在while循环中使用getchar（），打印一个语句两次..怎么样？

Question

I have a very simple program like this

int main()
{
    int opt;
    int n;
    int flag = 1;
    while(flag)
    {
        printf("m inside while.Press c to continue\n");
        if((opt = getchar())== 'c')
        {
            printf("choose a number\n");
            scanf(" %d",&n);
            switch(n)
            {
            case 0:
                printf("m zero\n");
                break;
            case 1:
               printf("entered one\n");
               break;
            case 3:
               printf("m exit\n");
               flag = 0;
               break;
            }
            printf("m broke\n");
        }
    }
    printf("m out\n");
    return 0;
}

I get output like this:

m inside while.Press c to continue
c
choose a number
1
entered one
m broke
m inside while.Press c to continue
m inside while.Press c to continue
c
choose a number

My doubt is why "m inside while.Press c to continue" gets printed twice after every loop??

Thanks in advance

Answer 1

This is because of \\n character left behind by previous scanf . When you input a number and press Enter key, an additional \\n character passed to the standard input buffer. scanf reads that nuber leaving behind \\n in the buffer. On next iteration of loop getchar reads \\n before pressing any character by you and hence m inside while.Press c to continue printed twice as \\n is not c .
Place this snippet of code just after the scanf statement in your while loop to eat up the newline characters

while(getchar() != '\n');  

This will eat up any number of \\n .
For more detailed explanation on the behavior of getchar read this answer .
You final code should be

 int main()
{
    int opt;
    int n;
    int flag = 1;
    while(flag)
    {
        printf("m inside while.Press c to continue\n");
        if((opt = getchar())== 'c')
        {
            printf("choose a number\n");
            scanf(" %d",&n);
            while(getchar() != '\n');
            switch(n)
            {
            case 0:
                printf("m zero\n");
                break;
            case 1:
               printf("entered one\n");
               break;
            case 3:
               printf("m exit\n");
               flag = 0;
               break;
            }
            printf("m broke\n");
        }
    }
    printf("m out\n");
    return 0;
}

Answer 2

After scanf reads the input there is a '\\n' still in the buffer you have to clear it otherwise it will be readed by getchar in the next time and as it's != 'c' it will prompt again:

Try this :

        printf("choose a number\n");
        scanf(" %d",&n);
        char c;
        while (c = getchar != '\n' && c != EOF);  // clear the buffer

Answer 3

while(flag)
{
    printf("m inside while.Press c to continue\n");
    while((opt=getchar()) != '\n') {
    if(opt == 'c')
    {
        printf("choose a number\n");
        scanf(" %d",&n);
        switch(n)
        {
        case 0:
            printf("m zero\n");
            break;
        case 1:
           printf("entered one\n");
           break;
        case 3:
           printf("m exit\n");
           flag = 0;
           break;
        }
        printf("m broke\n");
    }
    }
}