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Full Discussion: Pure C function pointer on printing vowels twice
Top Forums Programming Pure C function pointer on printing vowels twice Post 302997580 by yifangt on Tuesday 16th of May 2017 11:30:58 PM
Old 05-17-2017
Pure C function pointer on printing vowels twice
Have difficulty to understand this pure C code to only print vowels twice from input string. Questions are commented at the end of each place.
Code:
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <limits.h>
/*
 *Demonstrate the use of dispatch tables
 */

/*Print a char twice like putchar, return char if successful, or EOF on error.*/

int putcharTwice(int c)
{
    if (putchar(c) == EOF || putchar(c) == EOF)  {//Q1
    return EOF;
    } else {
    return c;
    }
}

#define NUM_CHARS (UCHAR_MAX + 1)    //UCHAR_MAX is in limits.h

int main(int argc, char **argv)
{
/*This declares table as array of function pointers */
    int (*table[NUM_CHARS]) (int);   //Q2
    int i;
    int c;

    for (i = 0; i < UCHAR_MAX; i++) {
    /* default is to call putchar */
    table[i] = putchar;
    }

    /*but lower-case vowels show up twice */
    table['a'] = putcharTwice; //Q3, and following 4 lines
    table['e'] = putcharTwice; //Q3
    table['i'] = putcharTwice; //Q3
    table['o'] = putcharTwice; //Q3
    table['u'] = putcharTwice; //Q3

    while ((c = getchar()) != EOF) {
    table[c] (c); //Q4,
    }
    return 0;
}

The code was compiled without any problem,
Code:
$ echo "The quick brown fox jumps over the lazy dog!" | ./a.out
$ Thee quuiick broown foox juumps ooveer thee laazy doog!

Q1: How does the logic work by repeating the same condition on both sides of "||" ? I tried using only one side of the OR condition, the code compiled without error but did not work as expected.

Q2:Even not quite sure how to ask this question. Is this a prototype of a function? But the calling is without any parameter see Q3.

Q3: Calling of the function does not have a parameter at all. How does it work?

Q4: I thought I understand this line, but I must have wrong catch because of Q2.

Thanks a lot!
 

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