CODEWITHSUNDEEP
cloopsnested-loopsdo-while
phatx88
phatx88

Reputation: 35

C: Nested do while loop not looping correctly

This code is supposed to -

But the inner loop is executed only once.

Why?

#include <stdio.h>
int main ()
{
    int x;
    char i, ans;
    i = '\0';
    do
    {
        i = '\0';
        x=0;
        printf("\nEnter sequence of character:");
        do
        {
            i = getchar();
            x++;
        }
        while(i!='\n');
        printf("\nNumber of characters entered is: %d", --x);
        printf("\nMore sequences (Y/N) ?");
        ans = getchar();
    }
    while(ans =='Y' || ans == 'y');

Upvotes: 1

Views: 298

Answers (3)

Theofilos Kiapidis
Theofilos Kiapidis

Reputation: 11

Not exactly sure, but I think that when the user presses enter to finish the 1st input character the input buffer keeps then enter button as the \n character. Try adding if(i == '\n') getChar(); after the x++;.

Upvotes: 1

Kitswas
Kitswas

Reputation: 1197

What happened:

  • getchar is a macro that gets a character from stdin.
  • The delimiter ('\n' in this case) is counted as a separate character that remains in the buffer and is retrieved the next time getchar() is called.
  • This causes inner loop to exit.

What could be done:

  • Insert the following after ans = getchar();
    i = getchar();
    if(i != '\n')
        ungetc(i,stdin);

New code explained:

  • ungetc(int x,FILE *stream) pushes a character back into input stream.
  • stdin is the standard input stream defined in <stdio.h>.
  • We are reading a character and putting it back if it is not '\n'.

Upvotes: 1

Vlad Rusu
Vlad Rusu

Reputation: 1479

After you read the answer yes/no (the line with ans = getchar();), you'll read an "y" and a "\n". You'll consume the "y" and process it, but the next iteration when you read i = getchar();, i will consume the remaining "\n", so will break that do-while loop.

Although it's not my favourite solution, a simple workaround is this:

#include <stdio.h>
int main ()
{
    int x;
    char i, ans;
    i = '\0';
    do
    {
        i = '\0';
        x=0;
        printf("\nEnter sequence of character:");
        do
        {
            i = getchar();
            x++;
        }
        while(i!='\n');
        printf("\nNumber of characters entered is: %d", --x);
        printf("\nMore sequences (Y/N) ?");
        ans = getchar();

        getchar();
    }
    while(ans =='Y' || ans == 'y');
}

So just consume that extra "\n". This will work only if you type "y" followed by "\n" in terminal. If you type any extra characters, you'll have undefined behaviour.

Note: In your version, try to type: "y1234" then enter when prompted if you want to input again. You'll see that in fact the nested do-while loop works and will count the 4 characters after "y".

Upvotes: 2

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