A loop statement is one which repeatedly executes, ensuring a given condition to be true prior to execution of each iteration, to perform some computations/work. As the condition turns false, loop terminates.
In C programming Language, loops are while statement, for statement, do-while statement
In while and for loops, conditional expression is tested a priory the execution of each iteration while in the do-while loop, condition is tested after execution of each iteration. This means in do-while loop, first iteration always executes even if condition happens to be false in first iteration!
Syntax
/* * In while, condition is tested in the beginning of each iteration */ while (condition) { statements; } for(initialization; condition; adjustment) { statements; } /* * In do-while: condition is tested in the end of each iteration */ do { statements; } while(condition);
Examples:
while loop
/* echo.c -- repeats input */ #include <stdio.h> int main(void) { int ch; /* * 1. getchar() function reads-in one character from the keyboard * at a time * 2. return type of getchar() is int * 3. getchar() returns integer value corresponding to char read * from the keyboard */ /* * ch is checked if it is not same as '!', and as match occurs, * loop terminates. */ while ((ch = getchar()) != '!') putchar(ch); return 0; }
for loop
/* echo_eof.c --- repeating input to end-of-file */ #include <stdio.h> int main(void) { int ch; /* * EOF is a character defined, as a Macro, in the stdio.h header file * EOF is a unique invisible character marks end of every file * adjustment statement not required so left blank */ for (ch = getchar(); ch != EOF; ) { putchar(); ch = getchar(); } return 0; }
do-while loop
/* echo.c -- repeats input */ #include <stdio.h> int main(void) { int ch; /* * 1. ch is checked if it is not same as '!', as match occurs, * loop terminates. * 2. Even if the first character reads-in is '!', it is printed * on the screen & then loop terminates! */ do { ch = getchar(); putchar(ch); } while(ch != '!') return 0; }
This is important to note that any Non-Zero value, integer, floating point values, as a condition is considered to be true while 0 is false! For examples:
while loop
/* echo.c -- repeats input Indefinitely */ #include <stdio.h> int main(void) { int ch; /* * 1. getchar() function reads-in one character from the keyboard * at a time * 2. return type of getchar() is int * 3. getchar() returns integer value corresponding to char read * from the keyboard */ /* * Non-Zero value as a condition causes loop to Indefinitely * Iterate Over. * key-in ctrl+c on Linux to terminate the program. */ while (500.345) { ch = getchar(); putchar(ch); } return 0;
for loop
/* echo_eof.c --- repeating input Indefinitely */ #include <stdio.h> int main(void) { int ch; /* * 1. initialization & adjustment statements not required so left blank * 2. By default, initialization, conditional & adjustment statements, * all three are Optional. If not given, they are considered TRUE. */ for ( ; 200 ; ) { putchar(); ch = getchar(); } return 0; }
do-while loop
/* echo.c -- repeats input indefinitely */ #include <stdio.h> int main(void) { int ch; do { ch = getchar(); putchar(ch); } while(1) return 0; }
