printf（）打印整个字符矩阵

Question

As marked in the code, the first printf() rightfully prints only the i-th line of the matrix. But outiside the loop, both printf() and strcat() act on the whole matrix from i-th line on as a single-lined string. This means that

printf("%s\n",m_cfr[0])

will print whole matrix, but m_cfr[i] will print whole matrix from the i-th line on. char* string is a single lined string with no spaces.

trasp(char* string)
{
    int row = strlen(string) / 5;
    char m[row][5];
    char m_cfr[row][5];
    char cfr[row*5];

    memset(cfr, 0, row * 5);

    int key[5] = {3, 1, 2, 0, 4};
    int k      = 0;

    for (i = 0 ; i < row ; i++)
    {
        strncpy(m[i], string + k, 5);

        m[i][5] = '\0';
        k      += 5;    
    }

    for (i = 0 ; i < row ; i++)
    {
        for (j = 0 ; j < 5 ; j++)
        {
            m_cfr[i][key[j]] = m[i][j];
        }
        m_cfr[i][5] = '\0';
        printf("%s\n", m_cfr[i]);  //--->prints only line i
    }    
    printf("%s\n", m_cfr[0]); //prints whole matrix    
    strcat(cfr, m_cfr[0]);   //concatenates whole matrix       
    printf("%s\n", cfr);
}

Answer 1

In your code, your array definition is

char m_cfr[row][5];

while you're accessing

 m_cfr[i][5] = '\0';
 /*       ^
          | 
          there is no 6th element
 */

You're facing off-by-one error . Out-of-bound memory access causes undefined behaviour .

Maybe you want to change the null-terminating statement to

m_cfr[i][4] = '\0'; //last one is null

Answer 2

%s expects a char* and prints everything until it encounters a \\0 . So,

printf("%s\n", m_cfr[i]);
printf("%s\n",m_cfr[0]);
strcat(cfr,m_cfr[0]);

All exhibit Undefined Behavior as m_cfr[i] , m_cfr[0] and m_cfr[0] are char s and not char* s and %s as well as both the arguments of strcat expects a char* . Also, as SouravGhosh points out , using

m_cfr[i][5] = '\0';

And

m[i][5] = '\0';

Are wrong.

To fix the former issue, use

printf("%s\n", &m_cfr[i]);
printf("%s\n",m_cfr);
strcat(cfr,&m_cfr[0]);

To print the whole string and concatenate the two strings in the arguments of strcat or if you wanted to print just the chars , use

printf("%c\n", m_cfr[i]);
printf("%c\n",m_cfr[0]);

As for the latter issue, use

char m[row][5]={{0}};
char m_cfr[row][5]={{0}};