[SOLVED] initializing a multidimensional array of strings

piobair · 03-19-2014, 10:21 AM

Initializing an array of unequal length strings can be done:

Code:

const char *string[] = {"s1", "s2.", "s3.."};
for(i=0; i<3; i++) printf("string[%d] = %s\n", i, string[i]);

What I really want to do is to have a multidimensional array of strings which will allow:

Code:

printf("subtopic %d of topic %d = %s\n", i_subtopic, topic, subtopic[topic][i_subtopic]);

I do know the size of topic, but I have an unequal length of strings
How do I initialize the character array subtopic?

danielbmartin · 03-19-2014, 10:27 AM

Quote:

Originally Posted by piobair

How do I initialize the character array subtopic?

When posting on this forum it is always helpful to identify which programming language you are using.

Daniel B. Martin

piobair · 03-19-2014, 10:34 AM

Sorry, I am writing in C. I thought that would be obvious from my example.

Quote:

Originally Posted by danielbmartin

When posting on this forum it is always helpful to identify which programming language you are using.

Daniel B. Martin

danielbmartin · 03-19-2014, 11:21 AM

Quote:

Originally Posted by piobair

I thought that would be obvious from my example.

Obvious to someone who knows C. Some of us don't. Participants in this forum use a wide variety of languages.

Daniel B. Martin

rtmistler · 03-19-2014, 12:10 PM

You can initialize a two-dimensional array; however in my test code I had to put a dimension on the array and specifically place it on the second dimension.

Code:

#include <stdio.h>

void main(void)
{
    const char *test[][3] = { { "1", "2", "3" }, { "a", "b", "c" } };

    printf("Test strings are: { %s %s %s } { %s %s %s }\n",
           test[0][0], test[0][1], test[0][2],
           test[1][0], test[1][1], test[1][2]);
}

piobair · 03-19-2014, 04:50 PM

Unfortunately, the sub-arrays are not the same size.
I did figure out how to do it:

Code:

define n_subtopics 4
char **subtopic[n_subtopics];
char *subtopic_0[] = ("s0", "s0.", "s0.."};
char *subtopic_1[] = ("s1", "s1.", "s1..", "s1..."};
char *subtopic_2[] = ("s2....", "s0...", "s0.."};.
char *subtopic_3[] = {"s3...", "s3."};
subtopic[0] = &subtopic_0[0];
subtopic[1] = &subtopic_1[0];
subtopic[2] = &subtopic_2[0];
subtopic[3] = &subtopic_3[0];

Quote:

Originally Posted by rtmistler

You can initialize a two-dimensional array; however in my test code I had to put a dimension on the array and specifically place it on the second dimension.

Code:

#include <stdio.h>

void main(void)
{
    const char *test[][3] = { { "1", "2", "3" }, { "a", "b", "c" } };

    printf("Test strings are: { %s %s %s } { %s %s %s }\n",
           test[0][0], test[0][1], test[0][2],
           test[1][0], test[1][1], test[1][2]);
}

piobair · 03-19-2014, 05:11 PM

I thought it was solved. Further testing found otherwise.

Code:

printf("%s =? %s\n", subtopic[0], subtopic_0[0]); // this works
printf("%s =? %s\n", subtopic[2], subtopic_2[1]); // this doesn't work

ntubski · 03-19-2014, 07:04 PM

Surely you mean

Code:

printf("%s =? %s\n", subtopic[0][0], subtopic_0[0]);
printf("%s =? %s\n", subtopic[2][1], subtopic_2[1]);

You'll get a type mismatch warning if you turn up the warning settings: -Wall -Wextra -Wformat=2

piobair · 03-19-2014, 09:03 PM

Yes, I did mean that. First, I had an incorrect index in my test problem, then a typo in transcribing.
On further testing, the problem solution that I offered earlier really does work. As red as my face is, I will wait a while before I declare that the problem is solved.

Quote:

Originally Posted by ntubski

Surely you mean

Code:

printf("%s =? %s\n", subtopic[0][0], subtopic_0[0]);
printf("%s =? %s\n", subtopic[2][1], subtopic_2[1]);

You'll get a type mismatch warning if you turn up the warning settings: -Wall -Wextra -Wformat=2

rtmistler · 03-20-2014, 07:39 AM

That's why it's always best to write a small piece of test code to validate exactly what you want to accomplish.

