Why is NULL printed here? - C-90 - c

Question

The code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define N 500
#define M 100
/*Version 1*/
void replace_word(char *word);
void add_new_word_dictionary(void);
void do_nothing(void);
void return_basic(void);
void check_word(void);
void compute_words(void);
void compute_characters(void);
void compute_ch_sp(void);
void compute_num_dif_words(void);
void create_istogram(void);
void save_file(void);
int get_choice(void);
void return_word(void);
void insert_text(int numwords, char matrix[N][M], int posit);
int main() {
int j;
int choice = 0;
char matrix[N][M];
char word[40] = { "t" };
while (1) {
choice = get_choice();
if (choice == 0) {
insert_text(M, matrix, 1);
}
if (choice == 1) {
add_new_word_dictionary();
}
if (choice == 2) {
do_nothing();
}
if (choice == 3) {
save_file();
}
if (choice == 4) {
compute_words();
}
if (choice == 5) {
break;
}
for (j = 0; j < M; j++) {
printf("%s", matrix[N][M]);
}
}
printf("\n End of Program \n");
return 0;
}
void replace_word(char *word) {
return;
}
void add_new_word_dictionary(void) {
char word[50] = { "s" };
printf("\nPlease enter the word\n");
scanf("\n%s", word);
printf("Your word is %s", word);
return;
}
void do_nothing(void) {
printf("\n do_nothing \n");
return;
}
void return_basic(void) {
printf("\n return basic \n");
return;
}
void check_word(void) {
printf("\n check word \n");
return;
}
void compute_words(void) {
printf("\n compute_words \n");
return;
}
void compute_characters(void) {
printf("\n compute characters \n");
}
void compute_ch_sp(void) {
printf("\n compute_ch_sp \n");
return;
}
void compute_num_dif_words(void) {
printf("\n compute_num_same_words \n");
return;
}
void create_istogram(void) {
printf("\n create istogram \n");
return;
}
void save_file(void) {
printf("\n save_file \n");
return;
}
int get_choice(void) {
int choice = 0;
printf("\n Select a choice from the below \n");
printf("\n Select 0 to add text \n");
printf("\n Select 1 to add new words in the dictionary \n");
printf("\n Select 2 to enter enter correction mode \n");
printf("\n Select 3 to save the text \n");
printf("\n Select 4 to see the statistics about your text \n");
printf("\n Select 5 to exit the program\n");
scanf("\n%d", &choice);
return choice;
}
void insert_text(int numwords, char matrix[N][M], int posit) {
int i;
int j;
char word2[40] = { "" };
while (strcmp(word2, "*T*E*L*O*S*")) {
printf("\n Add the word \n");
scanf("\n%s", word2);
if (posit + 1 > numwords) {
printf("\n Out of Bounds \n ");
}
for (i = numwords - 2; i >= posit; i--) {
strcpy(matrix[i + 1], matrix[i]);
if (!i)
break;
}
strcpy(matrix[posit], word2);
j++;
}
for (i = 0; i < j; i++) {
printf("%s", matrix[i]);
printf("\n j is %d\n", j);
}
return;
}
The problem: I have a function called insert_text. This function adds a string in the 1st position of an array (at least that is what I think it does) and it is called if choice is 0 and executes itself until the string *ΤELOS* is given. When in insert_text I print matrix I get a bunch of *(null)*s... I can count how many words matrix has (by declaring a variable j and incrementing by 1 inside the while loop, but that does not seem to work either. How can I fix this?

