C/Language Basics/For

A simple for loop

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int i;
 /* count by 2s */
 for( i = 0; i < 100; i += 2)
     printf("%d ", i);
 return 0;

}

      </source>

Demonstrate multiple loop control variables

<source lang="cpp">

1. include <stdio.h>
2. include <string.h>

void converge(char *targ, char *src); int main(void) {

 char target[80] = "This is a test";
 converge(target, "This is a test of converge().");
 printf("Final string: %s\n", target);
 return 0;

} /* copies one string into another.

  It copies characters to both the ends,
  converging at the middle. */

void converge(char *targ, char *src) {

 int i, j; 
 printf("%s\n", targ);
 for(i = 0, j = strlen(src); i <= j; i++, j--) {
   targ[i] = src[i];
   targ[j] = src[j];
   printf("%s\n", targ);
 }

}

      </source>

For loop: bigger increase

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int i;
 for(i = 0; i < 102; i = i + 5) 
     printf("%d ", i);
 return 0;

}

      </source>

For loop condition: loop will not execute

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int i;
 /* this loop will not execute */
 for(i = 11; i < 11; i = i + 1) 
     printf("%d ", i);
 printf("terminating");
 return 0;

}

      </source>

For loop: Draw a box

<source lang="cpp">

1. include <stdio.h>

void main() {

  int count = 0;
  printf("\n**************");     /* box top */
  for(count = 1 ; count <= 8 ; ++count)
    printf("\n*            *");   /* box sides   */
  printf("\n**************\n");   /* bottom of the box */

}

      </source>

For loop: List ten integers

<source lang="cpp">

1. include <stdio.h>

void main() {

  int count = 1;
  for( ; count <= 10 ; ++count)
    printf("\n %d", count);
  printf("\n Finished.\n");

}

      </source>

For loop with init value, stop condition and incr

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int i, j, k;
 for( k = 0; k < 10; k = k + 1) {
   printf("Enter a number: ");
   scanf("%d", &i);
   printf("Enter a non-zero number: ");
   scanf("%d", &j);
   if(j != 0) 
       printf("%d\n", i / j);
   if(j == 0) 
       printf("Cannot divide by zero\n");
 }
 return 0;

}

      </source>

For loop without first part

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int i;
 printf("Enter an integer: ");
 scanf("%d", &i);
 for(; i; i--) 
     printf("%d ", i);
 return 0;

}

      </source>

For loop without the third part

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int i;
 for(i=0; i<10; ) {
   printf("%d ", i);
   i++;
 }
 return 0;

}

      </source>

For : no first part

<source lang="cpp">

1. include <stdio.h>

void main() {

  long luckyNumber = 15; 
  int yourInput = 0;    
  int count = 3;    /* The maximum number of tries */

  for( ; count>0 ; --count) {
    printf("\nYou have %d tr%s left.", count, count == 1 ? "y" : "ies");
    printf("\nEnter: ");             /* Prompt for a guess  */
    scanf("%d", &yourInput);         /* Read in a guess     */
    /* Check for a correct guess */
    if (yourInput == luckyNumber)
    {
      printf("\nYou guessed it!\n");
      return;  /* End the program */
    }
    /* Check for an invalid guess */
    if(yourInput<1 || yourInput > 20)
      printf("I said between 1 and 20.\n ");
    else
      printf("Sorry. %d is wrong.\n", yourInput);
  }
  printf("\nYou have had three tries and failed. The number was %ld\n",
                                                                 luckyNumber);

