Dynamic array of pointers

This is a discussion on Dynamic array of pointers within the C Programming forums, part of the General Programming Boards category; Hi. I'm learning about pointers and as an exercise I'm trying to create a program that will store an unknown ...

  1. #1
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    Dynamic array of pointers

    Hi. I'm learning about pointers and as an exercise I'm trying to create a program that will store an unknown number of pointers to strings that will be input by the user in an array of pointers.
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    
    int main(void)
    {
        char input[100];
        char **strings;
    
        do {
            printf("Type something: ");
            fgets(input, sizeof(input), stdin);
    
            /* I don't know what to do now */
        } while (strcmp(input, "exit") != 0);
    
        /* free the memory that was allocated */
    
        return 0;
    }
    If I was using something like
    Code:
    char *strings[5000];
    then I'd know what to do but using **strings is confusing me. Anyone can give me a hint? Thanks.

  2. #2
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    **strings just means that strings is a pointer which can hold the memory address of some other pointer.
    Code:
    >+++++++++[<++++++++>-]<.>+++++++[<++++>-]<+.+++++++..+++.[-]>++++++++[<++++>-] <.>+++++++++++[<++++++++>-]<-.--------.+++.------.--------.[-]>++++++++[<++++>- ]<+.[-]++++++++++.

  3. #3
    and the hat of wrongness Salem's Avatar
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    Start with
    char **strings = NULL;

    Then keep two counters
    int numStrings = 0, maxStrings = 0;

    Then you do this
    Code:
    if ( numStrings == maxStrings ) {
      void *temp = realloc ( strings, (maxStrings+10) * sizeof *strings );
      if ( temp != NULL ) {
        strings = temp;
        maxStrings += 10;
      } else {
        // handle error
      }
    }
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  4. #4
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    Code:
    if ( numStrings == maxStrings ) {
      /* after this, I'll be able to store 10 strings in variable strings, right? */
      void *temp = realloc ( strings, (maxStrings+10) * sizeof *strings );
      if ( temp != NULL ) {
        strings = temp;
        maxStrings += 10;
      } else {
        // handle error
      }
    }

  5. #5
    and the hat of wrongness Salem's Avatar
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    Yes.

    And you increment numStrings after each one you add.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  6. #6
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    Is this correct?
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    
    int main(void)
    {
        char *p = NULL;
        char linha[100];
        char **strings = NULL;
        int c = 0;
        int nstrings = 0;
        int maxstrings = 0;
        void *temp = NULL;
    
        for (;;) {
            printf("Type something or exit to exit: ");
            fgets(linha, sizeof(linha), stdin);
    
            if ((p = strchr(linha, '\n')) != NULL)
                *p = '\0';
            else
                while ((c = getchar()) != '\n' && c != EOF);
    
            if (strcmp(linha, "exit") == 0)
                break;
    
            if (nstrings == maxstrings)
                temp = realloc(strings, (maxstrings + 10) * sizeof(*strings));
            if (temp) {
                strings = (char **)temp;
                maxstrings += 10;
            }
            p = malloc(strlen(linha) + 1);
            if (p)
                strcpy(p, linha);
            else
                break;
            strings[nstrings++] = p;
        }
    
        --nstrings;
    
        for (; nstrings >= 0; nstrings--) {
            printf("%s\n", strings[nstrings]);
            free(strings[nstrings]);
        }
    
        free(strings);
    
        return 0;
    }
    Thanks.

  7. #7
    and the hat of wrongness Salem's Avatar
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    You should check the return result of fgets(), and break if it's NULL
    You should also break if your realloc fails (like you break if your malloc fails)
    The cast (char**) is redundant.

    Apart from those easy to fix issues, it's looking good
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  8. #8
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    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    
    int main(void)
    {
        char *p = NULL;
        char linha[100];
        char **strings = NULL;
        int c = 0;
        int nstrings = 0;
        int maxstrings = 0;
        void *temp = NULL;
    
        for (;;) {
            printf("Type something or exit to exit: ");
            if (fgets(linha, sizeof(linha), stdin) == NULL)
                break;
    
            if ((p = strchr(linha, '\n')) != NULL)
                *p = '\0';
            else
                while ((c = getchar()) != '\n' && c != EOF);
    
            if (strcmp(linha, "exit") == 0)
                break;
    
            if (nstrings == maxstrings)
                temp = realloc(strings, (maxstrings + 10) * sizeof(*strings));
            
            if (temp) {
                strings = temp;
                maxstrings += 10;
            }
            else
                break;
    
            p = malloc(strlen(linha) + 1);
            if (p)
                strcpy(p, linha);
            else
                break;
            strings[nstrings++] = p;
        }
    
        --nstrings;
    
        for (; nstrings >= 0; nstrings--) {
            printf("%s\n", strings[nstrings]);
            free(strings[nstrings]);
        }
    
        free(strings);
    
        return 0;
    }
    Just for testing purposes, I entered all the letters of the alphabet, one letter per line. Here's my output:
    Code:
    Type something or exit to exit: a
    Type something or exit to exit: b
    Type something or exit to exit: c
    Type something or exit to exit: d
    Type something or exit to exit: e
    Type something or exit to exit: f
    Type something or exit to exit: g
    Type something or exit to exit: h
    Type something or exit to exit: i
    Type something or exit to exit: j
    Type something or exit to exit: k
    Type something or exit to exit: l
    Type something or exit to exit: m
    Type something or exit to exit: n
    Type something or exit to exit: o
    Type something or exit to exit: p
    Type something or exit to exit: q
    Type something or exit to exit: r
    Type something or exit to exit: s
    Type something or exit to exit: t
    Type something or exit to exit: u
    Type something or exit to exit: v
    Type something or exit to exit: w
    Type something or exit to exit: x
    Type something or exit to exit: y
    Type something or exit to exit: z
    Type something or exit to exit: exit
    z
    y
    x
    w
    v
    u
    t
    s
    r
    q
    p
    o
    n
    m
    l
    k
    j
    i
    h
    g
    f
    e
    d
    c
    A little window pops-up asking if I want to send the error to Microsoft. I can't see the error but I think it has something to do with memory freeing. Can someone help me find the problem, please?

  9. #9
    and the hat of wrongness Salem's Avatar
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    Start making braces compulsory in your code
    Code:
        if (nstrings == maxstrings) {
          temp = realloc(strings, (maxstrings + 10) * sizeof(*strings));
          if (temp) {
            strings = temp;
            maxstrings += 10;
          } else {
            break;
          }
        }
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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