C Tutorial/Statement/If statement — различия между версиями

Версия 14:21, 25 мая 2010

Содержание

1 Checking for a Range of Values
2 if Comparisons and Their Opposites
3 if statement
4 if statement: compare the value from user input
5 if statement with two else statements
6 If with else statement
7 Operators Used in if Comparisons
8 The if-else-if statement
9 The if statement: a compound statement
10 To execute only one statement, opening and closing braces are not required
11 Use compound conditions to check for upper and lowercase letters
12 Use isdigit Function in if statement
13 Using nested ifs

Checking for a Range of Values

#include <stdio.h>
main()
{
  int iResponse = 0;
  printf("Enter a number from 1 to 10: ");
  scanf("%d", &iResponse);
  if ( iResponse < 1 || iResponse > 10 )
    printf("\nNumber not in range\n");
  else
    printf("\nThank you\n");
}

Enter a number from 1 to 10: 2
      
      Thank you

if Comparisons and Their Opposites

if Comparison else Statement Executed By This Condition < >= == != > <= <= > >= < != ==

if statement

To conditionally execute statements, you can use the if or the if...else statement.

The general form of the if statement is

if(expr) {
    s1 ;
    s2 ;
    ....
  }

if statement: compare the value from user input

#include <stdio.h>
int main(void)
{
  int number = 0;
  printf("\nEnter an integer between 1 and 10: ");
  scanf("%d",&number);
  if (number > 5)
    printf("You entered %d which is greater than 5\n", number);
  if (number < 6)
    printf("You entered %d which is less than 6\n", number);
  return 0;
}

Enter an integer between 1 and 10: 5
     You entered 5 which is less than 6

if statement with two else statements

#include <stdio.h>
#include <stdlib.h>
 
int main()
{
    char num[2];
    int number;
 
    printf("Enter a number from 0 to 9:");
    gets(num);
    number=atoi(num);
 
    if(number<5)
    {
        printf("That number is less than 5!\n");
    }
    else if(number==5)
    {
        printf("You typed in 5!\n");
    }
    else
    {
        printf("That number is more than 5!\n");
    }
 
    return(0);
}

Enter a number from 0 to 9:2
      That number is less than 5!

If with else statement

The general format for an if-else statement is

if (condition)
   simple or compound statement  
else
   simple or compound statement.

Operators Used in if Comparisons

Comparison Meaning or Pronunciation "True" Example < Less than 1 < 5, 8 < 9 == Equal to 7 == 7, 1 == 1 > Greater than 7 > 5, 10 > 0 <= Less than or equal to 4 <= 5, 8 <= 8 >= Greater than or equal to 6 >= 5, 2 >= 2 != Not equal to 1 != 0, 1 != 3.99

The if-else-if statement

The general format for the if-else if statement is:

if (condition 1)
   simple or compound statement  
else if (condition 2)
   simple or compound statement  
else if ( condition 3)
   simple or compound statement  
   .....
else if ( conditon n )
   simple or compound statement

The if statement: a compound statement

If a compound statement is used, it must be enclosed in opening and closing braces.

#include<stdio.h>
main()
{
   int i=5, j =1;
   
    if (i > j){
        i = i + 1;
        printf("%d",i);
    }   
    else
        j = j +1;
}

To execute only one statement, opening and closing braces are not required

#include <stdio.h>
main(){
  int i = 5;
  if(i > 0)
     printf(" i > 0. \n");
     
}

i > 0.

Use compound conditions to check for upper and lowercase letters

#include <stdio.h>
main(){
    char cResponse = "\0";
    printf("Enter the letter A: ");
    scanf("%c", &cResponse);
    if ( cResponse== "A" || cResponse == "a" )
       printf("\nCorrect response\n");
    else
       printf("\nIncorrect response\n");
}

Enter the letter A: 1
      
      Incorrect response

Use isdigit Function in if statement

#include <stdio.h>
#include <ctype.h>
main()
{
  char cResponse = "\0";
  printf("\nPlease enter a letter: ");
  scanf("%c", &cResponse);
  if ( isdigit(cResponse)== 0 )
    printf("\nThank you\n");
  else
    printf("\nYou did not enter a letter\n");
}

Please enter a letter: 1
      
      You did not enter a letter

Using nested ifs

#include <stdio.h>
#include <limits.h>
int main(void)
{
  long test = 0L;              

  if( test % 2L == 0L) {
    printf("The number %ld is even", test);
    if ( (test/2L) % 2L == 0L)
    {
      printf("\nHalf of %ld is also even", test);
      printf("\nThat"s interesting isn"t it?\n");
    }
  }
  else
    printf("The number %ld is odd\n", test);
  return 0;
}

The number 0 is even
     Half of 0 is also even
     That"s interesting isn"t it?