}

      </source>

For: no init and loop backwards

<source lang="cpp">

1. include <stdio.h>
2. include <stdlib.h> /* For rand() and srand() */
3. include <time.h> /* For time() function */

void main() {

  int chosen = 0;          /* The lucky number                     */
  int guess = 0;           /* Stores a guess                       */
  int count = 3;           /* The maximum number of tries          */
  int limit = 20;          /* Upper limit for pseudo-random values */
  srand(time(NULL));       /* Use clock value as starting seed     */
  chosen = 1 + rand()%limit;  /* Random int 1 to limit             */

  printf("\nI have chosen a number between 1 and 20"
                                      " which you must guess.\n");
  for( ; count>0 ; --count)
  {
    printf("\nYou have %d tr%s left.", count, count == 1 ? "y" : "ies");
    printf("\nEnter a guess: "); /* Prompt for a guess  */
    scanf("%d", &guess);         /* Read in a guess     */
    /* Check for a correct guess */
    if (guess == chosen)
    {
      printf("\nYou guessed it!\n");
      return;  /* End the program */
    }
    /* Check for an invalid guess */
    if(guess<1 || guess > 20)
      printf("I said between 1 and 20.\n ");
    else
      printf("Sorry. %d is wrong.\n", guess);
  }
  printf("\nYou have had three tries and failed. The number was %ld\n",
                                                                 chosen);

}

      </source>

For statement: backwards

<source lang="cpp">

1. include <stdio.h>

void main() {

 long sum = 0L;  
 
 int count = 10;  /* The number of integers to be summed */
 
 int i = 0;      /* The loop counter                    */

 /* Sum integers from count to 1 */
 for (i = count ; i >= 1 ; sum += i-- );
 printf("\nTotal of the first %d numbers is %ld\n", count, sum);

}

      </source>

Infinite for loop with break

<source lang="cpp">

1. include <stdio.h>

int main(void) {

   char ch;
   
   for( ; ; ) { /* infinite for loop */
       printf("Load, Save, Edit, Quit?\n");
       do {
           printf ("Enter your selection: ");
           ch = getchar();
       } while(ch!="L" && ch!="S" && ch!="E" && ch!="Q");
       if(ch == "Q") 
           break;
   }
 return 0;

}

      </source>

More calculation in the for statement

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int num, i;
 printf("Enter the number to test: ");
 scanf("%d", &num);
 for( i = 2; i < (num / 2) + 1; i = i + 1)
   if( ( num % i ) == 0) 
       printf("%d ", i);
 return 0;

}

      </source>

More complex for loop: more statements

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int i, j;
 /* count to 49 */
 for( i = 0, j = 100; i < j; i++, j--)
       printf("%d ", i);
 return 0;

}

      </source>

Nested for

<source lang="cpp">

1. include <stdio.h>

void main() {

  long sum = 0L; 
  int i = 1;     /* Outer loop control variable     */
  int j = 1;     /* Inner loop control variable     */
  int count = 10; /* Number of sums to be calculated */
 
  for( i = 1 ; i <= count ; i++ )
  {
    sum = 0L;  /* Initialize sum for the inner loop */
    /* Calculate sum of integers from 1 to i */
    for(j = 1 ; j <= i ; j++ )
      sum += j;

    printf("\n%d\t%ld", i, sum); /* Output sum of 1 to i */
  }

}

      </source>

nested for Demo

<source lang="cpp">

1. include <stdio.h>

int main() {

   int row,column;
   
   for(row = 1; tow <= 10; row++) {
       for(column = 1; column <= 10; column++)
           printf("%6d", row * column);
       
       putchar("\n");
   }

}

      </source>

Nested for loop demo

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int answer, i, chances, right;
 for(i = 1; i < 11; i++) {
   printf(" %d + %d = ?", i , i);
   scanf("%d", &answer);
   if(answer == i + i) 
       printf("Right!\n");
   else {
     printf("Wrong.\n");
     printf("Try again.\n");
     right = 0;
     /* nested for */
     for(chances = 0; chances < 3 && !right; chances++) {
       printf(" %d + %d = ? ", i, i);
       scanf("%d", &answer);
       if(answer == i + i) {
         printf("Right!\n");
         right = 1;
       }
     }
     /* if answer still wrong, tell user */
     if(!right)
       printf("The answer is %d.\n", i + i);
   }
 }
 return 0;

