Manipulation of lines of text - c

Question

The below code tries to manipulate several lines of text one at the time.
1.My first issue is to write a loop to read several lines of text(using scanf()) and quit when the first character typed is a newline. These lines of text have some conditions: The first character must be a number between 2 and 6 followed by a space and a line of text(<80).This number will make "dance" the text.
2.My second issue is to figure out how to convert the letters from small to capital and viceversa according to the first number typed. I have to function to make these conversions but I don't know how to call them to change the text.For example: if I typed "3 apples and bananas" the correct output should be "AppLes And BanNas".As you see, the white spaces are ignored and the text always start with a capital letter.
#include <stdio.h>
#include <stdlib.h>
#include <string>
#include <ctype.h>
using namespace std;
void print_upper(string s1);
void print_lower(string s2);
void main(void)
{
char text[80];
text[0]='A';//Initialization
int count_rhythm;
while (text[0] != '\n'){//To make the loop run until a newline is typed
scanf(" %79[^\n]",text);
if(isdigit(text[0])) //To verify that the first character is a number
{
printf("\nGood");//Only to test
}
else
{
printf("\nWrong text\n");//Only to test
}
}
}
void print_upper(string s1)//Print capital letters
{
int k1;
for(k1=0; s1[k1]!='\0'; ++k1)
putchar(toupper(s1[k1]));
}
void print_lower(string s2)//Print small letters
{
int k2;
for(k2=0; s2[k2]='\0'; ++k2)
putchar(tolower(s2[k2]));
}

Answer 1

You could also define a function printNthUpper() which would take a string and and integer n that would specify which characters to print in upper case. The of the function would be a loop similar to the functions you already have, but with a conditional that compares the provided integer value and the index of a given letter to decide whether to
call toupper() (e.g. printf("%c", i%n == 0 ? toupper(s[i]) : s[i]);).

Answer 2

to write a loop to read several lines of text you can use condition based infinite loop in combination with fgets rather than using scanf.
char line[80];
char result[80]
while(1)
{
fgets(line,sizeof(line),stdin); //read line with fgets
puts(line);
if(line[0]=='\n')
break;
if((strlen(line)>=4) &&'2'< =line[0] && line[0] <= '6' && line[1]==' ')
{
strcpy(result,change_case_of_nth_char(line));// call change case of nth letter
}
else
{
//prompt user to enter input again
}
}
char *change_case_of_nth_char(char *str)
{
}