Selection statements
Selection is used to select which statements are to be performed next based on a condition being true or false.
Relational expressions
In the solution of many problems, different actions must be taken depending on the value of the data. The if statement in C I used to implement such s decision structure in its simplest form – that of selecting a statement to be executed only if a condition is satisfied.
Syntax:
if(condtion) statement executed if condition is true
When an executing program encounters the if statement, the condition is evaluated to determine its numerical value, which is then interpreted as either true or false. If the condition evaluates to any non-0 value (positive or negative), the condition is considered as a “true” condition and the statement following the if is executed; otherwise this statement is not executed.

Relational Operators In C Relational operator | Meaning | Example | < | Less than | age < 30 | > | Greater than | height > 6.2 | <= | Less than or equal to | taxable <= 200000 | >= | Greater than or equal to | temp >= 98.6 | == | Equal to | grade == 100 | != | Not equal to | number !=250 |

In creating relational expressions, the relational operators must be typed exactly as given in the above table. Thus, although the following relational expressions are all valid: age > 40 length <= 50 temp >= 98.6
3 < 4 flag == done day != 5
The following are invalid: length =< 50 /* operator out of order*/ 2.0 >> 3.3 /*invalid operator*/ flag = = done /*spaces are not allowed*/
Relational expressions are also known as conditions.
The if and if-else Statements
The simplest C selection statement is the one-way if statement having the syntax if ( expression ) statement;
In this construction, the statement following the if (expression ) is only executed if the expression has a non-0 value ( a true condition ). Examples of this statement are if ( age > 62 ) discount = .20;

if (grade > 70 ) ++passTotal;
In the example above the statement following the condition contained within the parenthesis is only executed if the relational expression is true; otherwise the statement following the condition is not executed at all.

#include <stdio.h> main() { int idNum; float miles; printf(“ Please type in car number and mileage :” ); scanf(“ %d %f”, &idNum, &miles); if( miles > 3000) printf(“ Car %d is over the limit.\n”, idNum); printf(“. . .end of program output. . .\n”); getch(); }
#include <stdio.h> main() { int idNum; float miles; printf(“ Please type in car number and mileage :” ); scanf(“ %d %f”, &idNum, &miles); if( miles > 3000) printf(“ Car %d is over the limit.\n”, idNum); printf(“. . .end of program output. . .\n”); getch(); }

Compound statements
Although only a single statement is permitted in an if statement, this statement can be a single compound statement. A compound statement is one or more statements contained between braces as shown below:
{
statement1; statement2; statement3;
}
The use of braces to enclose a set of individual statements creates a single block of statements, which may be used anywhere in a C program in place of a single statement. The general syntax of a compound within a one way if statement is:

if (expression)
{
statement1; statement2; .
.
.
}

The if-else Statement
The if-else statement directs the computer to select a sequence of one or more instructions based on the result of a comparison. The commonly used form of the if-else statement is: if (expression) statement1; else statement2;
#include <stdio.h> main() { int a=0; printf(“enter your age : ”); scanf(“%i”, &a); if(a >= 21 ){ printf(“You are an adult.\n”); else printf(“You are a teenager “); printf(“end. . .”); getch(); }

#include <stdio.h> main() { int a=0; printf(“enter your age : ”); scanf(“%i”, &a); if(a >= 21 ){ printf(“You are an adult.\n”); else printf(“You are a teenager “); printf(“end. . .”); getch(); }

The expression is evaluated first. If the value of the expression is non-0, which corresponds to the expression being true, statement1 is executed. If the value is 0 which corresponds to the expression being false the statement2 after the reserved word else is executed. Thus, one of the two statements either statement1 or statement2 is always executed depending on the value of the tested expression.