I recommend the following compile flags:

-ggdb -Werror -Wall -Wextra -Wswitch-default -Wswitch-enum -Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations -Wmissing-noreturn

I like ntubski's suggestion of -Wformat=2, I'll have to read up on that one and consider adding it to my common build process.

ntubski · 03-20-2014, 09:44 AM

Quote:

Originally Posted by piobair

then a typo in transcribing.

Yes, I find if I'm not copy+pasting directly from tested code, I'm all but guaranteed to make a mistake.

Quote:

Originally Posted by rtmistler

I recommend the following compile flags:

-ggdb -Werror -Wall -Wextra -Wswitch-default -Wswitch-enum -Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations -Wmissing-noreturn

Don't -Wall and -Wextra already cover the following ones? I dropped -Wformat=2 into my tmp/Makefile a while ago, I think it was for printf(a_string_variable) cases.

rtmistler · 03-20-2014, 10:35 AM

Quote:

Originally Posted by ntubski

Yes, I find if I'm not copy+pasting directly from tested code, I'm all but guaranteed to make a mistake.

Don't -Wall and -Wextra already cover the following ones? I dropped -Wformat=2 into my tmp/Makefile a while ago, I think it was for printf(a_string_variable) cases.

No -Wall doesn't cover all of those. The documentation for gcc describes what flags are covered by -Wall, in fact -Wformat is covered, but the variations on -Wformat such as =2 is not covered by -Wall.

Easiest two things to do are to look at the manpage for gcc and search for -Wall, but that easily shows what flags are covered. I comprehensive viewing of the flags will show you the list of all flags and when you see ones which you feel are good ideas, you can view the fine print which will say whether or not it is included by -Wall. That second option is what I did a while ago, but it's always worth a continued look.

piobair · 03-20-2014, 01:51 PM

Cut and paste of a program does work better than transcribing. Baby steps: I am a self-taught C programmer, and compiling with -Wall is not something that I have done before. I will have to study the rest of that list.

The following, using const char, is (IMHO) better than my previous offering:

Code:

// multidim_strings.c
#include <stdio.h>
extern void exit(int);
const char *subtopic_0[] = {"s0", "s0.", "s0.."};
const char *subtopic_1[] = {"s1", "s1.", "s1..", "s1..."};
const char *subtopic_2[] = {"s2....", "s2...", "s2.."};
const char *subtopic_3[] = {"s3...", "s3."};
const char **subtopic[] = {&subtopic_0[0], &subtopic_1[0], &subtopic_2[0], &subtopic_3[0]};

int main(){
  int i, j, size[] = {3, 4, 3, 2};
  for(i=0; i<4; i++){
    printf("N=%d ",(int)(sizeof(subtopic[i])));
    for(j=0; j< size[i]; j++)
      printf("%s  ", subtopic[i][j]);
    printf("\n");
  }
  exit(0);
}

I thought to use sizeof(subtopic[i]) in lieu of size[i], but sizeof(subtopic[i]) returns 8 for all cases of [i]. I do not understand why that is so.

rtmistler · 03-20-2014, 01:59 PM

I don't know the exact code syntax, but C++ is eminently better qualified to manage lists and arrays; similarly is python. For instance, just me tossing it out having not checked, you have a list declared, you also have methods for it such as .size() or .length(), I believe it would be .size() where that will give you an integer representation of the size of the list. Further, the greater part of C++ is that things like String types or something similar; classes actually; allow you to declare something as a String class or list of String classes and not worry about pre-declaring the dimensions. So you might want to keep what you do have, but look a bit at some of the available C++ classes you could use.

And to note further, gcc on a general Linux system should be able to deal with C++ code. Not sure if you're working on that type of platform, but just pointing that detail out.

ntubski · 03-20-2014, 02:17 PM

Quote:

Originally Posted by piobair

sizeof(subtopic[i]) returns 8 for all cases of [i]. I do not understand why that is so.

sizeof returns size in bytes, your size[] array is counting the number of elements. subtopic is an array of pointers, all pointers are 8 bytes (or 4 bytes on a 32 bit system).