C multidimensional array pointing problem? - c

Question

Trying to take strings as input and place it in 2d array. Why is this given code showing different behavior. The last for loop "arr[i][j]" is not printing the string.It is not even printing a character also.
Why this code does not work.only this code.Not a new way to write it
This code takes input just fine(or at least the way needed.each row a single string no white space)And when a short string is stored remaining are filled with null after carriage return. When the arr[] is passed in last for loop everything seems fine only when arr[][] is passed ,the problem arises.But again arr[][] is initialized as arr[1][0] then arr[2][0] so should not it work!
#include <stdio.h>
#include <stdbool.h>
int main(void){
int i,j,m;
scanf("%d",&m);
char arr[m][50];
for(i=0;i<m;i++){
for(j=0;j<50;j++){
printf("please enter a string");
scanf("%s",&arr[i][j]);
/*j is always 0. arr[i] takes string without space and store ending with null*/
break;
}
}
//Everything fine upto this,including storing a small continuous string in arr[i](where i<50) and null terminating*/
for(i=0;i<m;i++){
for(j=0;j<50;j++){
printf("%s\n",arr[i][j]);
break;
}
}
}

Answer 1

You program has several issues, like using wrong format specifier:
scanf("%s",&arr[i][j]);
arr[i][j] is a character and you are using %s format specifier. If you want your program should take string as input, you just need to do:
scanf("%s",arr[i]);
Since, you have given the size 50 characters, put a restriction in scanf() to not to read more than 49 characters (the remain one character space is for null terminating character) like this:
scanf("%49s",arr[i]);
^^
Beware with this, it does not discard the remaining input from input stream when the input characters are more than 49 and the remaining characters will be consumed by consecutive scanf() call.
If you want to drop the extra input which wasn't consumed by scanf(), one way of doing it is to read and discard the extra input using a loop, like this:
int c;
while((c = getchar()) != '\n' && c != EOF)
/* discard the character */;
In case if you have any doubt on how this will discard the extra input, I would suggest first go through getchar().
Putting all these together, you can do:
#include <stdio.h>
int main(void){
int i,m;
scanf("%d",&m);
char arr[m][50];
for(i=0;i<m;i++){
printf("please enter a string");
scanf("%49s",arr[i]);
int c;
while((c = getchar()) != '\n' && c != EOF) // <=== This loop read the extra input characters and discard them
/* discard the character */;
}
for(i=0;i<m;i++){
printf("%s\n",arr[i]);
}
return 0;
}
EDIT
The below edit is because OP updated question and added - Why this code does not work.only this code.Not a new way to write it
Above in my answer, I have already stated that you are using wrong format specifier in the scanf(). In this part of your code:
for(i=0;i<m;i++){
for(j=0;j<50;j++){ // <====== Nested for loop
printf("please enter a string");
scanf("%s",&arr[i][j]);
// since the nested loop is supposed to run 50 times, assuming you are trying to read character by character and using %s format specifier
break;
// the nested loop will break in the first iteration itself unconditionally, do you really need nested loop here!
}
}
Check the inline comments. Hope this might give an idea of the mistakes you are doing.
Seems that you want to read string character by character using scanf(). If this is the case than make sure to take care of null terminating character because, in C, strings are actually one-dimensional array of characters terminated by a null character '\0'.
You can do:
#include <stdio.h>
void discard_extra_input() {
int c;
while((c = getchar()) != '\n' && c != EOF)
/* discard the character */;
}
int main(void){
int i,j,m;
printf ("Enter number of strings: ");
scanf("%d",&m);
discard_extra_input();
char arr[m][50];
for(i=0;i<m;i++){
printf("please enter string number %d: ", i+1);
for(j=0;j<49;j++){
scanf("%c",&arr[i][j]);
if (arr[i][j] == '\n') {
//need to add null terminating character manually
arr[i][j] = '\0';
break;
}
}
if (j==49) {
// In case where the input from user is more than 50 characters,
// need to add null terminating character manually.
arr[i][j] = '\0';
// discard the extra input when input from user is more than 50 characters.
discard_extra_input();
}
}
for(i=0;i<m;i++){
for(j=0;j<50 && arr[i][j]!='\0';j++){
printf("%c",arr[i][j]);
}
printf ("\n");
}
return 0;
}
The code is self explanatory except one thing - call to discard_extra_input() function after first input from user scanf("%d",&m);. Reason -
Look at the statement:
scanf("%c",&arr[i][j]);
the %c format specifier will consume the leftover newline character '\n' from the input stream due to the ENTER key pressed after first input by the user (number of strings input from user). Hence, in order to discard it, calling discard_extra_input() function. In the other place it has been used to discard the characters when user entered string of size more than 49.
Hope this helps.

Answer 2

I know the code. But looking for specific ans. Where the problem lies with the code
The problem is here:
scanf("%s",&arr[i][j]);
and here:
printf("%s", arr[i][j]);
This is the specific answer you are looking for.
%s won't do any bound checking. It adds the characters starting from the memory location arr + i * m + j to arr + i * m + j + (length of input) + 1 (one extra char for the additional null character that scanf appends). Take a sample input. Assume an arbitrary starting address for arr and do the maths.
Also consider any writes beyond the allocated space for arr leads to undefined behavior.
Similarly printf("%s", arr[i][j]); will try to start reading from the address arr[i][j] till it finds a null character. It would usually lead to crash of the code because if your string has ascii characters, the address would be too low to point to any valid user-mapped memory.
If your code is working, its mostly because you already have a UB in your scanf.
Get a pen and paper and do some dry runs

Answer 3

This is a pretty simple problem buddy. You've got the idea right actually, that you need to use 2d array to store strings. Just that the usage is slightly wrong.
First of all let me tell you how 2d arrays need to be used to store in c. In your 2-D array, you've got rows and columns. Say, row represented by i and columns by j, i.e, each row arr[i] contains j elements. So in your context, each row arr[i] contains each string of upto 50 chars. So scanf should be just for arr[i]. And you need to loop with for, m times to accept m strings.
Same applies to printing as well.
Here is the working code:
#include <stdio.h>
#include <stdbool.h>
int main(void){
int i,j,m;
printf("\nenter m value:");
scanf("%d",&m);
char arr[m][50];
for(i=0;i<m;i++){
printf("\nplease enter a string no %d: ", (i+1));
scanf("%s",arr[i]);
}
printf("\nthe strings are: \n");
for(i=0;i<m;i++){
printf("\n%s\n",arr[i]);
}
}
And the output in case you want to cross check:
OUTPUT:
enter m value: 3
please enter a string no 1: qwerty
please enter a string no 2: asdfgh
please enter a string no 3: zxcvbn
the strings are:
qwerty
asdfgh
zxcvbn