using if is checking the password entered by the user; if the password is correct, the program allows the user to perform some action; if the password is entered incorrectly, then the program will not allow the user to access limited resources. Without the conditional statement, the program would execute the same way over and over again, no matter what input the user gave. If you use selection operators, the result of the program can be much more interesting, since it will depend directly on the user’s input data. Before you begin to understand the structure of the if statement, it is worth paying attention to the meanings TRUE and FALSE in the context of programming and computer terminology. The true value (TRUE) has a value other than zero, FALSE is equivalent to zero. When using comparison operators, the operator will return one if the comparison expression is true, or - 0 if the conditional expression is false. For example, the expression 3 == 2 will return the value 0, since three does not equal two. The expression 5 == 5 evaluates to true and will return the value 1. If you have trouble understanding this, try printing these expressions to the screen, for example: printf("%d", 7 == 0); During the programming process, you often have to compare some variables with others and, based on these comparisons, control the program flow. There is a whole list of operators that allows you to perform comparisons, here it is: You're probably familiar with these comparison operators, but just in case, I've shown them in the table above. They should not be difficult for you to understand; most of these operators you learned in school in mathematics lessons. Now you understand what TRUE and FALSE are, it's time to put the if select statement to the test. if structure: If (conditional expression) // here is one statement that will be executed if the conditional expression is true Here's a simple example of using the if statement: If (7 > 6) printf("Seven is greater than six"); In this example, the program evaluates the conditional expression - “is seven greater than six?” To see the result of this piece of code, simply paste it into the main() function and do not forget to include the stdio.h header, run the program and see the result - true. Construction of the selection operator if with curly braces: If (TRUE) ( /* all code placed inside the brackets will be executed */ ) If you do not use curly braces, then only one, the first statement, will relate to the body of the if statement. If you need to control several operators, you need to place them in curly braces. I recommend always putting parentheses after an if declaration - this is good programming practice and you will never get confused in your code, since such a declaration is the most understandable. Sometimes, when a conditional expression is FALSE, it would be convenient to have some code executed that is different from the code that is executed when the condition is TRUE. How is this done? If (TRUE) ( /* execute this code if the condition is true */ ) else ( /* execute this code if the condition is false */ ) Typically, else if statements are used when multiple selection is needed, that is, for example, several conditions are defined that can be true at the same time, but we only need one true conditional expression. You can use the if else statement immediately after the if statement, after its body. In such a case, if the condition of the first select statement is TRUE, then the else if clause will be ignored, whereas otherwise, if the condition of the first select statement is FALSE, the check in the else if clause will begin to be executed. That is, if the condition of one if statement is true, then the others will not be checked. Now, in order to firmly consolidate all this in our heads and understand it, let’s look at a simple example using selection operator constructs. #include Logical operators allow you to create more complex conditional expressions. For example, if you want to check whether your variable is greater than 0 but less than 10, then you just need to use the logical operator - AND. This is how it is done - var > 0 and var< 10 . В языке СИ есть точно такой же же оператор, только обозначается он иначе — && . Please suspend AdBlock on this site. Now that we have dealt with conditional expressions, we can move on to the main topic of the lesson - conditional operator. There are two main options here: Listing 1. // first option if (conditional_expression) operator_1; // second option if (conditional_expression) operator_1; else statement_2; Well, and pictures, of course. Where would we be without pictures? Fig. 1 Block diagrams of the if-else statement. This operator works like this. The value of the conditional expression is calculated. If the result is true, then operator_1 from the main branch is executed, and if false, then either nothing happens (in the first option) or operator_2 from the side branch is executed (in the second option). I suggest you understand it right away with examples. For example, what do you think the following code will output? Check your guess. Listing 2. #include Well, yes, that's right, it will print TRUE! . The condition is true. You haven’t forgotten that one is the truth, have you? I'll tell you something terrible now. Any non-zero number is perceived as true. Check it out for yourself. Okay, now here's an example. What do you think this program will output? Listing 3. #include I hope you gave the correct answer and were not confused by the line with the output FALSE! , which I added specifically to confuse you. Yes, this program will not output anything. The condition in the brackets is false, which means the statement will not be executed. Everything is according to the rules. Let's give one more example to reinforce this. Be extremely careful, I have prepared everything for you there. So what will this code output? Listing 4. #include I believe that you succeeded! If it doesn’t work out, don’t be discouraged – there will still be time to practice. Well, now let's talk about the nuances - as usual, they are there. Only ONE statement can be written in each branch of a conditional statement. Here, look at an example. Listing 5. #include It seems that the program should work like this. The user enters an integer. If the number is less than zero, then change its sign to the opposite one. Otherwise we do nothing. After this, we display the number on the console screen. Now