Answer 1

The printing code is incorrect: matrix is an array of N arrays of M characters, where you store null terminated C strings. As coded, you pass a single character just beyond the end of the array to printf for %s, which expects a string. The loop should be:
for (j = 0; j < N; j++) {
printf("%s ", matrix[j]);
}
Note that char matrix[N][M]; is uninitialized, so its contents will seem random. Initialize matrix as char matrix[N][M] = { "" };
Also note that in add_new_word_dictionary(), the scanf() conversion should be scanf("%49s", word); to prevent a potential buffer overflow if the user enters a very long word.
Same in insert_text, the code should be scanf("%39s", word2) ans you should test the return value to check for input errors.
Finally, arrays are indexed from 0 in C, so insert_text should be given a position of 0 and the number of words should be N, not M.
Both the test and the insertion loop have problems too.
Here is a modified version:
// call from main as insert_text(N, matrix, 0);
//
void insert_text(int numwords, char matrix[N][M], int posit) {
char word2[40];
int i, j = 0;
for (;;) {
printf("\n Add the word \n");
if (scanf("%39s", word2) != 1) {
break; // end of file or input error
}
if (!strcmp(word2, "TELOS")) {
break; // magic word
}
if (posit >= numwords) {
printf("\n Out of Bounds \n");
break;
}
for (i = numwords - 2; i >= posit; i--) {
strcpy(matrix[i + 1], matrix[i]);
}
strcpy(matrix[posit], word2);
j++;
}
for (i = 0; i < j; i++) {
printf("%s ", matrix[i]);
}
printf("\n j is %d\n", j);
}
Here is a modified version of the whole program:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define N 500
#define M 100
/*Version 1*/
void replace_word(char *word);
void add_new_word_dictionary(void);
void do_nothing(void);
void return_basic(void);
void check_word(void);
void compute_words(void);
void compute_characters(void);
void compute_ch_sp(void);
void compute_num_dif_words(void);
void create_istogram(void);
void save_file(void);
int get_choice(void);
void return_word(void);
void insert_text(int numwords, char matrix[N][M], int posit);
int main() {
int j, done = 0;
char matrix[N][M] = { "" };
while (!done) {
switch (get_choice()) {
case 0:
insert_text(N, matrix, 0);
break;
case 1:
add_new_word_dictionary();
break;
case 2:
do_nothing();
break;
case 3:
save_file();
break;
case 4:
compute_words();
break;
default:
done = 1;
break;
}
for (j = 0; j < N; j++) {
if (matrix[j][0])
printf("%s ", matrix[j]);
}
printf("\n");
}
printf("\n End of Program \n");
return 0;
}
void add_new_word_dictionary(void) {
char word[50];
printf("\nPlease enter the word\n");
if (scanf("%49s", word) == 1)
printf("Your word is %s\n", word);
}
void replace_word(char *word) { printf("\n replace word \n"); }
void do_nothing(void) { printf("\n do_nothing \n"); }
void return_basic(void) { printf("\n return basic \n"); }
void check_word(void) { printf("\n check word \n"); }
void compute_words(void) { printf("\n compute_words \n"); }
void compute_characters(void) { printf("\n compute characters \n"); }
void compute_ch_sp(void) { printf("\n compute_ch_sp \n"); }
void compute_num_dif_words(void) { printf("\n compute_num_same_words \n"); }
void create_istogram(void) { printf("\n create istogram \n"); }
void save_file(void) { printf("\n save_file \n"); }
int get_choice(void) {
int choice = -1;
printf("\nSelect a choice from the below \n");
printf("Select 0 to add text \n");
printf("Select 1 to add new words in the dictionary \n");
printf("Select 2 to enter enter correction mode \n");
printf("Select 3 to save the text \n");
printf("Select 4 to see the statistics about your text \n");
printf("Select 5 to exit the program\n");
scanf("%d", &choice);
return choice;
}
// call from main as insert_text(N, matrix, 0);
//
void insert_text(int numwords, char matrix[N][M], int posit) {
char word2[40];
int i;
for (;;) {
printf("\n Add the word \n");
if (scanf("%39s", word2) != 1) {
break; // end of file or input error
}
if (!strcmp(word2, "TELOS")) {
break; // magic word
}
if (posit >= numwords) {
printf("\n Out of Bounds \n");
break;
}
for (i = numwords - 2; i >= posit; i--) {
strcpy(matrix[i + 1], matrix[i]);
}
strcpy(matrix[posit], word2);
posit++;
}
for (i = 0; i < posit; i++) {
printf("%s ", matrix[i]);
}
printf("\n posit is %d\n", posit);
}