#include <bits/stdc++.h>
using namespace std;
int main()
{
    string s;
    cin >> s;
    int num = 0;
    for (int i = 0; i < s.length(); i++)
    {
        if (s[i] <= '9' && s[i] >= '0')
        {
            num++;
        }
    }
    cout << num;
    return 0;
}

#include <bits/stdc++.h> 
using namespace std;
int main()
{
    string s;
    cin >> s;
    int num = 0;
    for(int i = 0; i < s.length(); i++)
    {
        if(s[i] >= '0' and s[i] <= '9')
        {
            num++;
        }
    }
    cout << num;
    return 0;
}

#include<bits/stdc++.h>
using namespace std;
int ans;
int main()
{
    char c[10000005];
    scanf("%s",&c);
    int len=strlen(c);
    for(int i=0;i<len;i++)
        if(c[i]>='0'&&c[i]<='9')
            ans++;
    printf("%d",ans);
    return 0;
}

极简代码：

#include <bits/stdc++.h>
char c;
int s;
int main()
{
    while (std::cin >> c)
        if (c >= '0' && c <= '9') s++;
    std::cout << s;
    return 0;
}