turn your attention to the screen. Fig.2 Result of the program Listing 11 But there is a solution! And this decision is compound operator(). If we enclose multiple statements in curly braces, they will be treated as one single statement. Therefore, for the program to work correctly, we add a compound operator to it: Listing 6. #include Well, now everything is as it should be. Check it out yourself. By the way, from experience. I strongly advise you to always use curly braces, even if there is only one statement inside them. Very often this allows you to avoid stupid mistakes. Inside the if-else control constructs, you can use any language constructs, including another if-else construct. Like this for example: Listing 7. #include I think it's clear that using nested conditional statements, you can make a construct similar to the switch selection statement. The use of nested conditional statements introduces another feature. else always refers to the nearest if that does not have its own else For example: Listing 8. If (n > 0) if (a > b) z = a; else z = b; According to our rule, else refers to the inner (second) if . If we want else to apply to the outer (first) if , then we can use the compound operator. Listing 9. If (n > 0) ( if (a > b) z = a; ) else z = b; As I already mentioned, it is better to always use curly braces to avoid cases of misinterpretation of the entry. It is very difficult to find such errors in programs. Also pay attention to the placement of indents. I use them so that it is immediately clear from the code which branch belongs to which if. Conditional operator allows you to skip or execute a certain block of code depending on the result of calculating the specified expression - condition. A conditional statement can be said to be a decision point in a program; sometimes it is also called a branch statement. If you imagine that a program is a road, and the PHP interpreter is a traveler walking along it, then conditional statements can be thought of as crossroads where the program code branches into two or more roads, and at such crossroads the interpreter must choose which road to take next . The if statement is the simplest of the branch statements. The syntax of the if statement is: The if statement first evaluates the conditional expression specified in parentheses, the result of which is a Boolean value. If the result obtained is true, then the instruction is executed. If the expression returns false, then the instruction is not executed. An expression of any complexity can be used as a condition. If the body of the if statement uses only one instruction, then enclosing it in curly braces is possible, but not required. However, if you need to execute more than one instruction in the body of an if statement, then these several instructions must be enclosed in curly braces. Please note that there should not be a semicolon after the closing curly brace. The following code demonstrates the use of the if statement:
If statements can be nested within other if statements:
Pay attention to the last example: the instruction does not have to be written exactly under the if statement; if the instruction is not large in size, then it can be written in one line. And so we learned that the if statement allows you to execute instructions if the condition is true. If the condition is false, then no action is performed. However, it is often necessary to execute certain instructions if a certain condition is true and other instructions if the condition is false. It is for such cases that if else branching is used. It consists of an if statement followed by a block of statements and an else keyword followed by another block of statements. The syntax of the if else statement is: The else statement is optional. The block of instructions located after else is executed by default, i.e. when the conditional expression in if returns false . The else statement cannot be used separately from the if statement. The else block should only appear after the if statement; it can be considered the default action. Modifying our previous example slightly, we can see how the if else statement works if the condition returns false:
The if else statement can be nested. Such nested conditional statements occur quite often in practice. An if statement is nested if it is nested inside another if or else block. If your code uses multiple if statements in a row, the else always refers to the closest if:
The last else does not apply to if($a) because it is not in the inner block, so the closest one to it is if($i) . The else statement inside the block is related to if($b) because this if is the closest one to it. The if/else statement evaluates the value of a conditional expression and executes a particular piece of program code. But what if you need to execute one of many fragments? If you need to check several conditions in a row, then the elseif or else if construction is suitable for this (this is the same construction, just written differently). Formally, it is not an independent PHP construct - it is just a common programming style that consists of using repeated if/else statements. It allows additional conditions to be tested until true is found or the else block is reached. elseif/else if construct must appear after the if statement and before the else statement, if there is one.
Here three conditions are checked and, depending on the value of the $username variable, different actions are performed. There's really nothing special about this piece. It is simply a sequence of if statements, where each if statement is part of the else clause of the previous if statement. For those who have encountered this form of notation for the first time and do not really understand how it works, we will rewrite the same example, only in an equivalent syntactic form that fully shows the nesting of structures:
else operator
Here is an example of using the if else statement:else if construct
Let's look at interesting conditional expressions using logical operators.
When using if statements, you often need to test several different conditions, so it is important to understand the logical operators OR, NOT, and AND. Logical operators work exactly the same as comparison operators: they return 0 if the Boolean expression is false or 1 if the Boolean expression is true.
You can read more about logical operations in.If -- else statement template
if statement
if else statement
elseif/else if construct