converting a FOR loop to a DO WHILE loop in C++

Question

im trying to convert this into a do-while; it is supposed to show a pyramid based on a number that you have entered:

    int size,a,b,c;
    printf("Enter a Number: ");
    scanf("%d",&size);
    printf("\n");

    // for loop starts here
    for(a=1; a<=size; a++)
    {

     for(b=1; b<=size-a; b++)
     {
      printf(" ");
     }

     for(c=1; c<=(2*a)-1; c++)
     {
      printf("*");
     }

      printf("\n");
    }
     getch();
   }

this is what i have done so far:

    int size;
    int a=1;
    int b=1;
    int c=1;

    do {

     a++;

      do {
       c++;
       printf("*");
      } while(c<=((2*a)-1));

        do {
       printf(" ");
          b++;
      } while(b<=(size-a));

      printf("\n");

    } while(a<=size);

    getch();
   }

Aaaaand its not showing the same output, any suggestion guys? TIA :)

Is this really C++ ? Looks like pure C to me (without the usual nice formatting) — quantdev
– quantdev, Commented Aug 11, 2014 at 13:07
Have you properly initialized a, b and c in your second code segment? — wolfPack88
– wolfPack88, Commented Aug 11, 2014 at 13:08
It would very nice if you post the output of both pieces of code. — Raydel Miranda
– Raydel Miranda, Commented Aug 11, 2014 at 13:10

Cheers and hth. - Alf · Accepted Answer · 2014-08-11 13:19:54Z

11

A for loop

for (BEGIN; COND; INC)
{
    WORK;
}

Is equivalent to the while loop

{
    BEGIN;
    while (COND)
    {
        WORK;
        INC;
    }
}

… except for the effect of a continue statement in the WORK part (with a for loop the continue jumps to the INC), and except that names declared in the for loop’s BEGIN are in the same declarative region as those declared in the COND.

It doesn't make an elegant do-while loop, however.

{
    BEGIN;
    do
    {
        if (! COND) break;
        WORK;
        INC;
    } while (true)
}

Alternatively (although it repeats code)

{
    BEGIN;
    if (COND)
    {
        do
        {
            WORK;
            INC;
        } while (COND)
    }
}

edited Aug 11, 2014 at 13:19

Cheers and hth. - Alf

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answered Aug 11, 2014 at 13:09

Neil Kirk

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2 Comments

user3929662 Over a year ago

but we were told to only use "Do while" statement, but is it possible without using "if"? thanks :)

Baum mit Augen Over a year ago

No, obviously not because a for loop might have zero iterations (e.g. if size == 0) while do {} while has at least one. You need to somehow handle this.

Sneftel · Accepted Answer · 2014-08-11 13:15:01Z

0

Compilers actually tend to convert for-loops to the code equivalent of do-while loops. The result looks like this (using Neil Kirk's metavariables):

BEGIN;
if(COND) {
    do {
        WORK;
        INC;
    } while(COND);
}

Note that COND appears in two different places. If you examine the generated assembly for a for-loop, you'll usually see that effect, unless the optimizer is clever enough to figure out (or told by the programmer using __assume) that COND will never be false the first time through.

answered Aug 11, 2014 at 13:15

Sneftel

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Comments

Swapnil Salam · Accepted Answer · 2014-08-11 14:10:35Z

0

#include <stdio.h>
void main()
{
    int size;
    int i=1;
    int j;
    int k;
    printf("Enter the number:\n");
    scanf("%d",&size);
    do
    {
        j=i;
        do
        {
            printf(" ");  
            j++;
        }while(j<=size);
        k=1;
        do
        {
            printf("*");
            k++;
        }while(k<=(2*i)-1);
        printf("\n");
        i++;
    }while(i<=size);
}

answered Aug 11, 2014 at 14:10

Swapnil Salam

11 bronze badge

1 Comment

Swapnil Salam Over a year ago

This is what you wanted,isn't it?

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