为什么 <null> 每次我调用不同的功能时都会消失吗？

Question

Being a newbie at programming I make many mistakes but I cannot understand perfect pointers.I wrote a code in which into the function Add I add data about food schedule.After with function Modify I want my code to print the last added data.But here is the problem,it prints me everytime .Do you know how to fix that?

int login ( char name[50] );
void autoPass( char username[50] );
char pass[50];
int n = -1; //global counter
char *table[100][4];
void Add( int n );
void Modify( int n );

int main()
{
    char selection[7];
    printf( "\n\nChoose between: Add,Modify,View,Search,Sort,Exit\nSelection: " );
    scanf ( "%s",&selection );

    while ( ( strcmp( selection,"Exit" )  == 0 ) == 0 )
    {
        if (strcmp( selection,"Add" ) == 0 )
        {
            Add( n );
            printf( "Press any character to continue or Exit to finish.\n" );
            scanf ( "%s",&selection );
            printf( "check1" );
        }else if ( strcmp( selection,"Modify" ) == 0 )
        {
            Modify( n );
            printf( "Press any character to continue or Exit to finish.\n" );
            scanf ( "%s", &selection );
            printf( "check2" );
        }
    }
}



void Add (int n)
{
    int i,j;
    n++;
    for ( j = 0 ; j < 4 ; j++ )
    {
        if ( j == 0 )
        {
            printf ( "Enter a food:\n" );
            table[n][j] = (char*) malloc( 30 );
            scanf( "%s",table[n][j] );
        }else if (j == 1 )
        {
            printf("Enter calories:\n");
            table[n][j]=(char*) malloc(30);
            scanf("%s",table[n][j]);
        }else if ( j == 2 )
        {
            printf( "Enter the time you ate:\n" );
            table[n][j] = (char*) malloc( 30 );
            scanf ( "%s",table[n][j] );
        }else if ( j == 3 )
        {
            if (atof( table[n][j-1]) >= 5.00 && atof( table[n][j-1] ) <= 11.59 )
            {
                table[n][j] = "prwino";
            }else if( atof( table[n][j-1] ) >= 12.00 && atof( table[n][j-1] ) <= 19.59 )
            {
                table[n][j] = "mesimeriano";
            }else if ( atof( table[n][j-1] ) >= 20.00 && atof( table[n][j-1] ) <= 4.59 )
            {
                table[n][j] = "vradino";
            }
        }
    }
}

void Modify( int n )
{
    int i,j;
    for ( j=0 ; j < 4 ; j++ )
    {
       printf ( "%s",table[n][j]," " );
     }
     printf( "\n" );
 }

I expected my Modify function to print the data I added before with function Add.Instead of this everytime it is printing me .

Answer 1

Well your code seems to have some problems.

you should replace all scanf calls:

scanf ( "%s",&selection );

and change them with:

scanf ( "%s",selection ); /// Drop the &

You need to drop the & because the format %s expects argument of type char* but when you use &selection the argument becomes char (*)[7] which is not char* like you declare it.

Also you should ALWAYS check the return value of scanf :

if ( scanf ( "%6s",selection ) != 1 ) ///notice 6 there?
{
    printf("Error, scanf()\n" );///Is there to read only 6...
    exit( EXIT_FAILURE );
}

Another issue is inside void Add ( int n ) , there you increment n :

n++;

But you have declared also a n as global: int n = -1; //global counter int n = -1; //global counter

Which n are you expecting to increment?

Inside the modify() functions you have too many arguments:

printf ( "%s",table[n][j]," " );

Drop the last " " :

printf ( "%s",table[n][j] );

As a suggestion you can use strcasecmp instead of strcmp . You need to include strings.h for that.