The if-else Chain
Including one or more if-else statements within an if or if-else statement is referred to as nested if statements. if (expression1) statement1; else if(expression2) statement2; else if(expression3) statement3;
#include <stdio.h> main() { int grade=0; printf(“enter your grade : ”); scanf(“%i”, &grade); if( grade >= 100 ){ printf(“Excellent.\n”); else if(grade >= 90 ) printf(“Good job!\n “); else if(grade >= 80 ) printf(“Fair!\n “); else printf(“Sorry! You failed!”); printf(“end. . .”); getch(); }

#include <stdio.h> main() { int grade=0; printf(“enter your grade : ”); scanf(“%i”, &grade); if( grade >= 100 ){ printf(“Excellent.\n”); else if(grade >= 90 ) printf(“Good job!\n “); else if(grade >= 80 ) printf(“Fair!\n “); else printf(“Sorry! You failed!”); printf(“end. . .”); getch(); }

The construction is called an if-else chain, and is used extensively in many programming problems. In this construction, each condition is evaluated in order, and if any condition is true, the corresponding statement is executed and all further processing of the chain stops. Thus the final else statement is only executed if none of the previous conditions are satisfied. This serves as a default or catch-all case that is useful for detecting an impossible or error condition.

The switch statement
A switch statement is a specialized selection statement that can be used in place of an if-else chain where exact equality to one or more integer constants is required. Such cases occur with some regularity in real-world applications.
The main advantages of using a switch statement are to avoid using the equality operator, ==, avoids braces needed for compound statements internal to an if-else chain, and frequently results in simpler code. The general form of a switch statement is switch( integer_expression)
{
case value1: statement1; statement2; break; case value2: statement3; statement4; break; case value3: statement5; statement6; break; default: statement5;
}
In the syntax above the switch statement uses for new keywords: switch, case, default and break.
The keyword switch identifies the start of the switch statement. The integer_expression in parentheses following this word is evaluated first. The integer value of the expression is then compared to each of the values listed after the keyword case contained within the compound statement. A compilation error occurs if any of these values are not integer constants or variables.
The keyword case identifies the values that will be compared to the value of the switch expression. This expression’s value is compared to each of these case values in the order that these values are listed until a match is found. When a match occurs, execution begins with the statement immediately following the match.
Any number of case labels may be contained within a switch statement, in any order. If the value expression does not match any of the case values, however no statement is executed unless the keyword default is encountered. The word default is optional and operates the same as the last else in an if-else chain. If the value of the expression does not match any of these case values, program execution begins with the statement following the word default.
Once an entry point has been located by the switch statement, no further case evaluations are done; this means that unless a break statement is encountered, all statements that follow, until the closing brace of the switch statement, will be executed. This is the reason for the break statement, which identifies the end of a particular case and causes an immediate exit from the switch statement. Thus, just as the word case identifies possible starting points in the compound statement, the break statement determines termination points. If the break statements are omitted, all cases following the matching case value, including the default case, are executed.

#include <stdio.h> main() { int choice; float num1, num2; printf(“please type in two numbers : ”); scanf(“%f %f ”, &num1, &num2); printf(“enter your code:”); printf(“\n 1 for addition”); printf(“\n2 for subtraction”); printf(“\n3 for multiplication”); scanf(“%i”, &choice); switch( choice)
{
case 1: printf(“the sum of the 2 numbers entered is %f”, num1+num2); break; case 2: printf(“the difference of the 2 numbers entered is %f”, num1-num2); break; case 3: printf(“the product of the 2 numbers entered is %f”, num1*num2); break; getch();
}

#include <stdio.h> main() { int choice; float num1, num2; printf(“please type in two numbers : ”); scanf(“%f %f ”, &num1, &num2); printf(“enter your code:”); printf(“\n 1 for addition”); printf(“\n2 for subtraction”); printf(“\n3 for multiplication”); scanf(“%i”, &choice); switch( choice)
{
case 1: printf(“the sum of the 2 numbers entered is %f”, num1+num2); break; case 2: printf(“the difference of the 2 numbers entered is %f”, num1-num2); break; case 3: printf(“the product of the 2 numbers entered is %f”, num1*num2); break; getch();
}

When constructing a switch case statement, multiple case values can be stacked together, and the default label is always optional. switch (number)
{
case 1: printf(“have a good morning!\n”); break; case 2: printf(“have ahappy day!\n”); break; case3: case4: case5: printf(“have a nice nice evening!\n”);
}
Assuming the choice is a character variable, the following switch statement is valid: switch (choice)
{
case ‘a’: case ‘b’: printf(“you enter a letter!\n”); break; default: printf(“ it’s not a letter!\n);
}