The do-while loop in C & C++ Language
The do-while Loop
Unlike for loop and while loop, which test the loop condition at the top of the loop, the do-while loop checks its condition at the bottom of the loop. This means that a do-while loop always executes at least once. The general form of the do-while loop is
do{
statement;
} while(condition);
Although the curly braces are not necessary when only one statement is present, they are usually used to avoid confusion (to you, not the compiler) with the while. The do-while loop iterates until condition becomes false. The following do-while loop will read numbers from the keyboard until it finds a number less than or equal to 100.
do {
scanf(”%d”, &num);
} while(num > 100);
Perhaps the most common use of the do-while loop is in a menu selection function. When the user enters a valid response, it is returned as the value of the function. Invalid responses cause a reprompt.
void menu(void)
{
char ch;
printf(”1. Check Spelling\n”);
printf(”2. Correct Spelling Errors\n”);
printf(”3. Display Spelling Errors\n”);
printf(” Enter your choice: “);
do {
ch = getchar(); /* read the selection from
the keyboard */
switch(ch) {
case ‘1′:
check_spelling();
break;
case ‘2′:
correct_errors();
break;
case ‘3′:
display_errors();
break;
}
} while(ch!=’1′ && ch!=’2′ && ch!=’3′);
}
Here, the do-while loop is a good choice because you will always want a menu function to execute at least once. After the options have been displayed, the program will loop until a valid option is selected.
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