You'd probably need a while loop. This is the situation for which do-while would be good, but for some reason everyone's trying to get rid of do-while these days.
So you'd first do the random number as usual, then on the next line, start a while loop that continues while the number is forbidden. In this case, while (num == 7)
This is the kind of loop I'd want to put on one line, but for readability:
int num = random (1, 10);
while (num == 7)
{
num = random (1, 10);
}
If you're curious, the do-while version would be like:
int num;
do { num=random (1, 10); }
while (num == 7);

Most people seem to agree. They're more useful when the script blocks for user input, which really shouldn't be done outside of text-based games. They also make more sense in older versions of C, when you'd declare all the variables at the start of a function, but not assign them values until later.
Mostly, if I have to copy a line of code to be both before and inside a while loop, I'd rather use a do-while. But this is not necessary.

A do while loop is useful when you want something to run at least one time. fore example:

// this is c++ code
#include <iostream>
using namespace std;

void whileLoop(int x);
void doWhileLoop(int x);

int main()
{
    int x = 0;

whileLoop(x);
doWhileLoop(x);

    return 0;
}

void whileLoop(int x)
{
    // loop will check the condition and not run

    while (x > 0)
    {
        cout << "Running while loop. x is: " << x << endl; //
        x--; //decrement x
    }
}

void doWhileLoop(int x)
{
    // loop will run this time because it is a do while

    do
    {
        cout << "Running do while loop. x is: " << x << endl;
        x--; // decrement x
    }
    while (x > 0);
}

This is a bit contrived but i hope you get the point. do whiles can be very useful in many cases. They can help with debugging and protecting against your conditionals acting up because of weird input.