即使增加了变量的值，它在while循环中也不会改变

Question

    #include <stdio.h>
    #include<stdlib.h>
    #include<string.h>
    #include<ctype.h>
    int main()
    {
    char s[] = "We promptly judged antique ivory buckles for the next prize";

    for(int i=0;i<strlen(s);i++)
    {
        s[i] = tolower(s[i]);
    }
    int i=0;
    int n=1;
    int counter =0;
    char *s1;
    s1=(char*)malloc(sizeof(char)*n);
    s1[0] = s[0];

    int k=0;
    while(s[i]!='\0')
    {

            for( k=0;k<strlen(s1);++k)
            {
                if(s[i] == s1[k] && s[i] !=' ')
                ++counter;


            }
            if(counter==0 && s[i]!=' ' )
            {
                ++n;

              s1 = realloc(s1, sizeof(char) * (n));
              ++k;
              printf("%d :  %d\n",n,k);
              s1[k] = s[i];
            }


        ++i;
        counter =0;
        k=0;
    }
    s1 =realloc(s1 , sizeof(char)*(n+1));
    s1[n] = '\0';
    printf("%s\n",s1);

    if(n == 26)
    printf("yes");
    else
    printf("No");

    return 0;
}

This program seeks to check whether the word is pangram or not.. It takes a letter from the array s and stores it in array s1 if the letter is not repeated in the latter. Thus if the length of the array s1 after the while loop finishes is 26 it has all the letters possible. But when i tried printing the s1 array it shows only w has its contents.. Confused to the extreme...

Answer 1

Your ++k (inside the if statement) is not needed, remove it and it will work:

    #include <stdio.h>
    #include<stdlib.h>
    #include<string.h>
    #include<ctype.h>
    int main()
    {
    char s[] = "We promptly judged antique ivory buckles for the next prize";

    for(int i=0;i<strlen(s);i++)
    {
        s[i] = tolower(s[i]);
    }
    int i=0;
    int n=2;
    int counter =0;
    char *s1;
    s1=(char*)malloc(sizeof(char)*n);
    s1[0] = s[0];
    s1[1]='\0';

    int k=0;
    while(s[i]!='\0')
    {

            for( k=0;k<strlen(s1);++k)
            {
                if(s[i] == s1[k] && s[i] !=' ')
                    ++counter;

            }

            if(counter==0 && s[i]!=' ' )
            {
                ++n;

              s1 = realloc(s1, sizeof(char) * (n));
              /* ++k; */  /* <== REMOVE IT */
              printf("%d :  %d\n",n,k);
              s1[k] = s[i];
              s1[k+1]='\0';
            }


        ++i;
        counter =0;
        k=0;
    }
    s1 =realloc(s1 , sizeof(char)*(n+1));
    s1[n] = '\0';
    printf("%s\n",s1);

    if(n == 26)
    printf("yes");
    else
    printf("No");

    return 0;
}

k was already incremented at the end of the for statement, so the ++k later is the cause for the wrong result. Also s1 should be initialized as NULL terminated, and should be kept NULL terminated inside the if statement, as without it s1 likely to contain garbage.