}

      </source>

Prime number tester

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int num, i, is_prime;
 printf("Enter a number: ");
 scanf("%d", &num);
 
 is_prime = 1;
 
 for(i = 2; i <= num / 2; i = i + 1)
   if((num%i)==0) 
        is_prime = 0;
 if(is_prime==1) 
     printf(" is prime.");
 else 
     printf("not prime.");
 return 0;

}

      </source>

Reverse order of for loop

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int i, j;
 printf("Enter a number: ");
 scanf("%d", &i);
 for(j = i; j > 0; j--) 
     printf("%d\n", j);
 
 printf("\a");
 return 0;

}

      </source>

See the for loop terminating condition

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int i;
 for(i = 1; i < 11; i = i+1) 
     printf("%d ", i);
 
 printf("terminating");
 return 0;

}

      </source>

Set the init value for a for loop

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int i;
 for ( i = 17; i < 101; i = i + 1)
   if( ( i % 17) == 0) 
      printf("%d ", i);
 return 0;

}

      </source>

Simplest for loop

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int i;
 for( i = 1; i < 101; i++) 
     printf("%d ", i);
 return 0;

}

      </source>

Simplest for loop: user control the loop

<source lang="cpp">

1. include <stdio.h>

main() {

   int i,n;
   
   scanf("%d",&n); 
   
   for(i = 0; i < n; i = i+1) {
       printf("the numbers are %d \n",i); 
   }

}

      </source>

Three level nested for loop

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int i, j, k;
 
 for(i = 0; i < 3; i++)
   for(j = 0; j < 26; j++)
     for( k = 0; k < 2; k++) 
        printf("%c", "A" + j);
 return 0;

}

      </source>

Two condition for ending a for loop

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int magic; /* magic number */
 int guess; /* user"s guess */
 int i;
 magic = 1325;
 guess = 0;
 for(i = 0; i < 10 && guess != magic; i++) {
   printf("Enter your guess: ");
   scanf("%d", &guess);
   if(guess == magic) {
     printf("RIGHT!");
     printf(" %d is the magic number.\n", magic);
   }
   else {
     printf("wrong...");
     if(guess > magic)
       printf(" too high.\n");
     else 
       printf(" too low.\n");
   }
 }
 return 0;

}

      </source>

Use char as int in a for loop

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int i;
 char j;
 i = 0;
 for(j = 1; j < 101; j++) 
     i = j + i;
 printf("Total is: %d", i);
 return 0;

}

      </source>

Use for loop as the user selection"s controller

<source lang="cpp">

1. include <stdio.h>
2. include <conio.h>

int main(void) {

 int i;
 char ch;
 ch = "a"; /* initial value */
 for(i=0; ch != "q"; i++) {
   printf("pass: %d\n", i);
   ch = getchar();
 }
 return 0;

}

      </source>

Use function as for loop control variable

<source lang="cpp">

1. include <stdio.h>

int sqrnum(int num); int readnum(void); int prompt(void); int main(void) {

 int t;
 for(prompt(); t=readnum(); prompt())
   sqrnum(t);
 return 0;

} int prompt(void) {

 printf("Enter a number: ");
 return 0;

} int readnum(void) {

 int t;
 scanf("%d", &t);
 return t;

} int sqrnum(int num) {

 printf("%d\n", num*num);
 return num*num;

}

      </source>

Use int as the while loop controller

<source lang="cpp">

1. include <stdio.h>

main() {

   int i,n;
   scanf("%d",&n); 
   i = 0;
   
   do {
       printf("the numbers are %d \n",i);
       i = i +1;
   }while( i<n);

}

      </source>

Use integer as the for loop controller

<source lang="cpp">

1. include <stdio.h>

int main(void) {

 int i;
 for(i = 1; i < 101; i = i + 1) 
     printf("%d ", i);
 return 0;

}

      </source>