尝试在 C 中打印数组时不断出现乱码 output

Question

I am trying to write a program that reverses words in a sentence, the code is able to do that but also prints gibberish in between. (see below code for sample)

#include <stdio.h>
#define MAX_VALUE 50
int main(){
    char c[MAX_VALUE],
        b[MAX_VALUE];
    int i = 0, j = 0;
    while ((c[i] = getchar()) != '\n'){
        i++;
    }
    
    for(i = MAX_VALUE; i >= 0; i--){
        if (c[i] == ' '){
            for(j = i+1; j < MAX_VALUE; j++){
                if (c[j] != ' '){
                    printf("%c", c[j]);
                }
                else{
                    break;
                }
            }
            printf(" ");
        }
    }
    
    while (c[i] != ' '){
        printf("%c", c[i]);
        i++;
    }
}

Loop goes backwards when it detects a space it prints the word until it finds another space then goes backwards again from where it left off last

The input and output expected:

input: test a is this

output: this is a test

What I get:

input: test a is this

output:

this
`����l�,+�D     �=� is a test

Answer 1

When you set up your initial for loop you set i to MAX_VALUE which is out of bounds for a char[MAX_VALUE] array, where the last index would be MAX_VALUE - 1

Answer 2

This for loop

while ((c[i] = getchar()) != '\n'){
    i++;
}

can write outside the array c and store the new line character '\n' in the array.

And this output consisting from two lines

this
`����l�,+�D     �=� is a test

demonstrates that the new line character was stored in the array.

Also you forgot to append the entered sequence of characters with the zero terminating character '\0' to form a string.

You could write the while loop the following way

int ch;

while ( i < MAX_VALUE - 1 && ( ch = getchar() ) != '\n' && ch != EOF ){
    c[i++] = ch;
}
c[i] = '\0';

Or if you want store the entered sequence without the terminating zero character '\0' then the loop can look like

int ch;

while ( i < MAX_VALUE && ( ch = getchar() ) != '\n' && ch != EOF ){
    c[i++] = ch;
}

In this case the variable i will contain the number of entered symbols in the array and you should use its value instead of the value MAX_VALUE in the following loops.

This outer for loop

 for(i = MAX_VALUE; i >= 0; i--){

is incorrect because the user can enter a sequence of symbols that contains less than MAX_VALUE symbols. You should either use standard string function strlen (if you appended the sequence with the terminating zero character '\0' ) to determine the length of the string stored in the array or the value of the variable i as mentioned above that denotes the number of entered symbols.

And it is not enough to check this condition

if (c[i] == ' '){

because the leading spaces can be absent in the entered string. So you need to check also whether i is equal to 0 . Otherwise the output can be incorrect if the sequence contains for example only one word.

The inner for loop

for(j = i+1; j < MAX_VALUE; j++){

again is incorrect by the same reason of using MAX_VALUE in the condition.

Pay attention to that the array b

char c[MAX_VALUE],
    b[MAX_VALUE];

is not used in your program.

Using your approach I can suggest the following solution to output words of a string in the reverse order shown in the demonstrative program below.

#include <stdio.h>
#include <string.h>
#include <ctype.h>

int main( void ) 
{
    const char *s = "test  a  is  this";

    for( size_t i = strlen( s ); i != 0; )
    {
        while ( i != 0 && isblank( ( unsigned char )s[i-1]) )
        {
            putchar( s[--i] ); 
        }

        size_t j = i;

        while ( j != 0 && !isblank( ( unsigned char )s[j-1]) )
        {
            --j;
        }

        if ( j != i )
        {
            size_t last = i;
            i = j;
            while ( j != last )
            {
                putchar( s[j++] );
            }
        }
    }

    putchar( '\n' );
}

The program output is

this  is  a  test